openPMD C++ & Python API¶

openPMD is an open meta-data schema that provides meaning and self-description for data sets in science and engineering. See the openPMD standard for details of this schema.

This library provides a reference API for openPMD data handling. Since openPMD is a schema (or markup) on top of portable, hierarchical file formats, this library implements various backends such as HDF5, ADIOS1, ADIOS2 and JSON. Writing & reading through those backends and their associated files is supported for serial and MPI-parallel workflows.

Supported openPMD Standard Versions¶

openPMD-api is a library using semantic versioning for its public API. Please see this link for ABI-compatibility. The version number of openPMD-api is not related to the version of the openPMD standard.

The supported version of the openPMD standard are reflected as follows: standardMAJOR.apiMAJOR.apiMINOR.

openPMD-api version	supported openPMD standard versions
`2.0.0+`	`2.0.0+` (not released yet)
`1.0.0+`	`1.0.1-1.1.0` (not released yet)
`0.13.1-0.14.0` (beta)	`1.0.0-1.1.0`
`0.1.0-0.12.0` (alpha)	`1.0.0-1.1.0`

Code of Conduct¶

Our Pledge¶

In the interest of fostering an open and welcoming environment, we as contributors and maintainers pledge to making participation in our project and our community a harassment-free experience for everyone, regardless of age, body size, disability, ethnicity, sex characteristics, gender identity and expression, level of experience, education, socio-economic status, nationality, personal appearance, race, religion, or sexual identity and orientation.

Our Standards¶

Examples of behavior that contributes to creating a positive environment include:

Using welcoming and inclusive language
Being respectful of differing viewpoints and experiences
Gracefully accepting constructive criticism
Focusing on what is best for the community
Showing empathy towards other community members

Examples of unacceptable behavior by participants include:

The use of sexualized language or imagery and unwelcome sexual attention or advances
Trolling, insulting/derogatory comments, and personal or political attacks
Public or private harassment
Publishing others’ private information, such as a physical or electronic address, without explicit permission
Other conduct which could reasonably be considered inappropriate in a professional setting

Our Responsibilities¶

Project maintainers are responsible for clarifying the standards of acceptable behavior and are expected to take appropriate and fair corrective action in response to any instances of unacceptable behavior.

Project maintainers have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, or to ban temporarily or permanently any contributor for other behaviors that they deem inappropriate, threatening, offensive, or harmful.

Scope¶

This Code of Conduct applies both within project spaces and in public spaces when an individual is representing the project or its community. Examples of representing a project or community include using an official project e-mail address, posting via an official social media account, or acting as an appointed representative at an online or offline event. Representation of a project may be further defined and clarified by project maintainers.

Enforcement¶

Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting the project team at openpmd@plasma.ninja. All complaints will be reviewed and investigated and will result in a response that is deemed necessary and appropriate to the circumstances. The project team is obligated to maintain confidentiality with regard to the reporter of an incident. Further details of specific enforcement policies may be posted separately.

Project maintainers who do not follow or enforce the Code of Conduct in good faith may face temporary or permanent repercussions as determined by other members of the project’s leadership.

Attribution¶

This Code of Conduct is adapted from the Contributor Covenant, version 1.4, available at https://www.contributor-covenant.org/version/1/4/code-of-conduct.html

For answers to common questions about this code of conduct, see https://www.contributor-covenant.org/faq

Citation¶

openPMD (Open Standard for Particle-Mesh Data Files) is a community project with many people contributing to it. If you use openPMD and/or openPMD related software in your work, please credit it when publishing and/or presenting work performed with it in order to give back to the community.

openPMD-standard¶

The central definition of openPMD is the meta data schema defined in openPMD/openPMD-standard. The most general reference to openPMD is:

Tip

Axel Huebl, Remi Lehe, Jean-Luc Vay, David P. Grote, Ivo F. Sbalzarini, Stephan Kuschel, David Sagan, Christopher Mayes, Frederic Perez, Fabian Koller, and Michael Bussmann. “openPMD: A meta data standard for particle and mesh based data,” DOI:10.5281/zenodo.591699 (2015)

Since the openPMD-standard is an actively evolving meta data schema, a specific version of the openPMD standard might be used in your work. You can select a version-specific DOI from the release page and add the version number to the cited title, e.g.

Note

[author list as above] … “openPMD 1.1.0: A meta data standard for particle and mesh based data,” DOI:10.5281/zenodo.1167843 (2018)

openPMD-api¶

openPMD-api is a software library that provides a reference implementation of the openPMD-standard for popular data formats. It targets both desktop as well as high-performance computing environments.

It is good scientific practice to document all used software, including transient dependencies, with versions in, e.g. a methods section of a publication. As a software citation, you almost always want to refer to a specific version of openPMD-api in your work, as illustrated for version 0.10.3:

Tip

Fabian Koller, Franz Poeschel, Junmin Gu, and Axel Huebl. “openPMD-api 0.10.3: C++ & Python API for Scientific I/O with openPMD,” DOI:10.14278/rodare.209 (2019)

A list of all releases and DOIs can be found on the release page.

We also provide a DOI that refers to all releases of openPMD-api:

Note

Axel Huebl, Franz Poeschel, Fabian Koller, and Junmin Gu. “openPMD-api: C++ & Python API for Scientific I/O with openPMD” DOI:10.14278/rodare.27 (2018)

Dependent Software¶

The good way to control complex software environments is to install software through a package manager (see installation). Furthermore, openPMD-api provides functionality to simplify the documentation of its version and enabled backends:

C++14¶

#include <openPMD/openPMD.hpp>
#include <iostream>

namespace io = openPMD;

// ...
std::cout << "openPMD-api: "
          << io::getVersion() << std::endl;
std::cout << "openPMD-standard: "
          << io::getStandard() << std::endl;

std::cout << "openPMD-api backend variants: " << std::endl;
for( auto const & v : io::getVariants() )
    std::cout << "  " << v.first << ": "
              << v.second << std::endl;

Python¶

import openpmd_api as io

print("openPMD-api: {}"
      .format(io.__version__))
print("openPMD-api backend variants: {}"
      .format(io.variants))

Installation¶

Our community loves to help each other. Please report installation problems in case you should get stuck.

Choose one of the installation methods below to get started:

Using the Spack Package¶

A package for openPMD-api is available via the Spack package manager.

# optional:               +python +adios1 -adios2 -hdf5 -mpi
spack install openpmd-api
spack load -r openpmd-api

Using the Conda Package¶

A package for openPMD-api is available via the Conda package manager.

# optional:                      OpenMPI support  =*=mpi_openmpi*
# optional:                        MPICH support  =*=mpi_mpich*
conda create -n openpmd -c conda-forge openpmd-api
conda activate openpmd

Using the Brew Package¶

A package for openPMD-api is available via the Homebrew/Linuxbrew package manager.

brew tap openpmd/openpmd
brew install openpmd-api

Brew ship only the latest release, includes (Open)MPI support and lacks the ADIOS1 backend.

Using the PyPI Package¶

A package for openPMD-api is available via the Python Package Index (PyPI).

On very old macOS versions (<10.9) or on exotic processor architectures, this install method compiles from source against the found installations of HDF5, ADIOS1, ADIOS2, and/or MPI (in system paths, from other package managers, or loaded via a module system, …).

# we need pip 19 or newer
# optional:                   --user
python3 -m pip install -U pip

# optional:                        --user
python3 -m pip install openpmd-api

If MPI-support shall be enabled, we always have to recompile:

# optional:                                    --user
python3 -m pip install -U pip setuptools wheel
python3 -m pip install -U cmake

# optional:                                                                   --user
openPMD_USE_MPI=ON python3 -m pip install openpmd-api --no-binary openpmd-api

For some exotic architectures and compilers, you might need to disable a compiler feature called link-time/interprocedural optimization if you encounter linking problems:

export CMAKE_INTERPROCEDURAL_OPTIMIZATION=OFF
# optional:                                                --user
python3 -m pip install openpmd-api --no-binary openpmd-api

From Source with CMake¶

You can also install openPMD-api from source with CMake. This requires that you have all dependencies installed on your system. The developer section on build options provides further details on variants of the build.

Linux & OSX¶

git clone https://github.com/openPMD/openPMD-api.git

mkdir openPMD-api-build
cd openPMD-api-build

# optional: for full tests
../openPMD-api/share/openPMD/download_samples.sh

# for own install prefix append:
#   -DCMAKE_INSTALL_PREFIX=$HOME/somepath
# for options append:
#   -DopenPMD_USE_...=...
# e.g. for python support add:
#   -DopenPMD_USE_PYTHON=ON
cmake ../openPMD-api

cmake --build .

# optional
ctest

# sudo might be required for system paths
cmake --build . --target install

Windows¶

The process is basically similar to Linux & OSX, with just a couple of minor tweaks. Use ps ..\openPMD-api\share\openPMD\download_samples.ps1 to download sample files for tests (optional). Replace the last three commands with

cmake --build . --config Release

# optional
ctest -C Release

# administrative privileges might be required for system paths
cmake --build . --config Release --target install

Post “From Source” Install¶

If you installed to a non-system path on Linux or OSX, you need to express where your newly installed library can be found.

Adjust the lines below accordingly, e.g. replace $HOME/somepath with your install location prefix in -DCMAKE_INSTALL_PREFIX=.... CMake will summarize the install paths for you before the build step.

# install prefix         |------------|
export CMAKE_PREFIX_PATH=$HOME/somepath:$CMAKE_PREFIX_PATH
export LD_LIBRARY_PATH=$HOME/somepath/lib:$LD_LIBRARY_PATH
# Note that one some systems, /lib might need to be replaced with /lib64.

#                change path to your python MAJOR.MINOR version
export PYTHONPATH=$HOME/somepath/lib/python3.8/site-packages:$PYTHONPATH

Adding those lines to your $HOME/.bashrc and re-opening your terminal will set them permanently.

Set hints on Windows with the CMake printed paths accordingly, e.g.:

set CMAKE_PREFIX_PATH=C:\\Program Files\openPMD;%CMAKE_PREFIX_PATH%
set PATH=C:\\Program Files\openPMD\Lib;%PATH%
set PYTHONPATH=C:\\Program Files\openPMD\Lib\site-packages;%PYTHONPATH%

Changelog¶

0.14.0¶

Date: 2021-07-29

Resize, Dask, openpmd-pipe and new ADIOS2 Iteration Encoding

This release adds support for resizable data sets. For data-processing, support for Dask (parallel) and Pandas (serial) are added and lazy reader parsing of iterations is now supported. ADIOS2 adds an experimental variable-based iteration encoding. An openPMD Series can now be flushed from non-Series objects and write buffers can be requested upfront to avoid unnecessary data copies in some situations.

Changes to “0.13.4”¶

Features¶

Resizable datasets #829 #1020 #1060 #1063
lazy parsing of iterations #938
Expose internal buffers to writers #901
seriesFlush: Attributable, Writable, Mesh & ParticleSpecies #924 #925
ADIOS2:
- Implement new variableBased iteration encoding #813 #855 #926 #941 #1008
- Set a default QueueLimit of 2 in the ADIOS2/SST engine #971
- Add environment control: OPENPMD_ADIOS2_STATS_LEVEL #1003
Conda environment file conda.yaml added to repo #1004
CMake: Expose Python LTO Control #980
HDF5:
- HDF5 1.12.0 fallback APIs: no wrappers and more portable #1012
- Empiric for optimal chunk size #916
Python:
- ParticleSpecies: Read to pandas.DataFrame #923
- ParticleSpecies: Read to dask.dataframe #935 #951 #956 #958 #959 #1033
- Dask: Array #952
- pyproject.toml: build-backend #932
Tools: add openpmd-pipe.py command line tool #904 #1062 #1069
Support for custom geometries #1011
Default constructors for Series and SeriesIterator #955
Make WriteIterations::key_type public #999
ParticleSpecies & RecordComponent serialize #963

Bug Fixes¶

ADIOS2:
- bp4_steps test: actually use NullCore engine #933
- Always check the return status of IO::Open() and Engine::BeginStep() in ADIOS2 #1017 #1023
- More obvious error message if datatype cannot be found #1036
- Don’t implicitly open files #1045
- fix C++17 compilation #1067
HDF5:
- Support Parallel HDF5 built w/ CMake #1027
- HDF5Auxiliary: Check String Sizes #979
Tests:
- Check for existence of the correct files in ParallelIOtests #944
- FBPIC example filename #950
- CoreTest: Lambda outside unevaluated context #1057
availableChunks: improve open logic for early chunk reads #1035 #1045
CMake:
- custom copy for dependent files #1016
- library type control #930
Fix detection of loadChunk() calls with wrong type #1022
Don’t flush Series a second time after throwing an error #1018
Use Series::writeIterations() without explicit flushing #1030
Mesh: enable_if only floating point APIs #1042
Datatype: Fix std::array template #1040
PkgConfig w/ external variant #1050
warnings: Unused params and unreachable code #1053 #1055

Other¶

ADIOS2: require version 2.7.0+ #927
Catch2: 2.13.4+ #940
pybind11: require version 2.6.2+ #977
CI:
- Update & NVHPC #1052
- ICC/ICPC & ICX/ICPX #870
- Reintroduce Clang Sanitizer #947
- Brew Update #970
- Source Tools Update #978
- Use specific commit for downloaded samples #1049
- SerialIOTest: fix CI hang in sanitizer #1054 #1056
CMake:
- Require only C-in-CXX MPI component #710
- Unused setter in openpmd_option #1015
Docs:
- describe high-level concepts #997
- meaning of Writable::written() #946
- Iteration::close/flush fix typo #988
- makeConstant & parallel HDF5 #1041
- ADIOS2 memory usage for various encoding schemes #1009
- dev-branch centered development #928
- limit docutils to 0.16, Sphinx to <4.0 #976
- Sphinx: rsvg converter for LaTeX #1001
- Update GitHub issue templates #1034
- Add CITATION.cff #1070
- Benchmark 8b: “pack” parameter #1066
- Move quoted lines from IOTasks #1061
- describe iteration encodings #1064
- describe regexes for showing only attributes or datasets in new ADIOS2 schema #1068
Tests & Examples:
- ADIOS2 SST tests: start reader a second after the writer #981
- ADIOS2 Git sample #1019 #1051
- Parallel Benchmark (8): 4D is now 3D #1010 #1047
RecordComponent: Remove unimplemented scaling #954
MSVC: Proper __cplusplus macro #919
Make switchType more comfortable to use #931
Split Series into an internal and an external class #886 #936 #1031 #1065
Series: fileBased more consequently throws no_such_file_error #1059
Retrieve paths of objects in the openPMD hierarchy #966
Remove duplicate function declarations #998
License Header: Update 2021 #922
Add Dependabot #929
Update author order for 0.14.0+ #1005
Download samples: optional directory #1039

0.13.4¶

Date: 2021-05-13

Fix AppleClang & DPC++ Build

Fix a missing include that fails builds with Apple’s clang and Intel’s dpcpp compilers.

Changes to “0.13.3”¶

Bug Fixes¶

Variant.hpp: size_t include #972

0.13.3¶

Date: 2021-04-09

Fix Various Read Issues

This release fixes various bugs related to reading: a chunk fallback for constant components, skip missing patch records, a backend bug in each ADIOS2 & HDF5, and we made the Python load_chunk method more robust.

Changes to “0.13.2”¶

Bug Fixes¶

available_chunks() for constant components #942
Particle Patches: Do not emplace patch records if they don’t exist in the file being read #945
ADIOS2: decay ReadWrite mode into adios2::Mode::Read if the file exists #943
HDF5: fix segfault with libSplash files #962
Python: fix load_chunk to temporary #913

Other¶

Sphinx: limit docutils to 0.16
CI: remove a failing find command

0.13.2¶

Date: 2021-02-02

Fix Patch Read & Python store_chunk

This release fixes a regression with particle patches, related to Iteration::open() and ::close() functionality. Also, issues with the Python store_chunk method are addressed.

Changes to “0.13.1”¶

Bug Fixes¶

Read: check whether particle patches are dirty & handle gracefully #909
Python store_chunk:
- add support for complex types #915
- fix a use-after-free with temporary variables #912

Other¶

CMake: hint CMAKE_PREFIX_PATH as a warning for HDF5 #896

0.13.1¶

Date: 2021-01-08

Fix openPMD-ls & Iteration open/close

This release fixes regressions in the series “ls” functionality and tools, related to Iteration::open() and ::close() functionality. We also add support to read back complex numbers with JSON.

Changes to “0.13.0”¶

Bug Fixes¶

fix Iteration::close() and helper::listSeries` / list_series / openPMD-ls #878 #880 #882 #883 #884
setup.py: stay with Python_EXECUTABLE #875
FindPython.cmake: Avoid overspecifying Development.Module with CMake 3.18+ #868
ChunkInfo:
- fix includes #879
- tests: adapt sourceID to handle nondeterministic subfile order #871
ADIOS1: fix Iteration::open() #864
JSON: support complex datatype reads #885
Docs: fix formatting of first read/write #892

Other¶

bounds check: more readable error message #890
ADIOS2: add a missing space in an error message #881
Docs: released pypi wheels include windows #869
CI:
- LGTM: fix C++ #873
- Brew returns non-zero if already installed #877

0.13.0¶

Date: 2021-01-03

Streaming Support, Python, Benchmarks

This release adds first support for streaming I/O via ADIOS2’s SST engine. More I/O benchmarks have been added with realistic application load patterns. Many Python properties for openPMD attributes have been modernized, with slight breaking changes in Iteration and Mesh data order. This release requires C++14 and adds support for Python 3.9. With this release, we leave the “alpha” phase of the software and declare “beta” status.

Changes to “0.12.0-alpha”¶

Features¶

ADIOS2: streaming support (via ADIOS SST) #570
add ::availableChunks call to record component types #802 #835 #847
HDF5: control alignment via OPENPMD_HDF5_ALIGNMENT #830
JSON configuration on the dataset level #818
Python
- attributes as properties in Series, Mesh, Iteration, … #859
- add missing python interface (read/write) for machine #796
- add Record_Component.make_empty() #538
added tests 8a & 8b to do 1D/2D mesh writing and reading #803 #816 #834
PyPI: support for Windows wheels on x86-64 #853

Bug Fixes¶

fix Series attributes: read defaults #812
allow reading a file-based series with many iterations without crashing the number of file handles #822 #837
Python: Fix & replace Data_Order semantics #850
ADIOS1:
- add missing CLOSE_FILE IO task to parallel backend #785
ADIOS2:
- fix engine destruction order, anticipating release 2.7.0 #838
HDF5:
- support alternate form of empty records (FBPIC) #849
Intel ICC (icpc):
- fix export #788
- fix segfault in Iteration #789
fix & support ClangCL on Windows #832
CMake:
- Warnings: ICC & root project only #791
- Warnings: FindADIOS(1).cmake 2.8.12+ #841
- Warnings: less verbose on Windows #851

Other¶

switched to “beta” status: dropping the version -suffix
switch to C++14 #825 #826 #836
CMake:
- require version 3.15.0+ #857
- re-order dependency checks #810
Python: support 3.6 - 3.9 #828
NLohmann-JSON dependency updated to 3.9.1+ #839
pybind11 dependency updated 2.6.1+ #857
ADIOS2:
- less verbose about missing boolean helper attributes #801
- turn off statistics (Min/Max) #831
HDF5: better status checks & error messages #795
Docs:
- release cibuildwheel example #775
- Iteration::close() is MPI-collective #779
- overview compression ADIOS2 #781
- add comment on lib64/ #793
- typo in description for ADIOS1 #797
- conda: recommend fresh environment #799
- Sphinx/rst: fix warnings #809
- first read: slice example #819
CI:
- Travis -> GH Action #823 #827
- remove Cygwin #820
- sanitize only project (temporarily disabled) #800
- update LGTM environment #844
- clang-tidy updates #843
- set oldest supported macOS #854
Tests:
- add HiPACE parallel I/O pattern #842 #848
- cover FBPIC empty HDF5 #849
Internal: add Optional based on variantSrc::variant #806

0.12.0-alpha¶

Date: 2020-09-07

Complex Numbers, Close & Backend Options

This release adds data type support for complex numbers, allows to close iterations and adds first support for backend configuration options (via JSON), which are currently implemented for ADIOS2. Further installation options have been added (homebrew and CLI tool support with pip). New free standing functions and macro defines are provided for version checks.

Changes to “0.11.1-alpha”¶

Features¶

Record(Component): scalar(), constant(), empty() #711
Advanced backend configuration via JSON #569 #733
Support for complex floating point types #639
Functionality to close an iteration (and associated files) #746
Python:
- __init__.py facade #720
- add Mesh_Record_Component.position read-write property #713
- add openpmd-ls tool in pip installs and as module #721 #724
- more idiomatic unit properties #735
- add file_extensions property #768
CD:
- homebrew: add Formula (OSX/Linux) #724 #725
- PyPI: autodeploy wheels (OSX/Linux) #716 #719
version compare macro #747
getFileExtensions function #768
Spack environment file spack.yaml added to repo #737
openpmd-ls: add -v, --version option #771

Bug Fixes¶

flush() exceptions in ~Series/~..IOHandler do not abort anymore #709
Iteration/Attributable assignment operator left object in invalid state #769
Datatype.hpp: add missing include #764
readme: python example syntax was broken and outdated #722
examples:
- fix "weighting" record attribute (ED-PIC) #728
- fix & validate all created test/example files #738 #739
warnings:
- listSeries: unused params in try-catch #707
- fix Doxygen 1.18.8 and 1.18.20 warnings #766
- extended write example: remove MSVC warning #752

Other¶

CMake: require version 3.12.0+ #755
ADIOS2: require version 2.6.0+ #754
separate header for export macros #704
rename AccessType/Access_Type to Access #740 #743 #744
CI & tests:
- migration to travis-ci.com / GitHub app #703
- migrate to GitHub checkout action v2 #712
- fix OSX numpy install #714
- move .travis/ to .github/ci/ #715
- move example file download scripts to share/openPMD/ #715
- add GCC 9.3 builds #723
- add Cygwin builds #727
- add Clang 10.0 builds #759
- migrate Spack to use AppleClang #758
- style check scripts: eval-uable #757
- new Spack external package syntax #760
- python tests: testAttributes JSON backend coverage #767
listSeries: remove unused parameters in try-catch #706
safer internal *dynamic_cast of pointers #745
CMake: subproject inclusion cleanup #751
Python: remove redundant move in container #753
read example: show particle load #706
Record component: fix formatting #763
add .editorconfig file #762
MPI benchmark: doxygen params #653

0.11.1-alpha¶

Date: 2020-03-24

HDF5-1.12, Azimuthal Examples & Tagfile

This release adds support for the latest HDF5 release. Also, we add versioned Doxygen and a tagfile for external docs to our online manual.

Changes to “0.11.0-alpha”¶

Features¶

HDF5: Support 1.12 release #696
Doxygen: per-version index in Sphinx pages #697

Other¶

Examples:
- document azimuthal decomposition read/write #678
- better example namespace alias (io) #698
Docs: update API detail pages #699

0.11.0-alpha¶

Date: 2020-03-05

Robust Independent I/O

This release improves MPI-parallel I/O with HDF5 and ADIOS. ADIOS2 is now the default backend for handing .bp files.

Changes to “0.10.3-alpha”¶

Features¶

ADIOS2:
- new default for .bp files (over ADIOS1) #676
- expose engine #656
HDF5: OPENPMD_HDF5_INDEPENDENT=ON is now default in parallel I/O #677
defaults for date and software base attributes #657
Series::setSoftware() add second argument for version #657
free standing functions to query the API version and feature variants at runtime #665
expose determineFormat and suffix functions #684
CLI: add openpmd-ls tool #574

Bug Fixes¶

std::ostream& operator<< overloads are not declared in namespace std anymore #662
ADIOS1:
- ensure creation of files that only contain attributes #674
- deprecated in favor of ADIOS2 backend #676
- allow non-collective storeChunk() calls with multiple iterations #679
Pip: work-around setuptools/CMake bootstrap issues on some systems #689

Other¶

deprecated Series::setSoftwareVersion: set the version with the second argument of setSoftware() #657
ADIOS2: require version 2.5.0+ #656
nvcc:
- warning missing erase overload of Container child classes #648
- warning on unreachable code #659
- MPark.Variant: update C++14 hotfix #618 to upstream version #650
docs:
- typo in Python example for first read #649
- remove all Doxygen warnings and add to CI #654
- backend feature matrix #661
- document CMake’s FetchContent feature for developers #667
- more notes on HDF5 & ADIOS1 #685
migrate static checks for python code to GitHub actions #660
add MPICH tests to CI #670
Attribute constructor: move argument into place #663
Spack: ADIOS2 backend now enabled by default #664 #676
add independent HDF5 write test to CI #669
add test of multiple active Series #686

0.10.3-alpha¶

Date: 2019-12-22

Improved HDF5 Handling

More robust HDF5 file handling and fixes of local includes for more isolated builds.

Changes to “0.10.2-alpha”¶

Bug Fixes¶

Source files: fix includes #640
HDF5: gracefully handle already open files #643

Other¶

Better handling of legacy libSplash HDF5 files #641
new contributors #644

0.10.2-alpha¶

Date: 2019-12-17

Improved Error Messages

Thrown errors are now prefixed by the backend in use and ADIOS1 series reads are more robust.

Changes to “0.10.1-alpha”¶

Bug Fixes¶

Implement assignment operators for: IOTask, Mesh, Iteration, BaseRecord, Record #628
Missing virtual destructors added #632

Other¶

Backends: Prefix Error Messages #634
ADIOS1: Skip Invalid Scalar Particle Records #635

0.10.1-alpha¶

Date: 2019-12-06

ADIOS2 Open Speed and NVCC Fixes

This releases improves the initial time spend when parsing data series with the ADIOS2 backend. Compile problems when using the CUDA NVCC compiler in downstream projects have been fixed. We adopted a Code of Conduct in openPMD.

Changes to “0.10.0-alpha”¶

Features¶

C++: add Container::contains method #622

Bug Fixes¶

ADIOS2:
- fix C++17 build #614
- improve initial open speed of series #613
nvcc:
- ignore export of enum class Operation #617
- fix C++14 build #618

Other¶

community:
- code of conduct added #619
- all contributors listed in README #621
manylinux2010 build automation updated for Python 3.8 #615

0.10.0-alpha¶

Date: 2019-11-14

ADIOS2 Preview, Python & MPI Improved

This release adds a first (preview) implementation of ADIOS2 (BP4). Python 3.8 support as well as improved pip builds on macOS and Windows have been added. ADIOS1 and HDF5 now support non-collective (independent) store and load operations with MPI. More HPC compilers, such as IBM XL, ICC and PGI have been tested. The manual has been improved with more details on APIs, examples, installation and backends.

Changes to “0.9.0-alpha”¶

Features¶

ADIOS2: support added (v2.4.0+) #482 #513 #530 #568 #572 #573 #588 #605
HDF5: add OPENPMD_HDF5_INDEPENDENT for non-collective parallel I/O #576
Python:
- Python 3.8 support #581
- support empty datasets via Record_Component.make_empty #538
pkg-config: add static variable (true/false) to openPMD.pc package #580

Bug Fixes¶

Clang: fix pybind11 compile on older releases, such as AppleClang 7.3-9.0, Clang 3.9 #543
Python:
- OSX: fix dlopen issues due to missing @loader_path with pip/setup.py #595
- Windows: fix a missing DLL issue by building static with pip/setup.py #602
- import mpi4py first (MPICH on OSX issue) #596
- skip examples using HDF5 if backend is missing #544
- fix a variable shadowing in Mesh #582
- add missing .unit_dimension for records #611
ADIOS1: fix deadlock in MPI-parallel, non-collective calls to storeChunk() #554
xlC 16.1: work-around C-array initializer parsing issue #547
icc 19.0.0 and PGI 19.5: fix compiler ID identification #548
CMake: fix false-positives in FindADIOS.cmake module #609
Series: throws an error message if no file ending is specified #610

Other¶

Python: improve pip install instructions #594 #600
PGI 19.5: fix warning static constexpr: storage class first #546
JSON:
- the backend is now always enabled #564 #587
- NLohmann-JSON dependency updated to 3.7.0+ #556
gitignore: generalize CLion, more build dirs #549 #552
fix clang-tidy warnings: strcmp and modernize auto, const correctness #551 #560
ParallelIOTest: less code duplication #553
Sphinx manual:
- PDF Chapters #557
- draft for the API architecture design #186
- draft for MPI data and collective contract in API usage #583
- fix tables & missing examples #579
- “first write” explains unitDimension #592
- link to datasets used in examples #598
- fix minor formatting and include problems #608
README:
- add authors and acknowledgements #566
- correct a typo #584
- use $(which python3) for CMake Python option #599
- update ADIOS homepage & CMake #604
Travis CI:
- speedup dependency build #558
- -Werror only in build phase #565

0.9.0-alpha¶

Date: 2019-07-25

Improved Builds and Packages

This release improves PyPI releases with proper declaration of build dependencies (use pip 19.0+). For Makefile-based projects, an openPMD.pc file to be used with pkg-config is added on install. RecordComponent now supports a makeEmpty method to write a zero-extent, yet multi-dimensional record component. We are now building as shared library by default.

Changes to “0.8.0-alpha”¶

Features¶

C++: support empty datasets via RecordComponent::makeEmpty #528 #529
CMake:
- build a shared library by default #506
- generate pkg-config .pc file #532 #535 #537
Python:
- manylinux2010 wheels for PyPI #523
- add pyproject.toml for build dependencies (PEP-518) #527

Bug Fixes¶

MPark.Variant: work-around missing version bump #504
linker error concerning Mesh::setTimeOffset method template #511
remove dummy dataset writing from RecordComponent::flush() #528
remove dummy dataset writing from PatchRecordComponent::flush #512
allow flushing before defining position and positionOffset components of particle species #518 #519
CMake:
- make install paths cacheable on Windows #521
- HDF5 linkage is private #533
warnings:
- unused variable in JSON backend #507
- MSVC: Warning DLL Interface STDlib #508

Other¶

increase pybind11 dependency to 2.3.0+ #525
GitHub:
- auto-add labels #515
- issue template for install issues #526
- update badges #522
docs:
- link parallel python examples in manual #499
- improved Doxygen parsing for all backends #500
- fix typos #517

0.8.0-alpha¶

Date: 2019-03-09

Python mpi4py and Slice Support

We implemented MPI support for the Python frontend via mpi4py and added []-slice access to Record_Component loads and stores. A bug requiring write permissions for read-only series was fixed and memory provided by users is now properly checked for being contiguous. Introductory chapters in the manual have been greatly extended.

Changes to “0.7.1-alpha”¶

Features¶

Python:
- mpi4py support added #454
- slice protocol for record component #458

Bug Fixes¶

do not require write permissions to open Series read-only #395
loadChunk: re-enable range/extent checks for adjusted ranges #469
Python: stricter contiguous check for user-provided arrays #458
CMake tests as root: apply OpenMPI flag only if present #456

Other¶

increase pybind11 dependency to 2.2.4+ #455
Python: remove (inofficial) bindings for 2.7 #435
CMake 3.12+: apply policy CMP0074 for <Package>_ROOT vars #391 #464
CMake: Optional ADIOS1 Wrapper Libs #472
MPark.Variant: updated to 1.4.0+ #465
Catch2: updated to 2.6.1+ #466
NLohmann-JSON: updated to 3.5.0+ #467
Docs:
- PyPI install method #450 #451 #497
- more info on MPI #449
- new “first steps” section #473 #478
- update invasive test info #474
- more info on Access #483
- improved MPI-parallel write example #496

0.7.1-alpha¶

Date: 2018-01-23

Bug Fixes in Multi-Platform Builds

This release fixes several issues on OSX, during cross-compile and with modern compilers.

Changes to “0.7.0-alpha”¶

Bug Fixes¶

fix compilation with C++17 for python bindings #438
FindADIOS.cmake: Cross-Compile Support #436
ADIOS1: fix runtime crash with libc++ (e.g. OSX) #442

Other¶

CI: clang libc++ coverage #441 #444
Docs:
- additional release workflows for maintainers #439
- ADIOS1 backend options in manual #440
- updated Spack variants #445

0.7.0-alpha¶

Date: 2019-01-11

JSON Support, Interface Simplification and Stability

This release introduces serial JSON (.json) support. Our API has been unified with slight breaking changes such as a new Python module name (import openpmd_api from now on) as well as re-ordered store/loadChunk argument orders. Please see our new “upgrade guide” section in the manual how to update existing scripts. Additionally, many little bugs have been fixed. Official Python 3.7 support and a parallel benchmark example have been added.

Changes to “0.6.3-alpha”¶

Features¶

C++:
- storeChunk argument order changed, defaults added #386 #416
- loadChunk argument order changed, defaults added #408
Python:
- import openPMD renamed to import openpmd_api #380 #392
- store_chunk argument order changed, defaults added #386
- load_chunk defaults added #408
- works with Python 3.7 #376
- setup.py for sdist #240
Backends: JSON support added #384 #393 #338 #429
Parallel benchmark added #346 #398 #402 #411

Bug Fixes¶

spurious MPI C++11 API usage in ParallelIOTest removed #396
spurious symbol issues on OSX #427
new []/delete mismatch in ParallelIOTest #422
use-after-free in SerialIOTest #409
fix ODR issue in ADIOS1 backend corrupting the AbstractIOHandler vtable #415
fix race condition in MPI-parallel directory creation #419
ADIOS1: fix use-after-free in parallel I/O method options #421

Other¶

modernize IOTask’s AbstractParameter for slice safety #410
Docs: upgrade guide added #385
Docs: python particle writing example #430
CI: GCC 8.1.0 & Python 3.7.0 #376
CI: (re-)activate Clang-Tidy #423
IOTask: init all parameters’ members #420
KDevelop project files to .gitignore #424
C++:
- Mesh’s setAxisLabels|GridSpacing|GridGlobalOffset passed as const & #425
CMake:
- treat third party libraries properly as IMPORTED #389 #403
- Catch2: separate implementation and tests #399 #400
- enable check for more warnings #401

0.6.3-alpha¶

Date: 2018-11-12

Reading Varying Iteration Padding Reading

Support reading series with varying iteration padding (or no padding at all) as currently used in PIConGPU.

Changes to “0.6.2-alpha”¶

Bug Fixes¶

support reading series with varying or no iteration padding in filename #388

0.6.2-alpha¶

Date: 2018-09-25

Python Stride: Regression

A regression in the last fix for python strides made the relaxation not efficient for 2-D and higher.

Changes to “0.6.1-alpha”¶

Bug Fixes¶

Python: relax strides further

0.6.1-alpha¶

Date: 2018-09-24

Relaxed Python Stride Checks

Python stride checks have been relaxed and one-element n-d arrays are allowed for scalars.

Changes to “0.6.0-alpha”¶

Bug Fixes¶

Python:
- stride check too strict #369
- allow one-element n-d arrays for scalars in store, make_constant #314

Other¶

dependency change: Catch2 2.3.0+
Python: add extended write example #314

0.6.0-alpha¶

Date: 2018-09-20

Particle Patches Improved, Constant Scalars and Python Containers Fixed

Scalar records properly support const-ness. The Particle Patch load interface was changed, loading now all patches at once, and Python bindings are available. Numpy dtype is now a first-class citizen for Python Datatype control, being accepted and returned instead of enums. Python lifetime in garbage collection for containers such as meshes, particles and iterations is now properly implemented.

Changes to “0.5.0-alpha”¶

Features¶

Python:
- accept & return numpy.dtype for Datatype #351
- better check for (unsupported) numpy array strides #353
- implement Record_Component.make_constant #354
- implement Particle_Patches #362
comply with runtime constraints w.r.t. written status #352
load at once ParticlePatches.load() #364

Bug Fixes¶

dataOrder: mesh attribute is a string #355
constant scalar Mesh Records: reading corrected #358
particle patches: stricter load( idx ) range check #363, then removed in #364
Python: lifetime of Iteration.meshes/particles and Series.iterations members #354

Other¶

test cases for mixed constant/non-constant Records #358
examples: close handles explicitly #359 #360

0.5.0-alpha¶

Date: 2018-09-17

Refactored Type System

The type system for Datatype::``s was refactored. Integer types are now represented by ``SHORT, INT, LONG and LONGLONG as fundamental C/C++ types. Python support enters “alpha” stage with fixed floating point storage and Attribute handling.

Changes to “0.4.0-alpha”¶

Features¶

Removed Datatype::INT32 types with ::SHORT, ::INT equivalents #337
Attribute::get<...>() performs a static_cast now #345

Bug Fixes¶

Refactor type system and Attribute set/get
- integers #337
- support long double reads on MSVC #184
setAttribute: explicit C-string handling #341
Dataset: setCompression warning and error logic #326
avoid impact on unrelated classes in invasive tests #324
Python
- single precision support: numpy.float is an alias for builtins.float #318 #320
- Dataset method namings to underscores #319
- container namespace ambiguity #343
- set_attribute: broken numpy, list and string support #330

Other¶

CMake: invasive tests not enabled by default #323
store_chunk: more detailed type mismatch error #322
no_such_file_error & no_such_attribute_error: remove c-string constructor #325 #327
add virtual destructor to Attributable #332
Python: Numpy 1.15+ required #330

0.4.0-alpha¶

Date: 2018-08-27

Improved output handling

Refactored and hardened for fileBased output. Records are not flushed before the ambiguity between scalar and vector records are resolved. Trying to write globally zero-extent records will throw gracefully instead of leading to undefined behavior in backends.

Changes to “0.3.1-alpha”¶

Features¶

do not assume record structure prematurely #297
throw in (global) zero-extent dataset creation and write #309

Bug Fixes¶

ADIOS1 fileBased IO #297
ADIOS2 stub header #302
name sanitization in ADIOS1 and HDF5 backends #310

Other¶

CI updates: #291
- measure C++ unit test coverage with coveralls
- clang-format support
- clang-tidy support
- include-what-you-use support #291 export headers #300
- OSX High Sierra support #301
- individual cache per build # 303
- readable build names #308
remove superfluous whitespaces #292
readme: openPMD is for scientific data #294
override implies virtual #293
spack load: -r #298
default constructors and destructors #304
string pass-by-value #305
test cases with 0-sized reads & writes #135

0.3.1-alpha¶

Date: 2018-07-07

Refined fileBased Series & Python Data Load

A specification for iteration padding in filenames for fileBased series is introduced. Padding present in read iterations is detected and conserved in processing. Python builds have been simplified and python data loads now work for both meshes and particles.

Changes to “0.3.0-alpha”¶

Features¶

CMake:
- add openPMD::openPMD alias for full-source inclusion #277
- include internally shipped pybind11 v2.2.3 #281
- ADIOS1: enable serial API usage even if MPI is present #252 #254
introduce detection and specification %0\d+T of iteration padding #270
Python:
- add unit tests #249
- expose record components for particles #284

Bug Fixes¶

improved handling of fileBased Series and READ_WRITE access
expose Container constructor as protected rather than public #282
Python:
- return actual data in load_chunk #286

Other¶

docs:
- improve “Install from source” section #274 #285
- Spack python 3 install command #278

0.3.0-alpha¶

Date: 2018-06-18

Python Attributes, Better FS Handling and Runtime Checks

This release exposes openPMD attributes to Python. A new independent mechanism for verifying internal conditions is now in place. Filesystem support is now more robust on varying directory separators.

Changes to “0.2.0-alpha”¶

Features¶

CMake: add new openPMD_USE_VERIFY option #229
introduce VERIFY macro for pre-/post-conditions that replaces ASSERT #229 #260
serial Singularity container #236
Python:
- expose attributes #256 #266
- use lists for offsets & extents #266
C++:
- setAttribute signature changed to const ref #268

Bug Fixes¶

handle directory separators platform-dependent #229
recursive directory creation with existing base #261
FindADIOS.cmake: reset on multiple calls #263
SerialIOTest: remove variable shadowing #262
ADIOS1: memory violation in string attribute writes #269

Other¶

enforce platform-specific directory separators on user input #229
docs:
- link updates to https #259
- minimum MPI version #251
- title updated #235
remove MPI from serial ADIOS interface #258
better name for scalar record in examples #257
check validity of internally used pointers #247
various CI updates #246 #250 #261

0.2.0-alpha¶

Date: 2018-06-11

Initial Numpy Bindings

Adds first bindings for record component reading and writing. Fixes some minor CMake issues.

Changes to “0.1.1-alpha”¶

Features¶

Python: first NumPy bindings for record component chunk store/load #219
CMake: add new BUILD_EXAMPLES option #238
CMake: build directories controllable #241

Bug Fixes¶

forgot to bump version.hpp/__version__ in last release
CMake: Overwritable Install Paths #237

0.1.1-alpha¶

Date: 2018-06-07

ADIOS1 Build Fixes & Less Flushes

We fixed build issues with the ADIOS1 backend. The number of performed flushes in backends was generally minimized.

Changes to “0.1.0-alpha”¶

Bug Fixes¶

SerialIOTest: loadChunk template missing for ADIOS1 #227
prepare running serial applications linked against parallel ADIOS1 library #228

Other¶

minimize number of flushes in backend #212

0.1.0-alpha¶

Date: 2018-06-06

This is the first developer release of openPMD-api.

Both HDF5 and ADIOS1 are implemented as backends with serial and parallel I/O support. The C++11 API is considered alpha state with few changes expected to come. We also ship an unstable preview of the Python3 API.

Upgrade Guide¶

0.14.0¶

ADIOS 2.7.0 is now the minimally supported version for ADIOS2 support. Catch2 2.13.4 is now the minimally supported version for tests. pybind11 2.6.2 is now the minimally supported version for Python support.

In RecordComponent::loadChunk, the optional last argument targetUnitSI was removed as it has not been implemented yet and had thus no function.

0.13.0¶

Building openPMD-api now requires a compiler that supports C++14 or newer. Supported Python version are now 3.6 to 3.9. CMake 3.15.0 is now the minimally supported version for CMake.

Python¶

Reading the data_order of a mesh was broken. The old setter function (set_data_order) and read-only property (data_order) are now unified in a single, writable property:

import openpmd_api as io

series = io.Series("data%T.h5", io.Access.read_only)
rho = series.iterations[0].meshes["rho"]
rho.data_order = 'C'  # or 'F'

print(rho.data_order == 'C')  # True

Note: we recommend using 'C' order since version 2 of the openPMD-standard will simplify this option to 'C', too. For Fortran-ordered indices, please just invert the attributes axis_labels, grid_spacing and grid_global_offset accordingly.

The Iteration functions time, dt and time_unit_SI have been replaced with read-write properties of the same name, essentially without the ()-access. set_time, set_dt and set_time_unit_SI are now deprecated and will be removed in future versions of the library.

The already existing read-only Series properties openPMD, openPMD_extension, base_path, meshes_path, particles_path, particles_path, author, date, iteration_encoding, iteration_format and name are now declared as read-write properties. set_openPMD, set_openPMD_extension, set_base_path, set_meshes_path, set_particles_path, set_author, set_date, set_iteration_encoding, set_iteration_format and set_name are now deprecated and will be removed in future versions of the library.

The already existing read-only Mesh properties geometry, geometry_parameters, axis_labels, grid_spacing, grid_global_offset and grid_unit_SI are now declared as read-write properties. set_geometry, set_geometry_parameters, set_axis_labels, set_grid_spacing, set_grid_global_offset and set_grid_unit_SI are now deprecated and will be removed in future versions of the library.

The already existing read-only Attributable property comment is now declared as read-write properties. set_comment is now deprecated and will be removed in future versions of the library.

0.12.0-alpha¶

CMake 3.12.0 is now the minimally supported version for CMake. ADIOS 2.6.0 is now the minimally supported version for ADIOS2 support.

Python¶

The already existing read-only properties unit_dimension, unit_SI, and time_offset are now declared as read-write properties. set_unit_dimension, set_unit_SI, and set_time_offset are now deprecated and will be removed in future versions of the library.

Access_Type is now called Access. Using it by the old name is deprecated and will be removed in future versions of the library.

C++¶

AccessType is now called Access. Using it by the old name is deprecated and will be removed in future versions of the library.

0.11.0-alpha¶

ADIOS2 is now the default backend for .bp files. As soon as the ADIOS2 backend is enabled it will take precedence over a potentially also enabled ADIOS1 backend. In order to prefer the legacy ADIOS1 backend in such a situation, set an environment variable: export OPENPMD_BP_BACKEND="ADIOS1". Support for ADIOS1 is now deprecated.

Independent MPI-I/O is now the default in parallel HDF5. For the old default, collective parallel I/O, set the environment variable export OPENPMD_HDF5_INDEPENDENT="OFF". Collective parallel I/O makes more functionality, such as storeChunk and loadChunk, MPI-collective. HDF5 attribute writes are MPI-collective in either case, due to HDF5 restrictions.

Our Spack packages build the ADIOS2 backend now by default. Pass -adios2 to the Spack spec to disable it: spack install openpmd-api -adios2 (same for spack load -r).

The Series::setSoftwareVersion method is now deprecated and will be removed in future versions of the library. Use Series::setSoftware(name, version) instead. Similarly for the Python API, use Series.set_software instead of Series.set_software_version.

The automated example-download scripts have been moved from .travis/download_samples.sh (and .ps1) to share/openPMD/.

0.10.0-alpha¶

We added preliminary support for ADIOS2 in this release. As long as also the ADIOS1 backend is enabled it will take precedence for .bp files over the newer ADIOS2 backend. In order to enforce using the new ADIOS2 backend in such a situation, set an environment variable: export OPENPMD_BP_BACKEND="ADIOS2". We will change this default in upcoming releases to prefer ADIOS2.

The JSON backend is now always enabled. The CMake option -DopenPMD_USE_JSON has been removed (as it is always ON now).

Previously, omitting a file ending in the Series constructor chose a “dummy” no-operation file backend. This was confusing and instead a runtime error is now thrown.

0.9.0-alpha¶

We are now building a shared library by default. In order to keep build the old default, a static library, append -DBUILD_SHARED_LIBS=OFF to the cmake command.

0.7.0-alpha¶

Python¶

Module Name¶

Our module name has changed to be consistent with other openPMD projects:

# old name
import openPMD

# new name
import openpmd_api

`store_chunk` Method¶

The order of arguments in the store_chunk method for record components has changed. The new order allows to make use of defaults in many cases in order reduce complexity.

particlePos_x = np.random.rand(234).astype(np.float32)

d = Dataset(particlePos_x.dtype, extent=particlePos_x.shape)
electrons["position"]["x"].reset_dataset(d)

# old code
electrons["position"]["x"].store_chunk([0, ], particlePos_x.shape, particlePos_x)

# new code
electrons["position"]["x"].store_chunk(particlePos_x)
# implied defaults:
#                         .store_chunk(particlePos_x,
#                                      offset=[0, ],
#                                      extent=particlePos_x.shape)

`load_chunk` Method¶

The loadChunk<T> method with on-the-fly allocation has default arguments for offset and extent now. Called without arguments, it will read the whole record component.

E_x = series.iterations[100].meshes["E"]["x"]

# old code
all_data = E_x.load_chunk(np.zeros(E_x.shape), E_x.shape)

# new code
all_data = E_x.load_chunk()

series.flush()

C++¶

`storeChunk` Method¶

The order of arguments in the storeChunk method for record components has changed. The new order allows to make use of defaults in many cases in order reduce complexity.

std::vector< float > particlePos_x(234, 1.234);

Datatype datatype = determineDatatype(shareRaw(particlePos_x));
Extent extent = {particlePos_x.size()};
Dataset d = Dataset(datatype, extent);
electrons["position"]["x"].resetDataset(d);

// old code
electrons["position"]["x"].storeChunk({0}, extent, shareRaw(particlePos_x));

// new code
electrons["position"]["x"].storeChunk(particlePos_x);
/* implied defaults:
 *                        .storeChunk(shareRaw(particlePos_x),
 *                                    {0},
 *                                    {particlePos_x.size()})  */

`loadChunk` Method¶

The order of arguments in the pre-allocated data overload of the loadChunk method for record components has changed. The new order allows was introduced for consistency with storeChunk.

float loadOnePos;

// old code
electrons["position"]["x"].loadChunk({0}, {1}, shareRaw(&loadOnePos));

// new code
electrons["position"]["x"].loadChunk(shareRaw(&loadOnePos), {0}, {1});

series.flush();

The loadChunk<T> method with on-the-fly allocation got default arguments for offset and extent. Called without arguments, it will read the whole record component.

MeshRecordComponent E_x = series.iterations[100].meshes["E"]["x"];

// old code
auto all_data = E_x.loadChunk<double>({0, 0, 0}, E_x.getExtent());

// new code
auto all_data = E_x.loadChunk<double>();

series.flush();

Usage¶

Concepts¶

For a first high-level overview on openPMD, please see www.openPMD.org.

openPMD defines data series of particles and mesh based data in the openPMD-standard. See for example the openPMD base standard definition, version 1.1.0.

Records and Record Components¶

At the bottom of openPMD is an Record that stores data in record components. A record is an data set with common properties, e.g. the electric field $\vec E$ with three components $E_x, E_y, E_z$ can be a record. A density field could be another record - which is scalar as it only has one component.

In general, openPMD allows records with arbitrary number of components (tensors), as well as vector records and scalar records.

Meshes and Particles¶

Records can be either (structured) meshes, e.g. a gridded electric field as mentioned above, or particle records.

Mesh records are logically n-dimensional arrays. The openPMD standard (see above) supports a various mesh geometries, while more can be standardized in the future. Ongoing work also adds support for block-structured mesh-refinement.

Particle species on the other hand group a number of particle records that are themselves stored as (logical) 1d arrays in record components. Conceptually, one could also think as particles table or dataframe, where each row represents a particle.

Iteration and Series¶

Updates to records are stored in an Iteration. Iterations are numbered by integers, do not need to be consecutive and can for example be used to store a the evolution of records over time.

The collection of iterations is called a Series. openPMD-api implements various file-formats (backends) and encoding strategies for openPMD Series, from simple one-file-per-iteration writes over using the backend-provided support for internal updates of records to data streaming techniques.

Iteration encoding: The openPMD-api can encode iterations in different ways. The method Series::setIterationEncoding() (C++) or Series.set_iteration_encoding() (Python) may be used in writing for selecting one of the following encodings explicitly:

group-based iteration encoding: This encoding is the default. It creates a separate group in the hierarchy of the openPMD standard for each iteration. As an example, all data pertaining to iteration 0 may be found in group /data/0, for iteration 100 in /data/100.
file-based iteration encoding: A unique file on the filesystem is created for each iteration. The preferred way to create a file-based iteration encoding is by specifying an expansion pattern in the filepath argument of the constructor of the Series class. Creating a Series by the filepath "series_%T.json" will create files series_0.json, series_100.json and series_200.json for iterations 0, 100 and 200. A padding may be specified by "series_%06T.json" to create files series_000000.json, series_000100.json and series_000200.json. The inner group layout of each file is identical to that of the group-based encoding.
variable-based iteration encoding: This experimental encoding uses a feature of some backends (i.e., ADIOS2) to maintain datasets and attributes in several versions (i.e., iterations are stored inside variables). No iteration-specific groups are created and the corresponding layer is dropped from the openPMD hierarchy. In backends that do not support this feature, a series created with this encoding can only contain one iteration.

Spellings for constants in the C++ (IterationEncoding) and Python (Iteration_Encoding) API: groupBased, variableBased, fileBased, group_based, group_based, variable_based

Attributes¶

openPMD defines a minimal set of standardized meta-data Attributes to for scientific self-description and portability. Such attributes are showcased in the following section and include for example the physical quantities in a record, unit conversions, time and gridding information.

Besides the standardized attributes, arbitrary additional attributes can be added to openPMD data and openPMD-api supports adding use-defined attributes on every object of the herein described hierarchy.

Tip

Does all of this sound a bit too theoretical? Just jump to the next section and see an example in action.

First Write¶

Step-by-step: how to write scientific data with openPMD-api?

Include / Import¶

After successful installation, you can start using openPMD-api as follows:

C++14¶

#include <openPMD/openPMD.hpp>

// example: data handling
#include <numeric>  // std::iota
#include <vector>   // std::vector

namespace io = openPMD;

Python¶

import openpmd_api as io

# example: data handling
import numpy as np

Open¶

Write into a new openPMD series in myOutput/data_<00...N>.h5. Further file formats than .h5 (HDF5) are supported: .bp (ADIOS1/ADIOS2) or .json (JSON).

C++14¶

auto series = io::Series(
    "myOutput/data_%05T.h5",
    io::Access::CREATE);

Python¶

series = io.Series(
    "myOutput/data_%05T.h5",
    io.Access.create)

Iteration¶

Grouping by an arbitrary, positive integer number <N> in a series:

C++14¶

auto i = series.iterations[42];

Python¶

i = series.iterations[42]

Attributes¶

Everything in openPMD can be extended and user-annotated. Let us try this by writing some meta data:

C++14¶

series.setAuthor(
    "Axel Huebl <axelhuebl@lbl.gov>");
series.setMachine(
    "Hall Probe 5000, Model 3");
series.setAttribute(
    "dinner", "Pizza and Coke");
i.setAttribute(
    "vacuum", true);

Python¶

series.author = \
    "Axel Huebl <axelhuebl@lbl.gov>"
series.machine = "Hall Probe 5000, Model 3"
series.set_attribute(
    "dinner", "Pizza and Coke")
i.set_attribute(
    "vacuum", True)

Data¶

Let’s prepare some data that we want to write. For example, a magnetic field slice $\vec B(i, j)$ in two spatial dimensions with three components $(B_x, B_y, B_z)^\intercal$ of which the $B_y$ component shall be constant for all $(i, j)$ indices.

C++14¶

std::vector<float> x_data(
    150 * 300);
std::iota(
    x_data.begin(),
    x_data.end(),
    0.);

float y_data = 4.f;

std::vector<float> z_data(x_data);
for( auto& c : z_data )
    c -= 8000.f;

Python¶

x_data = np.arange(
    150 * 300,
    dtype=np.float
).reshape(150, 300)



y_data = 4.

z_data = x_data.copy() - 8000.

Record¶

An openPMD record can be either structured (mesh) or unstructured (particles). We prepared a vector field in 2D above, which is a mesh:

C++14¶

// record
auto B = i.meshes["B"];

// record components
auto B_x = B["x"];
auto B_y = B["y"];
auto B_z = B["z"];

auto dataset = io::Dataset(
    io::determineDatatype<float>(),
    {150, 300});
B_x.resetDataset(dataset);
B_y.resetDataset(dataset);
B_z.resetDataset(dataset);

Python¶

# record
B = i.meshes["B"]

# record components
B_x = B["x"]
B_y = B["y"]
B_z = B["z"]

dataset = io.Dataset(
    x_data.dtype,
    x_data.shape)
B_x.reset_dataset(dataset)
B_y.reset_dataset(dataset)
B_z.reset_dataset(dataset)

Units¶

Let’s describe this magnetic field $\vec B$ in more detail. Independent of the absolute unit system, a magnetic field has the physical dimension of [mass (M)¹ $\cdot$ electric current (I)^-1 $\cdot$ time (T)^-2].

Ouch, our magnetic field was measured in cgs units! Quick, let’s also store the conversion factor 10^-4 from Gauss (cgs) to Tesla (SI).

C++14¶

// unit system agnostic dimension
B.setUnitDimension({
    {io::UnitDimension::M,  1},
    {io::UnitDimension::I, -1},
    {io::UnitDimension::T, -2}
});

// conversion to SI
B_x.setUnitSI(1.e-4);
B_y.setUnitSI(1.e-4);
B_z.setUnitSI(1.e-4);

Python¶

# unit system agnostic dimension
B.unit_dimension = {
    io.Unit_Dimension.M:  1,
    io.Unit_Dimension.I: -1,
    io.Unit_Dimension.T: -2
}

# conversion to SI
B_x.unit_SI = 1.e-4
B_y.unit_SI = 1.e-4
B_z.unit_SI = 1.e-4

Tip

Annotating the physical dimension (unitDimension) of a record allows us to read data sets with arbitrary names and understand their purpose simply by dimensional analysis. The dimensional base quantities in openPMD are length (L), mass (M), time (T), electric current (I), thermodynamic temperature (theta), amount of substance (N), luminous intensity (J) after the international system of quantities (ISQ). The factor to SI (unitSI) on the other hand allows us to convert values between absolute unit systems.

Register Chunk¶

We can write record components partially and in parallel or at once. Writing very small data one by one is is a performance killer for I/O. Therefore, we register all data to be written first and then flush it out collectively.

C++14¶

B_x.storeChunk(
    io::shareRaw(x_data),
    {0, 0}, {150, 300});
B_z.storeChunk(
    io::shareRaw(z_data),
    {0, 0}, {150, 300});

B_y.makeConstant(y_data);

Python¶

B_x.store_chunk(x_data)

B_z.store_chunk(z_data)

B_y.make_constant(y_data)

Attention

After registering a data chunk such as x_data and y_data, it MUST NOT be modified or deleted until the flush() step is performed!

Flush Chunk¶

We now flush the registered data chunks to the I/O backend. Flushing several chunks at once allows to increase I/O performance significantly. After that, the variables x_data and y_data can be used again.

C++14¶

series.flush();

Python¶

series.flush()

Close¶

Finally, the Series is fully closed (and newly registered data or attributes since the last .flush() is written) when its destructor is called.

C++14¶

// destruct series object,
// e.g. when out-of-scope

Python¶

del series

First Read¶

Step-by-step: how to read openPMD data? We are using the examples files from openPMD-example-datasets (example-3d.tar.gz).

Include / Import¶

After successful installation, you can start using openPMD-api as follows:

C++14¶

#include <openPMD/openPMD.hpp>

// example: data handling & print
#include <vector>   // std::vector
#include <iostream> // std::cout
#include <memory>   // std::shared_ptr

namespace io = openPMD;

Python¶

import openpmd_api as io

# example: data handling
import numpy as np

Open¶

Open an existing openPMD series in data<N>.h5. Further file formats than .h5 (HDF5) are supported: .bp (ADIOS1/ADIOS2) or .json (JSON).

C++14¶

auto series = io::Series(
    "data%T.h5",
    io::Access::READ_ONLY);

Python¶

series = io.Series(
    "data%T.h5",
    io.Access.read_only)

Iteration¶

Grouping by an arbitrary, positive integer number <N> in a series. Let’s take the iteration 100:

C++14¶

auto i = series.iterations[100];

Python¶

i = series.iterations[100]

Attributes¶

openPMD defines a kernel of meta attributes and can always be extended with more. Let’s see what we’ve got:

C++14¶

std::cout << "openPMD version: "
    << series.openPMD() << "\n";

if( series.containsAttribute("author") )
    std::cout << "Author: "
        << series.author() << "\n";

Python¶

print("openPMD version: ",
      series.openPMD)

if series.contains_attribute("author"):
    print("Author: ",
          series.author)

Record¶

An openPMD record can be either structured (mesh) or unstructured (particles). Let’s read an electric field:

C++14¶

// record
auto E = i.meshes["E"];

// record components
auto E_x = E["x"];

Python¶

# record
E = i.meshes["E"]

# record components
E_x = E["x"]

Tip

You can check via i.meshes.contains("E") (C++) or "E" in i.meshes (Python) if an entry exists.

Units¶

Even without understanding the name “E” we can check the dimensionality of a record to understand its purpose.

C++14¶

// unit system agnostic dimension
auto E_unitDim = E.unitDimension();

// ...
// io::UnitDimension::M

// conversion to SI
double x_unit = E_x.unitSI();

Python¶

# unit system agnostic dimension
E_unitDim = E.unit_dimension

# ...
# io.Unit_Dimension.M

# conversion to SI
x_unit = E_x.unit_SI

Note

This example is not yet written :-)

In the future, units are automatically converted to a selected unit system (not yet implemented). For now, please multiply your read data (x_data) with x_unit to covert to SI, otherwise the raw, potentially awkwardly scaled data is taken.

Register Chunk¶

We can load record components partially and in parallel or at once. Reading small data one by one is is a performance killer for I/O. Therefore, we register all data to be loaded first and then flush it in collectively.

C++14¶

// alternatively, pass pre-allocated
std::shared_ptr< double > x_data =
    E_x.loadChunk< double >();

Python¶

# returns an allocated but
# invalid numpy array
x_data = E_x.load_chunk()

Attention

After registering a data chunk such as x_data for loading, it MUST NOT be modified or deleted until the flush() step is performed! You must not yet access x_data !

One can also request to load a slice of data:

C++14¶

Extent extent = E_x.getExtent();
extent.at(2) = 1;
std::shared_ptr< double > x_slice_data =
    E_x.loadChunk< double >(
        io::Offset{0, 0, 4}, extent);

Python¶

# we support slice syntax, too
x_slice_data = E_x[:, :, 4]

Don’t forget that we still need to flush().

Flush Chunk¶

We now flush the registered data chunks and fill them with actual data from the I/O backend. Flushing several chunks at once allows to increase I/O performance significantly. Only after that, the variables x_data and x_slice_data can be read, manipulated and/or deleted.

C++14¶

series.flush();

Python¶

series.flush()

Data¶

We can now work with the newly loaded data in x_data (or x_slice_data):

C++14¶

auto extent = E_x.getExtent();

std::cout << "First values in E_x "
        "of shape: ";
for( auto const& dim : extent )
    std::cout << dim << ", ";
std::cout << "\n";

for( size_t col = 0;
     col < extent[1] && col < 5;
     ++col )
    std::cout << x_data.get()[col]
              << ", ";
std::cout << "\n";

Python¶

extent = E_x.shape

print(
    "First values in E_x "
    "of shape: ",
    extent)


print(x_data[0, 0, :5])

Close¶

Finally, the Series is closed when its destructor is called. Make sure to have flush() ed all data loads at this point, otherwise it will be called once more implicitly.

C++14¶

// destruct series object,
// e.g. when out-of-scope

Python¶

del series

Serial Examples¶

The serial API provides sequential, one-process read and write access. Most users will use this for exploration and processing of their data.

Reading¶

C++¶

#include <openPMD/openPMD.hpp>

#include <iostream>
#include <memory>
#include <cstddef>


using std::cout;
using namespace openPMD;

int main()
{
    Series series = Series(
        "../samples/git-sample/data%T.h5",
        Access::READ_ONLY
    );
    cout << "Read a Series with openPMD standard version "
         << series.openPMD() << '\n';

    cout << "The Series contains " << series.iterations.size() << " iterations:";
    for( auto const& i : series.iterations )
        cout << "\n\t" << i.first;
    cout << '\n';

    Iteration i = series.iterations[100];
    cout << "Iteration 100 contains " << i.meshes.size() << " meshes:";
    for( auto const& m : i.meshes )
        cout << "\n\t" << m.first;
    cout << '\n';
    cout << "Iteration 100 contains " << i.particles.size() << " particle species:";
    for( auto const& ps : i.particles ) {
        cout << "\n\t" << ps.first;
        for( auto const& r : ps.second ) {
            cout << "\n\t" << r.first;
            cout << '\n';
        }
    }

    openPMD::ParticleSpecies electrons = i.particles["electrons"];
    std::shared_ptr<double> charge = electrons["charge"][openPMD::RecordComponent::SCALAR].loadChunk<double>();
    series.flush();
    cout << "And the first electron particle has a charge = " << charge.get()[0];
    cout << '\n';

    MeshRecordComponent E_x = i.meshes["E"]["x"];
    Extent extent = E_x.getExtent();
    cout << "Field E/x has shape (";
    for( auto const& dim : extent )
        cout << dim << ',';
    cout << ") and has datatype " << E_x.getDatatype() << '\n';

    Offset chunk_offset = {1, 1, 1};
    Extent chunk_extent = {2, 2, 1};
    auto chunk_data = E_x.loadChunk<double>(chunk_offset, chunk_extent);
    cout << "Queued the loading of a single chunk from disk, "
            "ready to execute\n";
    series.flush();
    cout << "Chunk has been read from disk\n"
         << "Read chunk contains:\n";
    for( size_t row = 0; row < chunk_extent[0]; ++row )
    {
        for( size_t col = 0; col < chunk_extent[1]; ++col )
            cout << "\t"
                 << '(' << row + chunk_offset[0] << '|' << col + chunk_offset[1] << '|' << 1 << ")\t"
                 << chunk_data.get()[row*chunk_extent[1]+col];
        cout << '\n';
    }

    auto all_data = E_x.loadChunk<double>();
    series.flush();
    cout << "Full E/x starts with:\n\t{";
    for( size_t col = 0; col < extent[1] && col < 5; ++col )
        cout << all_data.get()[col] << ", ";
    cout << "...}\n";

    /* The files in 'series' are still open until the object is destroyed, on
     * which it cleanly flushes and closes all open file handles.
     * When running out of scope on return, the 'Series' destructor is called.
     */
    return 0;
}

An extended example can be found in examples/6_dump_filebased_series.cpp.

Python¶

import openpmd_api as io


if __name__ == "__main__":
    series = io.Series("../samples/git-sample/data%T.h5",
                       io.Access.read_only)
    print("Read a Series with openPMD standard version %s" %
          series.openPMD)

    print("The Series contains {0} iterations:".format(len(series.iterations)))
    for i in series.iterations:
        print("\t {0}".format(i))
    print("")

    i = series.iterations[100]
    print("Iteration 100 contains {0} meshes:".format(len(i.meshes)))
    for m in i.meshes:
        print("\t {0}".format(m))
    print("")
    print("Iteration 100 contains {0} particle species:".format(
        len(i.particles)))
    for ps in i.particles:
        print("\t {0}".format(ps))
        print("With records:")
        for r in i.particles[ps]:
            print("\t {0}".format(r))

    # printing a scalar value
    electrons = i.particles["electrons"]
    charge = electrons["charge"][io.Mesh_Record_Component.SCALAR]
    series.flush()
    print("And the first electron particle has a charge {}"
          .format(charge[0]))
    print("")

    E_x = i.meshes["E"]["x"]
    shape = E_x.shape

    print("Field E.x has shape {0} and datatype {1}".format(
          shape, E_x.dtype))

    chunk_data = E_x[1:3, 1:3, 1:2]
    # print("Queued the loading of a single chunk from disk, "
    #       "ready to execute")
    series.flush()
    print("Chunk has been read from disk\n"
          "Read chunk contains:")
    print(chunk_data)
    # for row in range(2):
    #     for col in range(2):
    #         print("\t({0}|{1}|{2})\t{3}".format(
    #            row + 1, col + 1, 1, chunk_data[row*chunk_extent[1]+col])
    #         )
    #     print("")

    all_data = E_x.load_chunk()
    series.flush()
    print("Full E/x is of shape {0} and starts with:".format(all_data.shape))
    print(all_data[0, 0, :5])

    # The files in 'series' are still open until the object is destroyed, on
    # which it cleanly flushes and closes all open file handles.
    # One can delete the object explicitly (or let it run out of scope) to
    # trigger this.
    del series

Writing¶

C++¶

#include <openPMD/openPMD.hpp>

#include <iostream>
#include <memory>
#include <numeric>
#include <cstdlib>


using std::cout;
using namespace openPMD;

int main(int argc, char *argv[])
{
    // user input: size of matrix to write, default 3x3
    size_t size = (argc == 2 ? atoi(argv[1]) : 3);

    // matrix dataset to write with values 0...size*size-1
    std::vector<double> global_data(size*size);
    std::iota(global_data.begin(), global_data.end(), 0.);

    cout << "Set up a 2D square array (" << size << 'x' << size
         << ") that will be written\n";

    // open file for writing
    Series series = Series(
        "../samples/3_write_serial.h5",
        Access::CREATE
    );
    cout << "Created an empty " << series.iterationEncoding() << " Series\n";

    MeshRecordComponent rho =
      series
          .iterations[1]
          .meshes["rho"][MeshRecordComponent::SCALAR];
    cout << "Created a scalar mesh Record with all required openPMD attributes\n";

    Datatype datatype = determineDatatype(shareRaw(global_data));
    Extent extent = {size, size};
    Dataset dataset = Dataset(datatype, extent);
    cout << "Created a Dataset of size " << dataset.extent[0] << 'x' << dataset.extent[1]
         << " and Datatype " << dataset.dtype << '\n';

    rho.resetDataset(dataset);
    cout << "Set the dataset properties for the scalar field rho in iteration 1\n";

    series.flush();
    cout << "File structure and required attributes have been written\n";

    Offset offset = {0, 0};
    rho.storeChunk(shareRaw(global_data), offset, extent);
    cout << "Stored the whole Dataset contents as a single chunk, "
            "ready to write content\n";

    series.flush();
    cout << "Dataset content has been fully written\n";

    /* The files in 'series' are still open until the object is destroyed, on
     * which it cleanly flushes and closes all open file handles.
     * When running out of scope on return, the 'Series' destructor is called.
     */
    return 0;
}

An extended example can be found in examples/7_extended_write_serial.cpp.

Python¶

import openpmd_api as io
import numpy as np


if __name__ == "__main__":
    # user input: size of matrix to write, default 3x3
    size = 3

    # matrix dataset to write with values 0...size*size-1
    data = np.arange(size*size, dtype=np.double).reshape(3, 3)

    print("Set up a 2D square array ({0}x{1}) that will be written".format(
        size, size))

    # open file for writing
    series = io.Series(
        "../samples/3_write_serial_py.h5",
        io.Access.create
    )

    print("Created an empty {0} Series".format(series.iteration_encoding))

    print(len(series.iterations))
    rho = series.iterations[1]. \
        meshes["rho"][io.Mesh_Record_Component.SCALAR]

    dataset = io.Dataset(data.dtype, data.shape)

    print("Created a Dataset of size {0}x{1} and Datatype {2}".format(
        dataset.extent[0], dataset.extent[1], dataset.dtype))

    rho.reset_dataset(dataset)
    print("Set the dataset properties for the scalar field rho in iteration 1")

    series.flush()
    print("File structure has been written")

    rho[()] = data

    print("Stored the whole Dataset contents as a single chunk, " +
          "ready to write content")

    series.flush()
    print("Dataset content has been fully written")

    # The files in 'series' are still open until the object is destroyed, on
    # which it cleanly flushes and closes all open file handles.
    # One can delete the object explicitly (or let it run out of scope) to
    # trigger this.
    del series

Parallel Examples¶

The following examples show parallel reading and writing of domain-decomposed data with MPI.

The Message Passing Interface (MPI) is an open communication standard for scientific computing. MPI is used on clusters, e.g. large-scale supercomputers, to communicate between nodes and provides parallel I/O primitives.

Reading¶

C++¶

#include <openPMD/openPMD.hpp>

#include <mpi.h>

#include <iostream>
#include <memory>
#include <cstddef>


using std::cout;
using namespace openPMD;

int main(int argc, char *argv[])
{
    MPI_Init(&argc, &argv);

    int mpi_size;
    int mpi_rank;

    MPI_Comm_size(MPI_COMM_WORLD, &mpi_size);
    MPI_Comm_rank(MPI_COMM_WORLD, &mpi_rank);


    /* note: this scope is intentional to destruct the openPMD::Series object
     *       prior to MPI_Finalize();
     */
    {
        Series series = Series(
            "../samples/git-sample/data%T.h5",
            Access::READ_ONLY,
            MPI_COMM_WORLD
        );
        if( 0 == mpi_rank )
            cout << "Read a series in parallel with " << mpi_size << " MPI ranks\n";

        MeshRecordComponent E_x = series.iterations[100].meshes["E"]["x"];

        Offset chunk_offset = {
            static_cast< long unsigned int >(mpi_rank) + 1,
            1,
            1
        };
        Extent chunk_extent = {2, 2, 1};

        auto chunk_data = E_x.loadChunk<double>(chunk_offset, chunk_extent);

        if( 0 == mpi_rank )
            cout << "Queued the loading of a single chunk per MPI rank from disk, "
                    "ready to execute\n";
        series.flush();

        if( 0 == mpi_rank )
            cout << "Chunks have been read from disk\n";

        for( int i = 0; i < mpi_size; ++i )
        {
            if( i == mpi_rank )
            {
                cout << "Rank " << mpi_rank << " - Read chunk contains:\n";
                for( size_t row = 0; row < chunk_extent[0]; ++row )
                {
                    for( size_t col = 0; col < chunk_extent[1]; ++col )
                        cout << "\t"
                             << '(' << row + chunk_offset[0] << '|' << col + chunk_offset[1] << '|' << 1 << ")\t"
                             << chunk_data.get()[row*chunk_extent[1]+col];
                    cout << std::endl;
                }
            }

            // this barrier is not necessary but structures the example output
            MPI_Barrier(MPI_COMM_WORLD);
        }
    }

    // openPMD::Series MUST be destructed at this point
    MPI_Finalize();

    return 0;
}

Python¶

# IMPORTANT: include mpi4py FIRST
# https://mpi4py.readthedocs.io/en/stable/mpi4py.run.html
# on import: calls MPI_Init_thread()
# exit hook: calls MPI_Finalize()
from mpi4py import MPI

import openpmd_api as io


if __name__ == "__main__":
    # also works with any other MPI communicator
    comm = MPI.COMM_WORLD

    series = io.Series(
        "../samples/git-sample/data%T.h5",
        io.Access.read_only,
        comm
    )
    if 0 == comm.rank:
        print("Read a series in parallel with {} MPI ranks".format(
              comm.size))

    E_x = series.iterations[100].meshes["E"]["x"]

    chunk_offset = [comm.rank + 1, 1, 1]
    chunk_extent = [2, 2, 1]

    chunk_data = E_x.load_chunk(chunk_offset, chunk_extent)

    if 0 == comm.rank:
        print("Queued the loading of a single chunk per MPI rank from disk, "
              "ready to execute")
    series.flush()

    if 0 == comm.rank:
        print("Chunks have been read from disk")

    for i in range(comm.size):
        if i == comm.rank:
            print("Rank {} - Read chunk contains:".format(i))
            for row in range(chunk_extent[0]):
                for col in range(chunk_extent[1]):
                    print("\t({}|{}|1)\t{:e}".format(
                        row + chunk_offset[0],
                        col + chunk_offset[1],
                        chunk_data[row, col, 0]
                    ), end='')
                print("")

        # this barrier is not necessary but structures the example output
        comm.Barrier()

    # The files in 'series' are still open until the object is destroyed, on
    # which it cleanly flushes and closes all open file handles.
    # One can delete the object explicitly (or let it run out of scope) to
    # trigger this.
    # In any case, this must happen before MPI_Finalize() is called
    # (usually in the mpi4py exit hook).
    del series

Writing¶

C++¶

#include <openPMD/openPMD.hpp>

#include <mpi.h>

#include <iostream>
#include <memory>
#include <vector>   // std::vector


using std::cout;
using namespace openPMD;

int main(int argc, char *argv[])
{
    MPI_Init(&argc, &argv);

    int mpi_size;
    int mpi_rank;

    MPI_Comm_size(MPI_COMM_WORLD, &mpi_size);
    MPI_Comm_rank(MPI_COMM_WORLD, &mpi_rank);

    /* note: this scope is intentional to destruct the openPMD::Series object
     *       prior to MPI_Finalize();
     */
    {
        // global data set to write: [MPI_Size * 10, 300]
        // each rank writes a 10x300 slice with its MPI rank as values
        auto const value = float(mpi_size);
        std::vector<float> local_data(
            10 * 300, value);
        if( 0 == mpi_rank )
            cout << "Set up a 2D array with 10x300 elements per MPI rank (" << mpi_size
                 << "x) that will be written to disk\n";

        // open file for writing
        Series series = Series(
            "../samples/5_parallel_write.h5",
            Access::CREATE,
            MPI_COMM_WORLD
        );
        if( 0 == mpi_rank )
          cout << "Created an empty series in parallel with "
               << mpi_size << " MPI ranks\n";

        MeshRecordComponent mymesh =
            series
                .iterations[1]
                .meshes["mymesh"][MeshRecordComponent::SCALAR];

        // example 1D domain decomposition in first index
        Datatype datatype = determineDatatype<float>();
        Extent global_extent = {10ul * mpi_size, 300};
        Dataset dataset = Dataset(datatype, global_extent);

        if( 0 == mpi_rank )
            cout << "Prepared a Dataset of size " << dataset.extent[0]
                 << "x" << dataset.extent[1]
                 << " and Datatype " << dataset.dtype << '\n';

        mymesh.resetDataset(dataset);
        if( 0 == mpi_rank )
            cout << "Set the global Dataset properties for the scalar field mymesh in iteration 1\n";

        // example shows a 1D domain decomposition in first index
        Offset chunk_offset = {10ul * mpi_rank, 0};
        Extent chunk_extent = {10, 300};
        mymesh.storeChunk(local_data, chunk_offset, chunk_extent);
        if( 0 == mpi_rank )
            cout << "Registered a single chunk per MPI rank containing its contribution, "
                    "ready to write content to disk\n";

        series.flush();
        if( 0 == mpi_rank )
            cout << "Dataset content has been fully written to disk\n";
    }

    // openPMD::Series MUST be destructed at this point
    MPI_Finalize();

    return 0;
}

Python¶

# IMPORTANT: include mpi4py FIRST
# https://mpi4py.readthedocs.io/en/stable/mpi4py.run.html
# on import: calls MPI_Init_thread()
# exit hook: calls MPI_Finalize()
from mpi4py import MPI

import openpmd_api as io
import numpy as np


if __name__ == "__main__":
    # also works with any other MPI communicator
    comm = MPI.COMM_WORLD

    # global data set to write: [MPI_Size * 10, 300]
    # each rank writes a 10x300 slice with its MPI rank as values
    local_value = comm.size
    local_data = np.ones(10 * 300,
                         dtype=np.double).reshape(10, 300) * local_value
    if 0 == comm.rank:
        print("Set up a 2D array with 10x300 elements per MPI rank ({}x) "
              "that will be written to disk".format(comm.size))

    # open file for writing
    series = io.Series(
        "../samples/5_parallel_write_py.h5",
        io.Access.create,
        comm
    )
    if 0 == comm.rank:
        print("Created an empty series in parallel with {} MPI ranks".format(
              comm.size))

    mymesh = series.iterations[1]. \
        meshes["mymesh"][io.Mesh_Record_Component.SCALAR]

    # example 1D domain decomposition in first index
    global_extent = [comm.size * 10, 300]
    dataset = io.Dataset(local_data.dtype, global_extent)

    if 0 == comm.rank:
        print("Prepared a Dataset of size {} and Datatype {}".format(
              dataset.extent, dataset.dtype))

    mymesh.reset_dataset(dataset)
    if 0 == comm.rank:
        print("Set the global Dataset properties for the scalar field "
              "mymesh in iteration 1")

    # example shows a 1D domain decomposition in first index
    mymesh[comm.rank*10:(comm.rank+1)*10, :] = local_data
    if 0 == comm.rank:
        print("Registered a single chunk per MPI rank containing its "
              "contribution, ready to write content to disk")

    series.flush()
    if 0 == comm.rank:
        print("Dataset content has been fully written to disk")

    # The files in 'series' are still open until the object is destroyed, on
    # which it cleanly flushes and closes all open file handles.
    # One can delete the object explicitly (or let it run out of scope) to
    # trigger this.
    del series

Workflow¶

Deferred Data API Contract¶

IO operations are in general not performed by the openPMD API immediately after calling the corresponding API function. Rather, operations are enqueued internally and performed at so-called flush points. A flush point is a point within an application’s sequential control flow where the openPMD API must uphold the following guarantees:

In write mode, any change made to a user buffer whose data shall be stored in a dataset up to the flush point must be found in the written dataset.
In write mode, no change made to a user buffer whose data shall be stored in a dataset after the flush point must be found in the written dataset.
In read mode, a buffer into which data from a dataset should be filled, must not be altered by the openPMD API before the flush point.
In read mode, a buffer into which data from a dataset should be filled, must have been filled with the requested data after the flush point.

In short: operations requested by storeChunk() and loadChunk() must happen exactly at flush points.

Flush points are triggered by:

Calling Series::flush().
Calling Iteration::close( flush=true ). Flush point guarantees affect only the corresponding iteration.
Calling Writable::seriesFlush() or Attributable::seriesFlush().
The streaming API (i.e. Series.readIterations() and Series.writeIteration()) automatically before accessing the next iteration.

Attributes are (currently) unaffected by this:

In writing, attributes are stored internally by value and can afterwards not be accessed by the user.
In reading, attributes are parsed upon opening the Series / an iteration and are available to read right-away.

Streaming¶

Note

Data streaming is a novel backend and under active development. At the moment, the internal data format is still changing rapidly and is likely not compatible between releases of the openPMD-api.

The openPMD API includes a streaming-aware API as well as streaming-enabled backends (currently: ADIOS2).

Unlike in file-based backends, the order in which data is put out becomes relevant in streaming-based backends. Each iteration will be published as one atomical step by the streaming API (compare the concept of steps in ADIOS2).

Reading¶

The reading end of the streaming API enforces further restrictions that become necessary through the nature of streaming. It can be used to read any kind of openPMD-compatible dataset, stream-based and filesystem-based alike.

C++¶

The reading end of the streaming API is activated through use of Series::readIterations() instead of accessing the field Series::iterations directly. The returned object of type ReadIterations can be used in a C++11 range-based for loop to iterate over objects of type IndexedIteration. This class extends the Iteration class with a field IndexedIteration::iterationIndex, denoting this iteration’s index.

Iterations are implicitly opened by the Streaming API and Iteration::open() needs not be called explicitly. Users are encouraged to explicitly .close() the iteration after reading from it. Closing the iteration will flush all pending operations on that iteration. If an iteration is not closed until the beginning of the next iteration, it will be closed automatically.

Note that a closed iteration cannot be reopened. This pays tribute to the fact that in streaming mode, an iteration may be dropped by the data source once the data sink has finished reading from it.

#include <openPMD/openPMD.hpp>

#include <algorithm>
#include <array>
#include <iostream>
#include <memory>

using std::cout;
using namespace openPMD;

int
main()
{
#if openPMD_HAVE_ADIOS2
    using position_t = double;
    auto backends = openPMD::getFileExtensions();
    if( std::find( backends.begin(), backends.end(), "sst" ) == backends.end() )
    {
        std::cout << "SST engine not available in ADIOS2." << std::endl;
        return 0;
    }

    Series series = Series( "electrons.sst", Access::READ_ONLY );

    for( IndexedIteration iteration : series.readIterations() )
    {
        std::cout << "Current iteration: " << iteration.iterationIndex
                  << std::endl;
        Record electronPositions = iteration.particles[ "e" ][ "position" ];
        std::array< std::shared_ptr< position_t >, 3 > loadedChunks;
        std::array< Extent, 3 > extents;
        std::array< std::string, 3 > const dimensions{ { "x", "y", "z" } };

        for( size_t i = 0; i < 3; ++i )
        {
            std::string dim = dimensions[ i ];
            RecordComponent rc = electronPositions[ dim ];
            loadedChunks[ i ] = rc.loadChunk< position_t >(
                Offset( rc.getDimensionality(), 0 ), rc.getExtent() );
            extents[ i ] = rc.getExtent();
        }

        iteration.close();

        for( size_t i = 0; i < 3; ++i )
        {
            std::string dim = dimensions[ i ];
            Extent const & extent = extents[ i ];
            std::cout << "\ndim: " << dim << "\n" << std::endl;
            auto chunk = loadedChunks[ i ];
            for( size_t j = 0; j < extent[ 0 ]; ++j )
            {
                std::cout << chunk.get()[ j ] << ", ";
            }
            std::cout << "\n----------\n" << std::endl;
        }
    }

    return 0;
#else
    std::cout << "The streaming example requires that openPMD has been built "
                 "with ADIOS2."
              << std::endl;
    return 0;
#endif
}

Python¶

The reading end of the streaming API is activated through use of Series.read_iterations() instead of accessing the field Series.iterations directly. The returned object of type ReadIterations can be used in a Python range-based for loop to iterate over objects of type IndexedIteration. This class extends the Iteration class with a field IndexedIteration.iteration_index, denoting this iteration’s index.

Iterations are implicitly opened by the Streaming API and Iteration.open() needs not be called explicitly. Users are encouraged to explicitly .close() the iteration after reading from it. Closing the iteration will flush all pending operations on that iteration. If an iteration is not closed until the beginning of the next iteration, it will be closed automatically.

Note that a closed iteration cannot be reopened. This pays tribute to the fact that in streaming mode, an iteration may be dropped by the data source once the data sink has finished reading from it.

#!/usr/bin/env python
import openpmd_api as io
import sys

if __name__ == "__main__":
    if 'adios2' not in io.variants or not io.variants['adios2']:
        print('This example requires ADIOS2')
        sys.exit(0)

    series = io.Series("stream.sst", io.Access_Type.read_only)

    backends = io.file_extensions
    if "sst" not in backends:
        print("SST engine not available in ADIOS2.")
        sys.exit(0)

    for iteration in series.read_iterations():
        print("Current iteration {}".format(iteration.iteration_index))
        electronPositions = iteration.particles["e"]["position"]
        loadedChunks = []
        shapes = []
        dimensions = ["x", "y", "z"]

        for i in range(3):
            dim = dimensions[i]
            rc = electronPositions[dim]
            loadedChunks.append(rc.load_chunk([0], rc.shape))
            shapes.append(rc.shape)
        iteration.close()

        for i in range(3):
            dim = dimensions[i]
            shape = shapes[i]
            print("dim: {}".format(dim))
            chunk = loadedChunks[i]
            print(chunk)

Writing¶

The writing end of the streaming API enforces further restrictions that become necessary through the nature of streaming. It can be used to write any kind of openPMD-compatible dataset, stream-based and filesystem-based alike.

C++¶

The writing end of the streaming API is activated through use of Series::writeIterations() instead of accessing the field Series::iterations directly. The returned object of type WriteIterations wraps the field Series::iterations, but exposes only a restricted subset of functionality. Using WriteIterations::operator[]( uint64_t ) will automatically open a streaming step for the corresponding iteration.

Users are encouraged to explicitly .close() the iteration after writing to it. Closing the iteration will flush all pending operations on that iteration. If an iteration is not closed until the next iteration is accessed via WriteIterations::operator[]( uint64_t ), it will be closed automatically.

Note that a closed iteration cannot be reopened. This pays tribute to the fact that in streaming mode, an iteration is sent to the sink upon closing it and the data source can no longer modify it.

#include <openPMD/openPMD.hpp>

#include <algorithm>
#include <iostream>
#include <memory>
#include <numeric> // std::iota

using std::cout;
using namespace openPMD;

int
main()
{
#if openPMD_HAVE_ADIOS2
    using position_t = double;
    auto backends = openPMD::getFileExtensions();
    if( std::find( backends.begin(), backends.end(), "sst" ) == backends.end() )
    {
        std::cout << "SST engine not available in ADIOS2." << std::endl;
        return 0;
    }

    // open file for writing
    Series series = Series( "electrons.sst", Access::CREATE );

    Datatype datatype = determineDatatype< position_t >();
    constexpr unsigned long length = 10ul;
    Extent global_extent = { length };
    Dataset dataset = Dataset( datatype, global_extent );
    std::shared_ptr< position_t > local_data(
        new position_t[ length ],
        []( position_t const * ptr ) { delete[] ptr; } );

    WriteIterations iterations = series.writeIterations();
    for( size_t i = 0; i < 100; ++i )
    {
        Iteration iteration = iterations[ i ];
        Record electronPositions = iteration.particles[ "e" ][ "position" ];

        std::iota( local_data.get(), local_data.get() + length, i * length );
        for( auto const & dim : { "x", "y", "z" } )
        {
            RecordComponent pos = electronPositions[ dim ];
            pos.resetDataset( dataset );
            pos.storeChunk( local_data, Offset{ 0 }, global_extent );
        }
        iteration.close();
    }

    return 0;
#else
    std::cout << "The streaming example requires that openPMD has been built "
                 "with ADIOS2."
              << std::endl;
    return 0;
#endif
}

Python¶

The writing end of the streaming API is activated through use of Series.write_iterations() instead of accessing the field Series.iterations directly. The returned object of type WriteIterations wraps the field Series.iterations, but exposes only a restricted subset of functionality. Using WriteIterations.__getitem__(index) (i.e. the index operator series.writeIterations()[index]) will automatically open a streaming step for the corresponding iteration.

Users are encouraged to explicitly .close() the iteration after writing to it. Closing the iteration will flush all pending operations on that iteration. If an iteration is not closed until the next iteration is accessed via WriteIterations.__getitem__(index), it will be closed automatically.

Note that a closed iteration cannot be reopened. This pays tribute to the fact that in streaming mode, an iteration is sent to the sink upon closing it and the data source can no longer modify it.

#!/usr/bin/env python
import openpmd_api as io
import numpy as np
import sys

if __name__ == "__main__":
    if 'adios2' not in io.variants or not io.variants['adios2']:
        print('This example requires ADIOS2')
        sys.exit(0)

    series = io.Series("stream.sst", io.Access_Type.create)

    backends = io.file_extensions
    if "sst" not in backends:
        print("SST engine not available in ADIOS2.")
        sys.exit(0)

    datatype = np.dtype("double")
    length = 10
    global_extent = [10]
    dataset = io.Dataset(datatype, global_extent)

    iterations = series.write_iterations()
    for i in range(100):
        iteration = iterations[i]
        electronPositions = iteration.particles["e"]["position"]

        local_data = np.arange(i * length, (i + 1) * length, dtype=datatype)
        for dim in ["x", "y", "z"]:
            pos = electronPositions[dim]
            pos.reset_dataset(dataset)
            pos[()] = local_data
        iteration.close()

Benchmarks¶

Parallel benchmark 8¶

Build based on the helper functions in the benchmark utilities, this benchmark executes a simple parallel read-write test.

In particular, this test case writes and reads a 3D array of type uint64_t, sliced 1D along the first dimension.

openPMD::Extent total{
    100 * scale_up, // sliced along this axis over MPI ranks
    100,
    1000
};

The benchmark writes 10 iterations, meaning that in the strong-scaling case, always around 3/4 GB of data are produced. In the weak-scaling case, the data scales as $N * 3/4 \mathrm{GiB}$ with $N$ as the number of participating MPI ranks.

By default, the benchmarks executes as strong-scaling unless the -w/--weak option is passed as a command-line argument to the executable.

Parallel benchmarks 8a & 8b¶

The following examples show parallel reading and writing of domain-decomposed data with MPI.

The Message Passing Interface (MPI) is an open communication standard for scientific computing. MPI is used on clusters, e.g. large-scale supercomputers, to communicate between nodes and provides parallel I/O primitives.

Writing¶

Source: examples/8a_benchmark_write_parallel.cpp

This benchmark writes a few meshes and particles, either 1D, 2D or 3D.

The meshes are viewed as grid of mini blocks. As an example, we assume the mini blocks dimension are [16, 32, 32].

Next we define the grid based on the mini block. say, [32, 32, 16]. Then our actual mesh size is [16x32, 32x32, 32x16].

Here is a sample input file (“w.input”):

dim=3
balanced=true
ratio=1
steps=10
minBlock=16 32 32
grid=32 32 16

With the above input file, we will create an openPMD file with the above mesh using

3D data
balanced load
particle to mesh ratio = 1
10 iteration steps

Note: All files generated are group based. i.e. One file per iteration.

To run:

./8a_benchmark_write_parallel w.input

then the file generated are: ../samples/8a_parallel_3Db_*

Optional input parameter: pack

Often a processor will write out a few small blocks. Using the example above, if the writer side uses 1024 processors, each processor will handle 16 blocks from the 32x32x16 grid. These 16 blocks per processor can be packed from a selection of 1x1x16 blocks, or 2x2x4, etc. The pack parameter specifies this selection, e.g., "pack=1 1 16" or "pack=2 2 4". Without specifying this parameter, a default will be applied. This parameter does not expected to impact the performance of writing, it will likely make a difference for certain reading patterns if the underlying storage is using subfiles.

Reading¶

Source: examples/8b_benchmark_read_parallel.cpp

This benchmark is to read from the files written by 8a.

The options are: a file prefix, and a read pattern

For example, if the files are in the format of /path/8a_parallel_3Db_%07T.bp the input will be: /path/8a_parallel_3Db <options>

The Read options intent to measure overall processing time in the following categories:

Metadata only (option = m)
or data retrieval (after metadata loaded)

The data retrieval is furthur divided into:

slice the “rho” mesh
(options = sx/sy/sz depends on which direction. e.g. sx implies x=0.)
slice on the 3D magnetic field(e.g. find values for “Bx”, “By” and “Bz”)
(options = fx/fy/fz depends on which direction. e.g. fx implies x=0.)

So here are the options one can use to read a file:

m
sx
sy
sz
fx
fy
fz

For example, To read files generated by the above write commmand, metadata only:

./8b_benchmark_read_parallel ../samples/8a_parallel_3Db m

More complicated Writing options (Applies to ADIOS BP)¶

The ADIOS BP files uses subfiles to store data from each rank. We have an option to provide hint on how data should be divided per rank in the command line: the order of options are:

grid of minimal blocks|balance|particle2mesh ratio
minial blocks
use multiple blocks
num of timesteps,
dimensions
hint on work load arrangement.

Example: “mpirun -n 4 ./8a_benchmark_write_parallel 400801 16016 1 5 3 4004002 “

Here 4 ranks are used to write a 3D mesh, minimal block is [16,16,16], grid of minimal block is [8,4,4], so the actual mesh = [16x8, 16x4, 16x4]. Number of timestep = 5.

The hint is asking each rank to work on a [16x2, 16x4, 16x4] block. It precisely cover the mesh with 4 ranks, so will be applied.

Benchmark Utilities¶

Further benchmarks are fund in utilities.

All Examples¶

The full list of example scripts shown below is also contained in our examples/ folder.

Example data sets can be downloaded from: github.com/openPMD/openPMD-example-datasets. The following command will automatically install those into samples/ on Linux and OSX: curl -sSL https://git.io/JewVw | bash

C++¶

1_structure.cpp: creating a first series
2_read_serial.cpp: reading a mesh & a particle species
2a_read_thetaMode_serial.cpp: read an azimuthally decomposed mesh (and reconstruct it)
3_write_serial.cpp: writing a mesh
3a_write_thetaMode_serial.cpp: write an azimuthally decomposed mesh
3b_write_resizable_particles.cpp: write particles in a resizeable dataset
4_read_parallel.cpp: MPI-parallel mesh read
5_write_parallel.cpp: MPI-parallel mesh write
6_dump_filebased_series.cpp: detailed reading with a file-based series
7_extended_write_serial.cpp: particle writing with patches and constant records
10_streaming_write.cpp / 10_streaming_read.cpp: ADIOS2 data streaming
12_span_write.cpp: using the span-based API to save memory when writing

Benchmarks¶

8_benchmark_parallel.cpp: a MPI-parallel IO-benchmark
8a_benchmark_write_parallel.cpp: creates 1D/2D/3D arrays, with each rank having a few blocks to write to
8b_benchmark_read_parallel.cpp: read slices of meshes and particles

Python¶

2_read_serial.py: reading a mesh & a particle species
2a_read_thetaMode_serial.py: reading an azimuthally decomposed mesh (and reconstruct it)
3_write_serial.py: writing a mesh
3a_write_thetaMode_serial.py: write an azimuthally decomposed mesh
3b_write_resizable_particles.py: write particles in a resizeable dataset
4_read_parallel.py: MPI-parallel mesh read
5_write_parallel.py: MPI-parallel mesh write
7_extended_write_serial.py: particle writing with patches and constant records
9_particle_write_serial.py: writing particles
10_streaming_write.py / 10_streaming_read.py: ADIOS2 data streaming
11_particle_dataframe.py: reading data into Pandas dataframes or Dask for distributed analysis
12_span_write.py: using the span-based API to save memory when writing

Unit Tests¶

Our unit tests in the test/ folder might also be informative for advanced developers.

API Details¶

C++¶

Our Doxygen page provides an index of all C++ functionality.

Public Headers¶

#include ... the following headers to use openPMD-api:

Include	Description
`<openPMD/openPMD.hpp>`	Public facade header (serial and MPI)
`<openPMD/benchmark/...>`	Optional benchmark helpers

External Documentation¶

If you want to link to the openPMD-api doxygen index from an external documentation, you can find the Doxygen tag file here.

If you want to use this tag file with e.g. xeus-cling, add the following in its configuration directory:

{
    "url": "https://openpmd-api.readthedocs.io/en/<adjust-version-of-tag-file-here>/_static/doxyhtml/",
    "tagfile": "openpmd-api-doxygen-web.tag.xml"
}

Python¶

Public Headers¶

import ... the following python module to use openPMD-api:

Import	Description
`openpmd_api`	Public facade import (serial and MPI)

Note

As demonstrated in our python examples, MPI-parallel scripts must import from mpi4py import MPI prior to importing openpmd_api in order to initialize MPI first.

Otherwise, errors of the following kind will occur:

The MPI_Comm_test_inter() function was called before MPI_INIT was invoked.
This is disallowed by the MPI standard.
Your MPI job will now abort.

MPI¶

Collective Behavior¶

openPMD-api is designed to support both serial as well as parallel I/O. The latter is implemented through the Message Passing Interface (MPI).

A collective operation needs to be executed by all MPI ranks of the MPI communicator that was passed to openPMD::Series. Contrarily, independent operations can also be called by a subset of these MPI ranks. For more information, please see the MPI standard documents, for example MPI-3.1 in “Section 2.4 - Semantic Terms”.

Functionality	Behavior	Description
`Series`	collective	open and close
`::flush()`	collective	read and write
`Iteration` 1	independent	declare and open
`::open()` 4	collective	explicit open
`Mesh` 1	independent	declare, open, write
`ParticleSpecies` 1	independent	declare, open, write
`::setAttribute` 3	backend-specific	declare, write
`::getAttribute`	independent	open, reading
`RecordComponent` 1	independent	declare, open, write
`::resetDataset` 1 2	backend-specific	declare, write
`::makeConstant` 3	backend-specific	declare, write
`::storeChunk` 1	independent	write
`::loadChunk`	independent	read
`::availableChunks` 4	collective	read, immediate result

1(1,2,3,4,5,6): Individual backends, i.e. parallel HDF5, will only support independent operations if the default, non-collective (aka independent) behavior is kept. Otherwise these operations are collective.
2: Dataset declarations in parallel HDF5 are only non-collective if chunking is set to none (auto by default). Otherwise these operations are collective.
3(1,2): HDF5 only supports collective attribute definitions/writes; ADIOS1 and ADIOS2 attributes can be written independently. If you want to support all backends equally, treat as a collective operation. Note that openPMD represents constant record components with attributes, thus inheriting this for ::makeConstant.
4(1,2): We usually open iterations delayed on first access. This first access is usually the flush() call after a storeChunk/loadChunk operation. If the first access is non-collective, an explicit, collective Iteration::open() can be used to have the files already open.

Alternatively, iterations might be accessed for the first time by immediate operations such as ::availableChunks().

Tip

Just because an operation is independent does not mean it is allowed to be inconsistent. For example, undefined behavior will occur if ranks pass differing values to ::setAttribute or try to use differing names to describe the same mesh.

Efficient Parallel I/O Patterns¶

Note

This section is a stub. We will improve it in future versions.

Write as large data set chunks as possible in ::storeChunk operations.

Read in large, non-overlapping subsets of the stored data (::loadChunk). Ideally, read the same chunk extents as were written, e.g. through ParticlePatches (example to-do).

See the implemented I/O backends for individual tuning options.

JSON configuration¶

While the openPMD API intends to be a backend-independent implementation of the openPMD standard, it is sometimes useful to pass configuration parameters to the specific backend in use. For each backend, configuration options can be passed via a JSON-formatted string or via environment variables. A JSON option always takes precedence over an environment variable.

The fundamental structure of this JSON configuration string is given as follows:

{
    "adios": "put ADIOS config here",
    "adios2": "put ADIOS2 config here",
    "hdf5": "put HDF5 config here",
    "json": "put JSON config here"
}

This structure allows keeping one configuration string for several backends at once, with the concrete backend configuration being chosen upon choosing the backend itself.

The configuration is read in a case-sensitive manner. Generally, keys of the configuration are lower case. Parameters that are directly passed through to an external library and not interpreted within openPMD API (e.g. adios2.engine.parameters) are unaffected by this and follow the respective library’s conventions.

The configuration string may refer to the complete openPMD::Series or may additionally be specified per openPMD::Dataset, passed in the respective constructors. This reflects the fact that certain backend-specific parameters may refer to the whole Series (such as storage engines and their parameters) and others refer to actual datasets (such as compression).

A JSON configuration may either be specified as a regular string that can be parsed as a JSON object, or in the constructor of openPMD::Series alternatively as a path to a JSON-formatted text file. In the latter case, the file path must be prepended by an at-sign @.

For a consistent user interface, backends shall follow the following rules:

The configuration structures for the Series and for each dataset should be defined equivalently.
Any setting referring to single datasets should also be applicable globally, affecting all datasets.
If a setting is defined globally, but also for a concrete dataset, the dataset-specific setting should override the global one.
If a setting is passed to a dataset that only makes sense globally (such as the storage engine), the setting should be ignored except for printing a warning. Backends should define clearly which keys are applicable to datasets and which are not.

Backend-independent JSON configuration¶

The key defer_iteration_parsing can be used to optimize the process of opening an openPMD Series. By default, a Series is parsed eagerly, i.e. opening a Series implies reading all available iterations. Especially when a Series has many iterations, this can be a costly operation and users may wish to defer parsing of iterations to a later point adding {"defer_iteration_parsing": true} to their JSON configuration.

When parsing non-eagerly, each iteration needs to be explicitly opened with Iteration::open() before accessing. (Notice that Iteration::open() is generally recommended to be used in parallel contexts to avoid parallel file accessing hazards). Using the Streaming API (i.e. SeriesInterface::readIteration()) will do this automatically. Parsing eagerly might be very expensive for a Series with many iterations, but will avoid bugs by forgotten calls to Iteration::open(). In complex environments, calling Iteration::open() on an already open environment does no harm (and does not incur additional runtime cost for additional open() calls).

The key resizable can be passed to Dataset options. It if set to {"resizable": true}, this declares that it shall be allowed to increased the Extent of a Dataset via resetDataset() at a later time, i.e., after it has been first declared (and potentially written). For HDF5, resizable Datasets come with a performance penalty. For JSON and ADIOS2, all datasets are resizable, independent of this option.

Configuration Structure per Backend¶

ADIOS2¶

A full configuration of the ADIOS2 backend:

{
  "adios2": {
    "engine": {
      "type": "sst",
      "parameters": {
        "BufferGrowthFactor": "2.0",
        "QueueLimit": "2"
      }
    },
    "dataset": {
      "operators": [
        {
          "type": "blosc",
          "parameters": {
              "clevel": "1",
              "doshuffle": "BLOSC_BITSHUFFLE"
          }
        }
      ]
    }
  }
}

All keys found under adios2.dataset are applicable globally as well as per dataset, keys found under adios2.engine only globally. Explanation of the single keys:

adios2.engine.type: A string that is passed directly to adios2::IO:::SetEngine for choosing the ADIOS2 engine to be used. Please refer to the official ADIOS2 documentation for a list of available engines.
adios2.engine.parameters: An associative array of string-formatted engine parameters, passed directly through to adios2::IO::SetParameters. Please refer to the official ADIOS2 documentation for the allowable engine parameters.
adios2.engine.usesteps: Described more closely in the documentation for the ADIOS2 backend.
adios2.dataset.operators: This key contains a list of ADIOS2 operators, used to enable compression or dataset transformations. Each object in the list has two keys:
- type supported ADIOS operator type, e.g. zfp, sz
- parameters is an associative map of string parameters for the operator (e.g. compression levels)

Any setting specified under adios2.dataset is applicable globally as well as on a per-dataset level. Any setting under adios2.engine is applicable globally only.

HDF5¶

A full configuration of the HDF5 backend:

All keys found under hdf5.dataset are applicable globally (future: as well as per dataset). Explanation of the single keys:

adios2.dataset.chunks: This key contains options for data chunking via H5Pset_chunk. The default is "auto" for a heuristic. "none" can be used to disable chunking. Chunking generally improves performance and only needs to be disabled in corner-cases, e.g. when heavily relying on independent, parallel I/O that non-collectively declares data records.

Other backends¶

Do currently not read the configuration string. Please refer to the respective backends’ documentations for further information on their configuration.

Utilities¶

Command Line Tools¶

openPMD-api installs command line tools alongside the main library. These terminal-focused tools help to quickly explore, manage or manipulate openPMD data series.

`openpmd-ls`¶

List information about an openPMD data series.

The syntax of the command line tool is printed via:

openpmd-ls --help

With some pip-based python installations, you might have to run this as a module:

python -m openpmd_api.ls --help

`openpmd-pipe`¶

Redirect openPMD data from any source to any sink.

The script can be used in parallel via MPI. Datasets will be split into chunks of equal size to be loaded and written by the single processes.

Possible uses include:

Conversion of a dataset between two openPMD-based backends, such as ADIOS and HDF5.
Decompression and compression of a dataset.
Capture of a stream into a file.
Template for simpler loosely-coupled post-processing scripts.

The syntax of the command line tool is printed via:

openpmd-pipe --help

With some pip-based python installations, you might have to run this as a module:

python -m openpmd_api.pipe --help

Benchmark¶

The openPMD API provides utilities to quickly configure and run benchmarks in a flexible fashion. The starting point for configuring and running benchmarks is the class template Benchmark<DatasetFillerProvider>.

#include "openPMD/benchmark/mpi/Benchmark.hpp"

An object of this class template allows to preconfigure a number of benchmark runs to execute, each run specified by:

The compression configuration, consisting itself of the compression string and the compression level.

The backend to use, specified by the filename extension (e.g. “h5”, “bp”, “json”, …).

The type of data to write, specified by the openPMD datatype.

The number of ranks to use, not greater than the MPI size. An overloaded version of addConfiguration() exists that picks the MPI size.

The number n of iterations. The benchmark will effectively be repeated n times.

The benchmark object is globally (i.e. by its constructor) specified by:

The base path to use. This will be extended with the chosen backend’s filename extension. Benchmarks might overwrite each others’ files.

The total extent of the dataset across all MPI ranks.

The BlockSlicer, i.e. an object telling each rank which portion of the dataset to write to and read from. Most users will be content with the implementation provided by OneDimensionalBlockSlicer that will simply divide the dataset into hyperslabs along one dimension, default = 0. This implementation can also deal with odd dimensions that are not divisible by the MPI size.

A DatasetFillerProvider. DatasetFiller<T> is an abstract class template whose job is to create the write data of type T for one run of the benchmark. Since one Benchmark object allows to use several datatypes, a DatasetFillerProvider is needed to create such objects. DatasetFillerProvider is a template parameter of the benchmark class template and should be a templated functor whose operator()<T>() returns a shared_ptr<DatasetFiller<T>> (or a value that can be dynamically casted to it). For users seeking to only run the benchmark with one datatype, the class template SimpleDatasetFillerProvider<DF> will lift a DatasetFiller<T> to a DatasetFillerProvider whose operator()<T'>() will only successfully return if T and T' are the same type.

The MPI Communicator.

The class template RandomDatasetFiller<Distr, T> (where by default T = typename Distr::result_type) provides an implementation of the DatasetFiller<T> that lifts a random distribution to a DatasetFiller. The general interface of a DatasetFiller<T> is kept simple, but an implementation should make sure that every call to DatasetFiller<T>::produceData() takes roughly the same amount of time, thus allowing to deduct from the benchmark results the time needed for producing data.

The configured benchmarks are run one after another by calling the method Benchmark<...>::runBenchmark<Clock>(int rootThread). The Clock template parameter should meet the requirements of a trivial clock. Although every rank will return a BenchmarkReport<typename Clock::rep>, only the report of the previously specified root rank will be populated with data, i.e. all ranks’ data will be collected into one report.

Example Usage¶

#include <openPMD/openPMD.hpp>
#include <openPMD/benchmark/mpi/MPIBenchmark.hpp>
#include <openPMD/benchmark/mpi/RandomDatasetFiller.hpp>
#include <openPMD/benchmark/mpi/OneDimensionalBlockSlicer.hpp>

#if openPMD_HAVE_MPI
#   include <mpi.h>
#endif

#include <iostream>
#include <string>
#include <vector>


#if openPMD_HAVE_MPI
inline void
print_help( std::string const program_name )
{
    std::cout << "Usage: " << program_name << "\n";
    std::cout << "Run a simple parallel write and read benchmark.\n\n";
    std::cout << "Options:\n";
    std::cout << "    -w, --weak    run a weak scaling (default: strong scaling)\n";
    std::cout << "    -h, --help    display this help and exit\n";
    std::cout << "    -v, --version output version information and exit\n";
    std::cout << "\n";
    std::cout << "Examples:\n";
    std::cout << "    " << program_name << " --weak  # for a weak-scaling\n";
    std::cout << "    " << program_name << "  # for a strong scaling\n";
}

inline void
print_version( std::string const program_name )
{
    std::cout << program_name << " (openPMD-api) "
              << openPMD::getVersion() << "\n";
    std::cout << "Copyright 2017-2021 openPMD contributors\n";
    std::cout << "Authors: Franz Poeschel, Axel Huebl et al.\n";
    std::cout << "License: LGPLv3+\n";
    std::cout << "This is free software: you are free to change and redistribute it.\n"
                 "There is NO WARRANTY, to the extent permitted by law.\n";
}

int main(
    int argc,
    char *argv[]
)
{
    using namespace std;
    MPI_Init(
        &argc,
        &argv
    );

    // CLI parsing
    std::vector< std::string > str_argv;
    for( int i = 0; i < argc; ++i ) str_argv.emplace_back( argv[i] );
    bool weak_scaling = false;

    for (int c = 1; c < int(argc); c++) {
        if (std::string("--help") == argv[c] || std::string("-h") == argv[c]) {
            print_help(argv[0]);
            return 0;
        }
        if (std::string("--version") == argv[c] || std::string("-v") == argv[c]) {
            print_version(argv[0]);
            return 0;
        }
        if (std::string("--weak") == argv[c] || std::string("-w") == argv[c]) {
            weak_scaling = true;
        }
    }

    if (argc > 2) {
        std::cerr << "Too many arguments! See: " << argv[0] << " --help\n";
        return 1;
    }

    // For simplicity, use only one datatype in this benchmark.
    // Note that a single Benchmark object can be used to configure
    // multiple different benchmark runs with different datatypes,
    // given that you provide it with an appropriate DatasetFillerProvider
    // (template parameter of the Benchmark class).
    using type = uint64_t;
#if openPMD_HAVE_ADIOS1 || openPMD_HAVE_ADIOS2 || openPMD_HAVE_HDF5
    openPMD::Datatype dt = openPMD::determineDatatype<type>();
#endif


    int rank, size;
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );
    MPI_Comm_size( MPI_COMM_WORLD, &size );
    const unsigned scale_up = weak_scaling ? unsigned( size ) : 1u;

    // Total (in this case 3D) dataset across all MPI ranks.
    // Will be the same for all configured benchmarks.
    openPMD::Extent total{
        100 * scale_up,
        100,
        1000
    };

    // The blockslicer assigns to each rank its part of the dataset. The rank will
    // write to and read from that part. OneDimensionalBlockSlicer is a simple
    // implementation of the BlockSlicer abstract class that will divide the
    // dataset into hyperslab along one given dimension.
    // If you wish to partition your dataset in a different manner, you can
    // replace this with your own implementation of BlockSlicer.
    auto blockSlicer = std::make_shared<openPMD::OneDimensionalBlockSlicer>(0);

    // Set up the DatasetFiller. The benchmarks will later inquire the DatasetFiller
    // to get data for writing.
    std::uniform_int_distribution<type> distr(
        0,
        200000000
    );
    openPMD::RandomDatasetFiller<decltype(distr)> df{distr};

    // The Benchmark class will in principle allow a user to configure
    // runs that write and read different datatypes.
    // For this, the class is templated with a type called DatasetFillerProvider.
    // This class serves as a factory for DatasetFillers for concrete types and
    // should have a templated operator()<T>() returning a value
    // that can be dynamically casted to a std::shared_ptr<openPMD::DatasetFiller<T>>
    // The openPMD API provides only one implementation of a DatasetFillerProvider,
    // namely the SimpleDatasetFillerProvider being used in this example.
    // Its purpose is to leverage a DatasetFiller for a concrete type (df in this example)
    // to a DatasetFillerProvider whose operator()<T>() will fail during runtime if T does
    // not correspond with the underlying DatasetFiller.
    // Use this implementation if you only wish to run the benchmark for one Datatype,
    // otherwise provide your own implementation of DatasetFillerProvider.
    openPMD::SimpleDatasetFillerProvider<decltype(df)> dfp{df};

    // Create the Benchmark object. The file name (first argument) will be extended
    // with the backends' file extensions.
    openPMD::MPIBenchmark<decltype(dfp)> benchmark{
        "../benchmarks/benchmark",
        total,
        std::dynamic_pointer_cast<openPMD::BlockSlicer>(blockSlicer),
        dfp,
    };

    // Add benchmark runs to be executed. This will only store the configuration and not
    // run the benchmark yet. Each run is configured by:
    // * The compression scheme to use (first two parameters). The first parameter chooses
    //   the compression scheme, the second parameter is the compression level.
    // * The backend (by file extension).
    // * The datatype to use for this run.
    // * The number of iterations. Effectively, the benchmark will be repeated for this many
    //   times.
#if openPMD_HAVE_ADIOS1 || openPMD_HAVE_ADIOS2
    benchmark.addConfiguration("", 0, "bp", dt, 10);
#endif
#if openPMD_HAVE_HDF5
    benchmark.addConfiguration("", 0, "h5", dt, 10);
#endif

    // Execute all previously configured benchmarks. Will return a MPIBenchmarkReport object
    // with write and read times for each configured run.
    // Take notice that results will be collected into the root rank's report object, the other
    // ranks' reports will be empty. The root rank is specified by the first parameter of runBenchmark,
    // the default being 0.
    auto res =
        benchmark.runBenchmark<std::chrono::high_resolution_clock>();

    if( rank == 0 )
    {
        for( auto it = res.durations.begin();
             it != res.durations.end();
             it++ )
        {
            auto time = it->second;
            std::cout << "on rank " << std::get<res.RANK>(it->first)
                      << "\t with backend "
                      << std::get<res.BACKEND>(it->first)
                      << "\twrite time: "
                      << std::chrono::duration_cast<std::chrono::milliseconds>(
                          time.first
                      ).count() << "\tread time: "
                      << std::chrono::duration_cast<std::chrono::milliseconds>(
                          time.second
                      ).count() << std::endl;
        }
    }

    MPI_Finalize();
}
#else
int main(void)
{
    return 0;
}
#endif

Backends¶

Overview¶

This section provides an overview of features in I/O backends.

Feature	ADIOS1	ADIOS2	HDF5	JSON
Operating Systems	Linux, OSX	Linux, OSX, Windows
Serial	supported	supported	supported	supported
MPI-parallel	supported	supported	supported	no
Dataset deletion	no	no	supported	supported
Compression	upcoming	supported	upcoming	no
Streaming/Staging	not exposed	upcoming	no	no
Portable Files	limited	awaiting	yes	yes
PByte-scalable	yes	yes	no	no
Performance	A	TBD	B	C
Native File Format	`.bp` (BP3)	`.bp` (BP4)	`.h5`	`.json`

supported/yes: implemented and accessible for users of openPMD-api
upcoming: planned for upcoming releases of openPMD-api
limited: for example, limited to certain datatypes
awaiting: planned for upcoming releases of a dependency
TBD: to be determined (e.g. with upcoming benchmarks)

Selected References¶

Axel Huebl, Rene Widera, Felix Schmitt, Alexander Matthes, Norbert Podhorszki, Jong Youl Choi, Scott Klasky, and Michael Bussmann. On the Scalability of Data Reduction Techniques in Current and Upcoming HPC Systems from an Application Perspective, ISC High Performance 2017: High Performance Computing, pp. 15-29, 2017. arXiv:1706.00522, DOI:10.1007/978-3-319-67630-2_2

JSON¶

openPMD supports writing to and reading from JSON files. The JSON backend is always available.

JSON File Format¶

A JSON file uses the file ending .json. The JSON backend is chosen by creating a Series object with a filename that has this file ending.

The top-level JSON object is a group representing the openPMD root group "/". Any openPMD group is represented in JSON as a JSON object with two reserved keys:

attributes: Attributes associated with the group. This key may be null or not be present at all, thus indicating a group without attributes.

platform_byte_widths (root group only): Byte widths specific to the writing platform. Will be overwritten every time that a JSON value is stored to disk, hence this information is only available about the last platform writing the JSON value.

All datasets and subgroups contained in this group are represented as a further key of the group object. attributes and platform_byte_widths have hence the character of reserved keywords and cannot be used for group and dataset names when working with the JSON backend. Datasets and groups have the same namespace, meaning that there may not be a subgroup and a dataset with the same name contained in one group.

Any openPMD dataset is a JSON object with three keys:

attributes: Attributes associated with the dataset. May be null or not present if no attributes are associated with the dataset.

datatype: A string describing the type of the stored data.

data A nested array storing the actual data in row-major manner. The data needs to be consistent with the fields datatype and extent. Checking whether this key points to an array can be (and is internally) used to distinguish groups from datasets.

Attributes are stored as a JSON object with a key for each attribute. Every such attribute is itself a JSON object with two keys:

datatype: A string describing the type of the value.

value: The actual value of type datatype.

Restrictions¶

For creation of JSON serializations (i.e. writing), the restrictions of the JSON backend are equivalent to those of the JSON library by Niels Lohmann used by the openPMD backend.

Numerical values, integral as well as floating point, are supported up to a length of 64 bits. Since JSON does not support special floating point values (i.e. NaN, Infinity, -Infinity), those values are rendered as null.

Instructing openPMD to write values of a datatype that is too wide for the JSON backend does not result in an error:

If casting the value to the widest supported datatype of the same category (integer or floating point) is possible without data loss, the cast is performed and the value is written. As an example, on a platform with sizeof(double) == 8, writing the value static_cast<long double>(std::numeric_limits<double>::max()) will work as expected since it can be cast back to double.

Otherwise, a null value is written.

Upon reading null when expecting a floating point number, a NaN value will be returned. Take notice that a NaN value returned from the deserialization process may have originally been +/-Infinity or beyond the supported value range.

Upon reading null when expecting any other datatype, the JSON backend will propagate the exception thrown by Niels Lohmann’s library.

The (keys) names "attributes", "data" and "datatype" are reserved and must not be used for base/mesh/particles path, records and their components.

A parallel (i.e. MPI) implementation is not available.

Example¶

The example code in the usage section will produce the following JSON serialization when picking the JSON backend:

{
  "attributes": {
    "basePath": {
      "datatype": "STRING",
      "value": "/data/%T/"
    },
    "iterationEncoding": {
      "datatype": "STRING",
      "value": "groupBased"
    },
    "iterationFormat": {
      "datatype": "STRING",
      "value": "/data/%T/"
    },
    "meshesPath": {
      "datatype": "STRING",
      "value": "meshes/"
    },
    "openPMD": {
      "datatype": "STRING",
      "value": "1.1.0"
    },
    "openPMDextension": {
      "datatype": "UINT",
      "value": 0
    }
  },
  "data": {
    "1": {
      "attributes": {
        "dt": {
          "datatype": "DOUBLE",
          "value": 1
        },
        "time": {
          "datatype": "DOUBLE",
          "value": 0
        },
        "timeUnitSI": {
          "datatype": "DOUBLE",
          "value": 1
        }
      },
      "meshes": {
        "rho": {
          "attributes": {
            "axisLabels": {
              "datatype": "VEC_STRING",
              "value": [
                "x"
              ]
            },
            "dataOrder": {
              "datatype": "STRING",
              "value": "C"
            },
            "geometry": {
              "datatype": "STRING",
              "value": "cartesian"
            },
            "gridGlobalOffset": {
              "datatype": "VEC_DOUBLE",
              "value": [
                0
              ]
            },
            "gridSpacing": {
              "datatype": "VEC_DOUBLE",
              "value": [
                1
              ]
            },
            "gridUnitSI": {
              "datatype": "DOUBLE",
              "value": 1
            },
            "position": {
              "datatype": "VEC_DOUBLE",
              "value": [
                0
              ]
            },
            "timeOffset": {
              "datatype": "FLOAT",
              "value": 0
            },
            "unitDimension": {
              "datatype": "ARR_DBL_7",
              "value": [
                0,
                0,
                0,
                0,
                0,
                0,
                0
              ]
            },
            "unitSI": {
              "datatype": "DOUBLE",
              "value": 1
            }
          },
          "data": [
            [
              0,
              1,
              2
            ],
            [
              3,
              4,
              5
            ],
            [
              6,
              7,
              8
            ]
          ],
          "datatype": "DOUBLE"
        }
      }
    }
  },
  "platform_byte_widths": {
    "BOOL": 1,
    "CHAR": 1,
    "DOUBLE": 8,
    "FLOAT": 4,
    "INT": 4,
    "LONG": 8,
    "LONGLONG": 8,
    "LONG_DOUBLE": 16,
    "SHORT": 2,
    "UCHAR": 1,
    "UINT": 4,
    "ULONG": 8,
    "ULONGLONG": 8,
    "USHORT": 2
  }
}

ADIOS1¶

openPMD supports writing to and reading from ADIOS1 .bp files. For this, the installed copy of openPMD must have been built with support for the ADIOS1 backend. To build openPMD with support for ADIOS, use the CMake option -DopenPMD_USE_ADIOS1=ON. For further information, check out the installation guide, build dependencies and the build options.

Note

This backend is deprecated, please use ADIOS2 instead.

I/O Method¶

ADIOS1 has several staging methods for alternative file formats, yet natively writes to .bp files. We currently implement the MPI_AGGREGATE transport method for MPI-parallel write (POSIX for serial write) and ADIOS_READ_METHOD_BP for read.

Backend-Specific Controls¶

The following environment variables control ADIOS1 I/O behavior at runtime. Fine-tuning these is especially useful when running at large scale.

environment variable	default	description
`OPENPMD_ADIOS_NUM_AGGREGATORS`	`1`	Number of I/O aggregator nodes for ADIOS1 `MPI_AGGREGATE` transport method.
`OPENPMD_ADIOS_NUM_OST`	`0`	Number of I/O OSTs for ADIOS1 `MPI_AGGREGATE` transport method.
`OPENPMD_ADIOS_HAVE_METADATA_FILE`	`1`	Online creation of the adios journal file (`1`: yes, `0`: no).
`OPENPMD_BP_BACKEND`	`ADIOS2`	Chose preferred `.bp` file backend if `ADIOS1` and `ADIOS2` are available.

Please refer to the ADIOS1 manual, section 6.1.5 for details on I/O tuning.

In case both the ADIOS1 backend and the ADIOS2 backend are enabled, set OPENPMD_BP_BACKEND to ADIOS1 to enforce using ADIOS1. If only the ADIOS1 backend was compiled but not the ADIOS2 backend, the default of OPENPMD_BP_BACKEND is automatically switched to ADIOS1. Be advised that ADIOS1 only supports .bp files up to the internal version BP3, while ADIOS2 supports BP3, BP4 and later formats.

Best Practice at Large Scale¶

A good practice at scale is to disable the online creation of the metadata file. After writing the data, run bpmeta on the (to-be-created) filename to generate the metadata file offline (repeat per iteration for file-based encoding). This metadata file is needed for reading, while the actual heavy data resides in <metadata filename>.dir/ directories.

Further options depend heavily on filesystem type, specific file striping, network infrastructure and available RAM on the aggregator nodes. If your filesystem exposes explicit object-storage-targets (OSTs), such as Lustre, try to set the number of OSTs to the maximum number available and allowed per job (e.g. non-full), assuming the number of writing MPI ranks is larger. A good number for aggregators is usually the number of contributing nodes divided by four.

For fine-tuning at extreme scale or for exotic systems, please refer to the ADIOS1 manual and talk to your filesystem admins and the ADIOS1 authors. Be aware that extreme-scale I/O is a research topic after all.

Limitations¶

Note

You cannot initialize and use more than one openPMD::Series with ADIOS1 backend at the same time in a process, even if both Series operate on totally independent data. This is an upstream bug in ADIOS1 that we cannot control: ADIOS1 cannot be initialized more than once, probably because it shares some internal state.

Note

The way we currently implement ADIOS1 in openPMD-api is sub-ideal and we close/re-open file handles way too often. Consequently, this can lead to severe performance degradation unless fixed. Mea culpa, we did better in the past (in PIConGPU). Please consider using our ADIOS2 backend instead, on which we focus our developments these days.

Note

ADIOS1 does not support attributes that are arrays of complex types.

Selected References¶

Hasan Abbasi, Matthew Wolf, Greg Eisenhauer, Scott Klasky, Karsten Schwan, and Fang Zheng. Datastager: scalable data staging services for petascale applications, Cluster Computing, 13(3):277–290, 2010. DOI:10.1007/s10586-010-0135-6
Ciprian Docan, Manish Parashar, and Scott Klasky. DataSpaces: An interaction and coordination framework or coupled simulation workflows, In Proc. of 19th International Symposium on High Performance and Distributed Computing (HPDC’10), June 2010. DOI:10.1007/s10586-011-0162-y
Qing Liu, Jeremy Logan, Yuan Tian, Hasan Abbasi, Norbert Podhorszki, Jong Youl Choi, Scott Klasky, Roselyne Tchoua, Jay Lofstead, Ron Oldfield, Manish Parashar, Nagiza Samatova, Karsten Schwan, Arie Shoshani, Matthew Wolf, Kesheng Wu, and Weikuan Yu. Hello ADIOS: the challenges and lessons of developing leadership class I/O frameworks, Concurrency and Computation: Practice and Experience, 26(7):1453–1473, 2014. DOI:10.1002/cpe.3125
Robert McLay, Doug James, Si Liu, John Cazes, and William Barth. A user-friendly approach for tuning parallel file operations, In Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis, SC’14, pages 229–236, IEEE Press, 2014. DOI:10.1109/SC.2014.24
Axel Huebl, Rene Widera, Felix Schmitt, Alexander Matthes, Norbert Podhorszki, Jong Youl Choi, Scott Klasky, and Michael Bussmann. On the Scalability of Data Reduction Techniques in Current and Upcoming HPC Systems from an Application Perspective, ISC High Performance 2017: High Performance Computing, pp. 15-29, 2017. arXiv:1706.00522, DOI:10.1007/978-3-319-67630-2_2

ADIOS2¶

openPMD supports writing to and reading from ADIOS2 .bp files. For this, the installed copy of openPMD must have been built with support for the ADIOS2 backend. To build openPMD with support for ADIOS2, use the CMake option -DopenPMD_USE_ADIOS2=ON. For further information, check out the installation guide, build dependencies and the build options.

I/O Method¶

ADIOS2 has several engines for alternative file formats and other kinds of backends, yet natively writes to .bp files. The openPMD API uses the BP4 engine as the default file engine and the SST engine for streaming support. We currently leverage the default ADIOS2 transport parameters, i.e. POSIX on Unix systems and FStream on Windows.

Steps¶

ADIOS2 is optimized towards organizing the process of reading/writing data into IO steps. In order to activate steps, it is imperative to use the Streaming API (which can be used for either file-based or streaming-based workflows).

ADIOS2 release 2.6.0 contained a bug (fixed in ADIOS 2.7.0, see PR #2348) that disallows random-accessing steps in file-based engines. With this ADIOS2 release, files written with steps may only be read using the streaming API. In order to keep compatibility with older codes reading ADIOS2 files, step-based processing must currently be opted in to via use of the JSON parameter adios2.engine.usesteps = true when using a file-based engine such as BP3 or BP4.

Upon reading a file, the ADIOS2 backend will automatically recognize whether it has been written with or without steps, ignoring the JSON option mentioned above. Steps are mandatory for streaming-based engines and trying to switch them off will result in a runtime error.

Note

ADIOS2 will in general dump data to disk/transport only upon closing a file/engine or a step. If not using steps, users are hence strongly encouraged to use file-based iteration layout (by creating a Series with a filename pattern such as simData_%06T.bp) and enforce dumping to disk by Iteration::close()-ing an iteration after writing to it. Otherwise, out-of-memory errors are likely to occur.

Backend-Specific Controls¶

The ADIOS2 SST engine for streaming can be picked by specifying the ending .sst instead of .bp.

The following environment variables control ADIOS2 I/O behavior at runtime. Fine-tuning these is especially useful when running at large scale.

environment variable	default	description
`OPENPMD_ADIOS2_HAVE_PROFILING`	`1`	Turns on/off profiling information right after a run.
`OPENPMD_ADIOS2_HAVE_METADATA_FILE`	`1`	Online creation of the adios journal file (`1`: yes, `0`: no).
`OPENPMD_ADIOS2_NUM_SUBSTREAMS`	`0`	Number of files to be created, 0 indicates maximum number possible.
`OPENPMD_ADIOS2_ENGINE`	`File`	ADIOS2 engine
`OPENPMD2_ADIOS2_SCHEMA`	`0`	ADIOS2 schema version (see below)
`OPENPMD_ADIOS2_STATS_LEVEL`	`0`	whether to generate statistics for variables in ADIOS2. (`1`: yes, `0`: no).
`OPENPMD_BP_BACKEND`	`ADIOS2`	Chose preferred `.bp` file backend if `ADIOS1` and `ADIOS2` are available.

Please refer to the ADIOS2 documentation for details on I/O tuning.

In case the ADIOS2 backend was not compiled but only the deprecated ADIOS1 backend, the default of OPENPMD_BP_BACKEND will fall back to ADIOS1. Be advised that ADIOS1 only supports .bp files up to the internal version BP3, while ADIOS2 supports BP3, BP4 and later formats.

Notice that the ADIOS2 backend is alternatively configurable via JSON parameters.

Due to performance considerations, the ADIOS2 backend configures ADIOS2 not to compute any dataset statistics (Min/Max) by default. Statistics may be activated by setting the JSON parameter adios2.engine.parameters.StatsLevel = "1".

The ADIOS2 backend overrides the default unlimited queueing behavior of the SST engine with a more cautious limit of 2 steps that may be held in the queue at one time. The default behavior may be restored by setting the JSON parameter adios2.engine.parameters.QueueLimit = "0".

Best Practice at Large Scale¶

A good practice at scale is to disable the online creation of the metadata file. After writing the data, run bpmeta on the (to-be-created) filename to generate the metadata file offline (repeat per iteration for file-based encoding). This metadata file is needed for reading, while the actual heavy data resides in <metadata filename>.dir/ directories. Note that such a tool is not yet available for ADIOS2, but the bpmeta utility provided by ADIOS1 is capable of processing files written by ADIOS2.

Further options depend heavily on filesystem type, specific file striping, network infrastructure and available RAM on the aggregator nodes. A good number for substreams is usually the number of contributing nodes divided by four.

For fine-tuning at extreme scale or for exotic systems, please refer to the ADIOS2 manual and talk to your filesystem admins and the ADIOS2 authors. Be aware that extreme-scale I/O is a research topic after all.

Experimental new ADIOS2 schema¶

We are experimenting with a breaking change to our layout of openPMD datasets in ADIOS2. It is likely that we will in future use ADIOS attributes only for a handful of internal flags. Actual openPMD attributes will be modeled by ADIOS variables of the same name. In order to distinguish datasets from attributes, datasets will be suffixed by /__data__.

We hope that this will bring several advantages:

Unlike ADIOS attributes, ADIOS variables are mutable.
ADIOS variables are more closely related to the concept of ADIOS steps. An ADIOS variable that is not written to in one step is not seen by the reader. This will bring more manageable amounts of metadata for readers to parse through.

The new layout may be activated for experimental purposes in two ways:

Via the JSON parameter adios2.schema = 20210209.
Via the environment variable export OPENPMD2_ADIOS2_SCHEMA=20210209.

The ADIOS2 backend will automatically recognize the layout that has been used by a writer when reading a dataset.

Tip

This schema does not use ADIOS2 attributes anymore, thus bpls -a and bpls -A attribute switches do not show openPMD attributes. Their functionality can be emulated via regexes:

Print datasets and attributes: Default behavior
Print datasets only: bpls -e '.*/__data__$'
Print attributes only: bpls -e '^(.(?!/__data__$))*$'

Memory usage¶

The IO strategy in ADIOS2 is to stage all written data in a large per-process buffer. This buffer is drained to storage only at specific times:

When an engine is closed.
When a step is closed.

The usage pattern of openPMD, especially the choice of iteration encoding influences the memory use of ADIOS2. The following graphs are created from a real-world application using openPMD (PIConGPU) using KDE Heaptrack. Ignore the 30GB initialization phases.

file-based iteration encoding: A new ADIOS2 engine is opened for each iteration and closed upon Iteration::close(). Each iteration has its own buffer:

Memory usage of file-based iteration encoding

variable-based iteration encoding and group-based iteration encoding with steps: One buffer is created and reused across all iterations. It is drained to disk when closing a step. If carefully selecting the correct InitialBufferSize, this is merely one single allocation held across all iterations. If selecting the InitialBufferSize too small, reallocations will occur. As usual with std::vector (which ADIOS2 uses internally), a reallocation will occupy both the old and new memory for a short time, leading to small memory spikes. These memory spikes can easily lead to out-of-memory (OOM) situations, motivating that the InitialBufferSize should not be chosen too small. Both behaviors are depicted in the following two pictures:

Memory usage of variable-based iteration encoding

Memory usage of variable-based iteration encoding with bad ``InitialBufferSize``

group-based iteration encoding without steps: This encoding should be avoided in ADIOS2. No data will be written to disk before closing the Series, leading to a continuous buildup of memory, and most likely to an OOM situation:

Memory usage of group-based iteration without using steps

Selected References¶

Hasan Abbasi, Matthew Wolf, Greg Eisenhauer, Scott Klasky, Karsten Schwan, and Fang Zheng. Datastager: scalable data staging services for petascale applications, Cluster Computing, 13(3):277–290, 2010. DOI:10.1007/s10586-010-0135-6
Ciprian Docan, Manish Parashar, and Scott Klasky. DataSpaces: An interaction and coordination framework or coupled simulation workflows, In Proc. of 19th International Symposium on High Performance and Distributed Computing (HPDC’10), June 2010. DOI:10.1007/s10586-011-0162-y
Qing Liu, Jeremy Logan, Yuan Tian, Hasan Abbasi, Norbert Podhorszki, Jong Youl Choi, Scott Klasky, Roselyne Tchoua, Jay Lofstead, Ron Oldfield, Manish Parashar, Nagiza Samatova, Karsten Schwan, Arie Shoshani, Matthew Wolf, Kesheng Wu, and Weikuan Yu. Hello ADIOS: the challenges and lessons of developing leadership class I/O frameworks, Concurrency and Computation: Practice and Experience, 26(7):1453–1473, 2014. DOI:10.1002/cpe.3125
Robert McLay, Doug James, Si Liu, John Cazes, and William Barth. A user-friendly approach for tuning parallel file operations, In Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis, SC’14, pages 229–236, IEEE Press, 2014. DOI:10.1109/SC.2014.24
Axel Huebl, Rene Widera, Felix Schmitt, Alexander Matthes, Norbert Podhorszki, Jong Youl Choi, Scott Klasky, and Michael Bussmann. On the Scalability of Data Reduction Techniques in Current and Upcoming HPC Systems from an Application Perspective, ISC High Performance 2017: High Performance Computing, pp. 15-29, 2017. arXiv:1706.00522, DOI:10.1007/978-3-319-67630-2_2

HDF5¶

openPMD supports writing to and reading from HDF5 .h5 files. For this, the installed copy of openPMD must have been built with support for the HDF5 backend. To build openPMD with support for HDF5, use the CMake option -DopenPMD_USE_HDF5=ON. For further information, check out the installation guide, build dependencies and the build options.

I/O Method¶

HDF5 internally either writes serially, via POSIX on Unix systems, or parallel to a single logical file via MPI-I/O.

Backend-Specific Controls¶

The following environment variables control HDF5 I/O behavior at runtime.

environment variable	default	description
`OPENPMD_HDF5_INDEPENDENT`	`ON`	Sets the MPI-parallel transfer mode to collective (`OFF`) or independent (`ON`).
`OPENPMD_HDF5_ALIGNMENT`	`1`	Tuning parameter for parallel I/O, choose an alignment which is a multiple of the disk block size.
`OPENPMD_HDF5_CHUNKS`	`auto`	Defaults for `H5Pset_chunk`: `"auto"` (heuristic) or `"none"` (no chunking).
`H5_COLL_API_SANITY_CHECK`	unset	Set to `1` to perform an `MPI_Barrier` inside each meta-data operation.

OPENPMD_HDF5_INDEPENDENT: by default, we implement MPI-parallel data storeChunk (write) and loadChunk (read) calls as none-collective MPI operations. Attribute writes are always collective in parallel HDF5. Although we choose the default to be non-collective (independent) for ease of use, be advised that performance penalties may occur, although this depends heavily on the use-case. For independent parallel I/O, potentially prefer using a modern version of the MPICH implementation (especially, use ROMIO instead of OpenMPI’s ompio implementation). Please refer to the HDF5 manual, function H5Pset_dxpl_mpio for more details.

OPENPMD_HDF5_ALIGNMENT This sets the alignment in Bytes for writes via the H5Pset_alignment function. According to the HDF5 documentation: For MPI IO and other parallel systems, choose an alignment which is a multiple of the disk block size. On Lustre filesystems, according to the NERSC documentation, it is advised to set this to the Lustre stripe size. In addition, ORNL Summit GPFS users are recommended to set the alignment value to 16777216(16MB).

OPENPMD_HDF5_CHUNKS This sets defaults for data chunking via H5Pset_chunk. Chunking generally improves performance and only needs to be disabled in corner-cases, e.g. when heavily relying on independent, parallel I/O that non-collectively declares data records.

H5_COLL_API_SANITY_CHECK: this is a HDF5 control option for debugging parallel I/O logic (API calls). Debugging a parallel program with that option enabled can help to spot bugs such as collective MPI-calls that are not called by all participating MPI ranks. Do not use in production, this will slow parallel I/O operations down.

Selected References¶

GitHub issue #554
Axel Huebl, Rene Widera, Felix Schmitt, Alexander Matthes, Norbert Podhorszki, Jong Youl Choi, Scott Klasky, and Michael Bussmann. On the Scalability of Data Reduction Techniques in Current and Upcoming HPC Systems from an Application Perspective, ISC High Performance 2017: High Performance Computing, pp. 15-29, 2017. arXiv:1706.00522, DOI:10.1007/978-3-319-67630-2_2

Development¶

Contribution Guide¶

GitHub¶

The best starting point is the GitHub issue tracker.

For existing tasks, the labels good first issue and help wanted are great for contributions. In case you want to start working on one of those, just comment in it first so no work is duplicated.

New contributions in form of pull requests always need to go in the dev (development) branch.

Maintainers organize prioritites and progress in the projects tab.

Style Guide¶

For coding style, please try to follow the guides in ComputationalRadiationPhysics/contributing for new code.

Repository Structure¶

Branches¶

dev: the development branch where all features start from and are merged to
release-X.Y.Z: release candidate for version X.Y.Z with an upcoming release, receives updates for bug fixes and documentation such as change logs but usually no new features

Directory Structure¶

include/
- C++ header files
- set -I here
- prefixed with project name
- auxiliary/
  - internal auxiliary functionality
- helper/, benchmark/
  - user-facing helper functionality
src/
- C++ source files
- cli/
  - user-facing command line tools
lib/
- python/
  - modules, e.g. additional python interfaces and helpers
  - set PYTHONPATH here
examples/
- read and write examples
samples/
- example files; need to be added manually with: share/openPMD/download_samples.sh (or .ps1)
share/openPMD/
- download scripts for example files
- cmake/
  - cmake scripts
- thirdParty/
  - included third party software
test/
- unit tests which are run with ctest (make test)
.github/
- GitHub issue/pull request templates
- workflows/
  - GitHub Action scripts for continuous integration checks
- ci/
  - service-agnostic configurations for continuous integration
docs/
- documentation files

Design Overview¶

Note

This section is a stub. Please open a pull-request to improve it or open an issue with open questions.

This library consists of three conceptual parts:

The backend, concerned with elementary low-level I/O operations.
The I/O-Queue, acting as a communication mechanism and buffer between the other two parts.
The user-facing frontend, enforcing the openPMD standard, keeping a logical state of the data, and synchronizing that state with persistent data by scheduling I/O-Tasks.

Backend¶

One of the main goals of this library is to provide a high-level common interface to synchronize persistent data with a volatile representation in memory. This includes handling data in any number of supported file formats transparently. Therefore, enabling users to handle hierarchical, self-describing file formats while disregarding the actual nitty-gritty details of just those file formats, required the reduction of possible operations reduced to a common set of IOTasks:

    CREATE_FILE,
    OPEN_FILE,
    CLOSE_FILE,
    DELETE_FILE,

    CREATE_PATH,
    CLOSE_PATH,
    OPEN_PATH,
    DELETE_PATH,
    LIST_PATHS,

    CREATE_DATASET,
    EXTEND_DATASET,
    OPEN_DATASET,
    DELETE_DATASET,
    WRITE_DATASET,
    READ_DATASET,
    LIST_DATASETS,
    GET_BUFFER_VIEW,

    DELETE_ATT,
    WRITE_ATT,
    READ_ATT,
    LIST_ATTS,

    ADVANCE,
    AVAILABLE_CHUNKS //!< Query chunks that can be loaded in a dataset

Every task is designed to be a fully self-contained description of one such atomic operation. By describing a required minimal step of work (without any side-effect), these operations are the foundation of the unified handling mechanism across suitable file formats. The actual low-level exchange of data is implemented in IOHandlers, one per file format (possibly two if handlingi MPI-parallel work is possible and requires different behaviour). The only task of these IOHandlers is to execute one atomic IOTask at a time. Ideally, additional logic is contained to improve performance by keeping track of open file handles, deferring and coalescing parts of work, avoiding redundant operations. It should be noted that while this is desirable, sequential consistency must be guaranteed (see I/O-Queue.)

Note this paragraph is a stub: AbstractParameter and subclasses as typesafe descriptions of task parameters, Writable as unique identification in task, corresponding to node in frontend hierarchy (tree-like structure), subclass of AbstractIOHandler to ensure simple extensibilty, and only two public interface methods (enqueue() and flush()) to hide separate behaviour & state AbstractFilePosition as a format-dependent location inside persistent data (e.g. node-id / path string) should be entirely agnostic to openPMD and just treat transferred data as raw bytes without knowledge

I/O-Queue¶

To keep coupling between openPMD logic and actual low-level I/O to a minimum, a sequence of atomic I/O-Tasks is used to transfer data between logical and physical representation. Individual tasks are scheduled by frontend application logic and stored in a data structure that allows for FIFO order processing (in future releases, this order might be relaxed). Tasks are not executed during their creation, but are instead buffered in this queue. Disk accesses can be coalesced and high access latencies can be amortized by performing multiple tasks bunched together. At appropriate points in time, the used backend processes all pending tasks (strict, single-threaded, synchronous FIFO is currently used in all backends, but is not mandatory as long as consistency with that order can be guaranteed).

A typical sequence of tasks that are scheduled during the read of an existing file could look something like this:

1.  OPEN_FILE
2.  READ_ATT     // 'openPMD'
3.  READ_ATT     // 'openPMDextension'
4.  READ_ATT     // 'basePath'
### PROCESS ELEMENTS ###
5.  LIST_ATTS    // in '/'
### PROCESS ELEMENTS ###
5.1 READ_ATT     // 'meshesPath', if in 5.
5.2 READ_ATT     // 'particlesPath', if in 5.
### PROCESS ELEMENTS ###
6.  OPEN_PATH    // 'basePath'
7.  LIST_ATTS    // in 'basePath'
### PROCESS ELEMENTS ###
7.X READ_ATT     // every 'att' in 7.
8.  LIST_PATHS   // in 'basePath'
### PROCESS ELEMENTS ###
9.X OPEN_PATH    // every 'path' in 8.
...

Note that (especially for reading), pending tasks might have to be processed between any two steps to guarantee data consistency. That is because action might have to be taken conditionally on read or written values, openPMD conformity checked to fail fast, or a processing of the tasks be requested by the user explicitly.

As such, FIFO-equivalence with the scheduling order must be satisfied. A task that is not located at the head of the queue (i.e. does not have the earliest schedule time of all pending tasks) is not guaranteed to succeed in isolation. Currently, this can only guaranteed by sequentially performing all tasks scheduled prior to it in chronological order. To give two examples where this matters:

Reading value chunks from a dataset only works after the dataset has been opened. Due to limitations in some of the backends and the atomic nature of the I/O-tasks in this API (i.e. operations without side effects), datatype and extent of a dataset are only obtained by opening the dataset. For some backends this information is required for chunk reading and thus must be known prior to performing the read.
Consecutive chunk writing and reading (to the same dataset) mirrors classical RAW data dependence.
The two chunks might overlap, in which case the read has to reflect the value changes introduced by the write.

Atomic operations contained in this queue are …

Frontend¶

While the other two components are primarily concerned with actual I/O, this one is the glue and constraint logic that lets a user build the in-memory view of the hierarchical file structure. Public interfaces should be limited to this part (exceptions may arise, e.g. format-dependent dataset parameters). Where the other parts contain virtually zero knowledge about openPMD, this one contains all of it and none of the low-level I/O.

Writable (mixin) base class of every front-end class, used to tree structure used in backend

Attributable (mixin) class that allows attaching meta-data to tree nodes (openPMD attributes)

Attribute a variadic datastore for attributes supported across backends

Container serves two purposes

python-esque access inside hierarchy groups (foo[“bar”][“baz”])
only way for user to construct objects (private constructors), forces them into the correct hierarchy (no dangling objects)

all meta-data access stores in the Attributable part of an object and follows the syntax

Object& setFoo(Foo foo);
Foo foo() const;

(future work: use CRTP)

openPMD frontend classes are designed as handles that user code may copy an arbitrary amount of times, all copies sharing common resources. In an undergoing effort, we are moving to a PIMPL-based pattern to make this design more explicit. This serves to separate data from implementation, demonstrated by the class hierarchy of Series:

SeriesData contains all actual data members of Series, representing the resources shared between instances. Its copy/move constructors/assignment operators are deleted, thus pinning it at one memory location. It has no implementation.
SeriesInterface defines the interface of Series. Its only data member is a pointer to its associated instance of SeriesData. Its purpose is to serve as a parent class for any class that implements the Series interface. The implementing class should deal with resource management for the instance of SeriesData, SeriesInterface itself performs no resource management for the data and will assume that the pointer points at a valid instance. (This PIMPL-based design allows us to avoid a similar template-heavy design based on CRTP.)
SeriesInternal is the class created by inheriting directly from SeriesData and SeriesInterface. It is intended for internal usage, pinned at a memory location just like SeriesData, but carrying an implementation. Its constructor and destructor define the setup and tear-down of shared resources for Series.
Series is a wrapper around a shared pointer to SeriesInternal and also derives from SeriesInterface. It implements handle semantics, serving as interface to users.

Other classes within our object model of the openPMD hierarchy are planned to follow in this design.

The class hierarchy of Attributable follows a similar design, with some differences due to its nature as a mixin class that is not instantiated directly:

AttributableData serves the same purpose as SeriesData.
AttributableInterface serves the same purpose as SeriesData.
Equivalents to SeriesInternal and Series do not exist since a combination of AttributableData and AttributableInterface is added as a mixin to other classes. As a common base class exposed to user code, AttributableInterface is aliased as Attributable.
For classes not yet following the PIMPL-based design, LegacyAttributable wraps around a shared pointer to AttributableData and derives from AttributableInterface. The Attributable mixin can be added to those classes by deriving from LegacyAttributable.
The Attributable mixin is added to Series by deriving SeriesData from AttributableData and SeriesInterface from AttributableInterface.

The Series class is the entry point to the openPMD-api. An instance of this class always represents an entire series (of iterations), regardless of how this series is modeled in storage or transport (e.g. as multiple files, as a stream, as variables containing multiple snapshots of a dataset or as one file containing all iterations at once).

How to Write a Backend¶

Adding support for additional types of file storage or data transportation is possible by creating a backend. Backend design has been kept independent of the openPMD-specific logic that maintains all constraints within a file. This should allow easy introduction of new file formats with only little knowledge about the rest of the system.

File Formats¶

To get started, you should create a new file format in include/openPMD/IO/Format.hpp representing the new backend. Note that this enumeration value will never be seen by users of openPMD-api, but should be kept short and concise to improve readability.

enum class Format
{
     JSON
};

In order to use the file format through the API, you need to provide unique and characteristic filename extensions that are associated with it. This happens in src/Series.cpp:

Format
determineFormat(std::string const& filename)
{
    if( auxiliary::ends_with(filename, ".json") )
        return Format::JSON;
}

std::string
cleanFilename(std::string const& filename, Format f)
{
    switch( f )
    {
        case Format::JSON:
            return auxiliary::replace_last(filename, ".json", "");
    }
}

std::function< bool(std::string const&) >
matcher(std::string const& name, Format f)
{
    switch( f )
    {
        case Format::JSON:
        {
            std::regex pattern(auxiliary::replace_last(name + ".json$", "%T", "[[:digit:]]+"));
            return [pattern](std::string const& filename) -> bool { return std::regex_search(filename, pattern); };
        }
    }
}

Unless your file format imposes additional restrictions to the openPMD constraints, this is all you have to do in the frontend section of the API.

IO Handler¶

Now that the user can specify that the new backend is to be used, a concrete mechanism for handling IO interactions is required. We call this an IOHandler. It is not concerned with any logic or constraints enforced by openPMD, but merely offers a small set of elementary IO operations.

On the very basic level, you will need to derive a class from AbstractIOHandler:

/* file: include/openPMD/IO/JSON/JSONIOHandler.hpp */
#include "openPMD/IO/AbstractIOHandler.hpp"

namespace openPMD
{
class JSONIOHandler : public AbstractIOHandler
{
public:
    JSONIOHandler(std::string const& path, Access);
    virtual ~JSONIOHandler();

    std::future< void > flush() override;
}
} // openPMD

/* file: src/IO/JSON/JSONIOHandler.cpp */
#include "openPMD/IO/JSON/JSONIOHandler.hpp"

namespace openPMD
{
JSONIOHandler::JSONIOHandler(std::string const& path, Access at)
        : AbstractIOHandler(path, at)
{ }

JSONIOHandler::~JSONIOHandler()
{ }

std::future< void >
JSONIOHandler::flush()
{ return std::future< void >(); }
} // openPMD

Familiarizing your backend with the rest of the API happens in just one place in src/IO/AbstractIOHandlerHelper.cpp:

#if openPMD_HAVE_MPI
std::shared_ptr< AbstractIOHandler >
createIOHandler(
    std::string const& path,
    Access at,
    Format f,
    MPI_Comm comm
)
{
    switch( f )
    {
        case Format::JSON:
            std::cerr << "No MPI-aware JSON backend available. "
                         "Falling back to the serial backend! "
                         "Possible failure and degraded performance!" << std::endl;
            return std::make_shared< JSONIOHandler >(path, at);
    }
}
#endif

std::shared_ptr< AbstractIOHandler >
createIOHandler(
    std::string const& path,
    Access at,
    Format f
)
{
    switch( f )
    {
        case Format::JSON:
            return std::make_shared< JSONIOHandler >(path, at);
    }
}

In this state, the backend will do no IO operations and just act as a dummy that ignores all queries.

IO Task Queue¶

Operations between the logical representation in this API and physical storage are funneled through a queue m_work that is contained in the newly created IOHandler. Contained in this queue are IOTask s that have to be processed in FIFO order (unless you can prove sequential execution guarantees for out-of-order execution) when AbstractIOHandler::flush() is called. A recommended skeleton is provided in AbstractIOHandlerImpl. Note that emptying the queue this way is not required and might not fit your IO scheme.

Using the provided skeleton involves

deriving an IOHandlerImpl for your IOHandler and
delegating all flush calls to the IOHandlerImpl:

/* file: include/openPMD/IO/JSON/JSONIOHandlerImpl.hpp */
#include "openPMD/IO/AbstractIOHandlerImpl.hpp"

namespace openPMD
{
class JSONIOHandlerImpl : public AbstractIOHandlerImpl
{
public:
    JSONIOHandlerImpl(AbstractIOHandler*);
    virtual ~JSONIOHandlerImpl();

    void createFile(Writable*, Parameter< Operation::CREATE_FILE > const&) override;
    void createPath(Writable*, Parameter< Operation::CREATE_PATH > const&) override;
    void createDataset(Writable*, Parameter< Operation::CREATE_DATASET > const&) override;
    void extendDataset(Writable*, Parameter< Operation::EXTEND_DATASET > const&) override;
    void openFile(Writable*, Parameter< Operation::OPEN_FILE > const&) override;
    void openPath(Writable*, Parameter< Operation::OPEN_PATH > const&) override;
    void openDataset(Writable*, Parameter< Operation::OPEN_DATASET > &) override;
    void deleteFile(Writable*, Parameter< Operation::DELETE_FILE > const&) override;
    void deletePath(Writable*, Parameter< Operation::DELETE_PATH > const&) override;
    void deleteDataset(Writable*, Parameter< Operation::DELETE_DATASET > const&) override;
    void deleteAttribute(Writable*, Parameter< Operation::DELETE_ATT > const&) override;
    void writeDataset(Writable*, Parameter< Operation::WRITE_DATASET > const&) override;
    void writeAttribute(Writable*, Parameter< Operation::WRITE_ATT > const&) override;
    void readDataset(Writable*, Parameter< Operation::READ_DATASET > &) override;
    void readAttribute(Writable*, Parameter< Operation::READ_ATT > &) override;
    void listPaths(Writable*, Parameter< Operation::LIST_PATHS > &) override;
    void listDatasets(Writable*, Parameter< Operation::LIST_DATASETS > &) override;
    void listAttributes(Writable*, Parameter< Operation::LIST_ATTS > &) override;
}
} // openPMD

/* file: include/openPMD/IO/JSON/JSONIOHandler.hpp */
#include "openPMD/IO/AbstractIOHandler.hpp"
#include "openPMD/IO/JSON/JSONIOHandlerImpl.hpp"

namespace openPMD
{
class JSONIOHandler : public AbstractIOHandler
{
public:
    /* ... */
private:
    JSONIOHandlerImpl m_impl;
}
} // openPMD

/* file: src/IO/JSON/JSONIOHandler.cpp */
#include "openPMD/IO/JSON/JSONIOHandler.hpp"

namespace openPMD
{
/*...*/
std::future< void >
JSONIOHandler::flush()
{
    return m_impl->flush();
}
} // openPMD

Each IOTask contains a pointer to a Writable that corresponds to one object in the openPMD hierarchy. This object may be a group or a dataset. When processing certain types of IOTasks in the queue, you will have to assign unique FilePositions to these objects to identify the logical object in your physical storage. For this, you need to derive a concrete FilePosition for your backend from AbstractFilePosition. There is no requirement on how to identify your objects, but ids from your IO library and positional strings are good candidates.

/* file: include/openPMD/IO/JSON/JSONFilePosition.hpp */
#include "openPMD/IO/AbstractFilePosition.hpp"

namespace openPMD
{
struct JSONFilePosition : public AbstractFilePosition
{
    JSONFilePosition(uint64_t id)
        : id{id}
    { }

    uint64_t id;
};
} // openPMD

From this point, all that is left to do is implement the elementary IO operations provided in the IOHandlerImpl. The Parameter structs contain both input parameters (from storage to API) and output parameters (from API to storage). The easy way to distinguish between the two parameter sets is their C++ type: Input parameters are std::shared_ptr s that allow you to pass the requested data to their destination. Output parameters are all objects that are not std::shared_ptr s. The contract of each function call is outlined in include/openPMD/IO/AbstractIOHandlerImpl.

/* file: src/IO/JSON/JSONIOHandlerImpl.cpp */
#include "openPMD/IO/JSONIOHandlerImpl.hpp"

namespace openPMD
{
void
JSONIOHandlerImpl::createFile(Writable* writable,
                              Parameter< Operation::CREATE_FILE > const& parameters)
{
    if( !writable->written )
    {
        path dir(m_handler->directory);
        if( !exists(dir) )
            create_directories(dir);

        std::string name = m_handler->directory + parameters.name;
        if( !auxiliary::ends_with(name, ".json") )
            name += ".json";

        uint64_t id = /*...*/
        VERIFY(id >= 0, "Internal error: Failed to create JSON file");

        writable->written = true;
        writable->abstractFilePosition = std::make_shared< JSONFilePosition >(id);
    }
}
/*...*/
} // openPMD

Note that you might have to keep track of open file handles if they have to be closed explicitly during destruction of the IOHandlerImpl (prominent in C-style frameworks).

Build Dependencies¶

openPMD-api depends on a series of third-party projects. These are currently:

Required¶

CMake 3.15.0+
C++14 capable compiler, e.g. g++ 5.0+, clang 5.0+, VS 2017+

Shipped internally¶

The following libraries are shipped internally in share/openPMD/thirdParty/ for convenience:

MPark.Variant 1.4.0+ (BSL-1.0)
Catch2 2.13.4+ (BSL-1.0)
pybind11 2.6.2+ (new BSD)
NLohmann-JSON 3.9.1+ (MIT)

Optional: I/O backends¶

JSON
HDF5 1.8.13+
ADIOS1 1.13.1+
ADIOS2 2.7.0+

while those can be build either with or without:

MPI 2.1+, e.g. OpenMPI 1.6.5+ or MPICH2

Optional: language bindings¶

Python:
- Python 3.6 - 3.9
- pybind11 2.6.2+
- numpy 1.15+
- mpi4py 2.1+ (optional, for MPI)
- pandas 1.0+ (optional, for dataframes)
- dask 2021+ (optional, for dask dataframes)

Quick Install with Spack¶

Quickly install all dependencies with a Spack anonymous environment. Go in the base directory and type:

spack env activate -d .
spack install

Build Options¶

Variants¶

The following options can be added to the cmake call to control features. CMake controls options with prefixed -D, e.g. -DopenPMD_USE_MPI=OFF:

CMake Option	Values	Description
`openPMD_USE_MPI`	AUTO/ON/OFF	Parallel, Multi-Node I/O for clusters
`openPMD_USE_HDF5`	AUTO/ON/OFF	HDF5 backend (`.h5` files)
`openPMD_USE_ADIOS1`	AUTO/ON/OFF	ADIOS1 backend (`.bp` files up to version BP3)
`openPMD_USE_ADIOS2`	AUTO/ON/OFF	ADIOS2 backend (`.bp` files in BP3, BP4 or higher)
`openPMD_USE_PYTHON`	AUTO/ON/OFF	Enable Python bindings
`openPMD_USE_INVASIVE_TESTS`	ON/OFF	Enable unit tests that modify source code ¹
`openPMD_USE_VERIFY`	ON/OFF	Enable internal VERIFY (assert) macro independent of build type ²
`openPMD_INSTALL`	ON/OFF	Add installation targets
`Python_EXECUTABLE`	(newest found)	Path to Python executable

¹ e.g. changes C++ visibility keywords, breaks MSVC

² this includes most pre-/post-condition checks, disabling without specific cause is highly discouraged

Shared or Static¶

By default, we will build as a shared library and install also its headers. You can only build a static (libopenPMD.a or openPMD.lib) or a shared library (libopenPMD.so or openPMD.dylib or openPMD.dll) at a time.

The following options switch between static and shared builds and control if dependencies are linked dynamically or statically:

CMake Option	Values	Description
`openPMD_BUILD_SHARED_LIBS`	ON/OFF	Build the C++ API as shared library
`HDF5_USE_STATIC_LIBRARIES`	ON/OFF	Require static HDF5 library
`ADIOS_USE_STATIC_LIBS`	ON/OFF	Require static ADIOS1 library

Note that python modules (openpmd_api.cpython.[...].so or openpmd_api.pyd) are always dynamic libraries. Therefore, if you want to build the python module and prefer static dependencies, make sure to provide all of dependencies build with position independent code (-fPIC). The same requirement is true if you want to build a shared C++ API library with static dependencies.

Debug¶

By default, the Release version is built. In order to build with debug symbols, pass -DCMAKE_BUILD_TYPE=Debug to your cmake command.

Shipped Dependencies¶

Additionally, the following libraries are shipped internally for convenience. These might get installed in your CMAKE_INSTALL_PREFIX if the option is ON.

The following options allow to switch to external installs of dependencies:

CMake Option	Values	Installs	Library	Version
`openPMD_USE_INTERNAL_VARIANT`	ON/OFF	Yes	MPark.Variant	1.4.0+
`openPMD_USE_INTERNAL_CATCH`	ON/OFF	No	Catch2	2.13.4+
`openPMD_USE_INTERNAL_PYBIND11`	ON/OFF	No	pybind11	2.6.2+
`openPMD_USE_INTERNAL_JSON`	ON/OFF	No	NLohmann-JSON	3.9.1+

Tests, Examples and Command Line Tools¶

By default, tests, examples and command line tools are built. In order to skip building those, pass OFF to these cmake options:

CMake Option	Values	Description
`openPMD_BUILD_TESTING`	ON/OFF	Build tests
`openPMD_BUILD_EXAMPLES`	ON/OFF	Build examples
`openPMD_BUILD_CLI_TOOLS`	ON/OFF	Build command-line tools

Sphinx¶

In the following section we explain how to contribute to this documentation.

If you are reading the HTML version on http://openPMD-api.readthedocs.io and want to improve or correct existing pages, check the “Edit on GitHub” link on the right upper corner of each document.

Alternatively, go to docs/source in our source code and follow the directory structure of reStructuredText (.rst) files there. For intrusive changes, like structural changes to chapters, please open an issue to discuss them beforehand.

Build Locally¶

This document is build based on free open-source software, namely Sphinx, Doxygen (C++ APIs as XML) and Breathe (to include doxygen XML in Sphinx). A web-version is hosted on ReadTheDocs.

The following requirements need to be installed (once) to build our documentation successfully:

cd docs/

# doxygen is not shipped via pip, install it externally,
# from the homepage, your package manager, conda, etc.
# example:
sudo apt-get install doxygen graphviz

# python tools & style theme
python -m pip install -r requirements.txt # --user

With all documentation-related software successfully installed, just run the following commands to build your docs locally. Please check your documentation build is successful and renders as you expected before opening a pull request!

# skip this if you are still in docs/
cd docs/

# render the `.rst` files and replace their macros within
#   enjoy the breathe errors on things it does not understand from doxygen :)
make html

# open it, e.g. with firefox :)
firefox build/html/index.html

# now again for the pdf :)
make latexpdf

# open it, e.g. with okular
build/latex/openPMD-api.pdf

Useful Links¶

A primer on writing restFUL files for sphinx

Why You Shouldn’t Use “Markdown” for Documentation

Markdown Limitations in Sphinx

Maintenance¶

Release Channels¶

Spack¶

Our recommended HPC release channel when in need for MPI. Also very useful for Linux and OSX desktop releases. Supports all variants of openPMD-api via flexible user-level controls. The same install workflow used to bundle this release also comes in handy to test a development version quickly with power-users.

Example workflow for a new release:

https://github.com/spack/spack/pull/14018

Please ping @ax3l in your pull-request.

Conda-Forge¶

Our primary release channel for desktops via a fully automated binary distribution. Provides the C++ and Python API for users. Supports Windows, OSX, and Linux. Packages are built with and without MPI, the latter is the default variant.

Example workflow for a new release:

https://github.com/conda-forge/openpmd-api-feedstock/pull/41

Brew¶

We maintain a homebrew tap for openPMD. Provides the C++ and Python API for users. Supports OSX and Linux. Its source-only Formula for the latest release includes (Open)MPI support and lacks the ADIOS1 backend.

Example workflow for a new release:

https://github.com/openPMD/homebrew-openPMD/commit/839c458f1e8fa2a40ad0b4fd7d0d239d1062f867

PyPI¶

Our PyPI release provides our Python bindings in a self-contained way, without providing access to the C++ API. On PyPI, we upload a source package with all build-variants to default (AUTO), but MPI (OFF) unless activated. Furthermore, we build portable, serial (non-MPI) binary wheels for Linux (manylinux2010), macOS (10.9+) and Windows.

The deployment of our binary wheels is automated via cibuildwheel. Update the version number with a new git tag in the wheels branch to trigger an automated deployment to pypi.org/project/openPMD-api . A push (merge) to this branch will build and upload all wheels together with the source distribution through twine.

The same pip install workflow used to bundle this release also comes in handy to test a development version quickly with power-users.

Example workflow for a new release:

https://github.com/openPMD/openPMD-api/pull/774

ReadTheDocs¶

Activate the new version in Projects - openPMD-api - Versions, which triggers its build.

And after the new version was built, and if this version was not a backport to an older release series, set the new default version in Admin - Advanced Settings.

Doxygen¶

In order to update the latest Doxygen C++ API docs, located under http://www.openPMD.org/openPMD-api/, do:

# assuming a clean source tree
git checkout gh-pages

# stash anything that the regular branches have in `.gitignore`
git stash --include-untracked

# optional first argument is branch/tag on mainline repo, default: dev
./update.sh
git add .
git commit
git push

# go back
git checkout -
git stash pop

Note that we publish per-release versions of the Doxygen HTML pages automatically on ReadTheDocs.