Roadmap: Wishlist of new features

[franzpoeschel]

A wishlist of features after the upcoming releases:

Near-Term

• Implement openPMD 2.0
  • Custom hierarchies
    see description openPMD/openPMD-standard#115
• New ADIOS2 schema with support for modifiable attributes
  #1310
• non-MPI parallel reads in DASK with ADIOS ornladios/ADIOS2#3651
• New ADIOS2 JoinedArray support for particles #1374 #1382
  ornladios/ADIOS2#3466
• Deprecation and later removal of RecordComponent::SCALAR
  #1154
• Full support for steps in ADIOS2, full support for variable-based iteration encoding. This implies:
  • Default enabled steps
  • Support random-accessing steps
  • Skip duplicate iterations at read time
  • Support reopening closed iterations #1606
• Julia bindings #1025
• better object model for default attributes #1439
• distributed initialization of a read-access dataset in ADIOS2 within non-MPI contexts (e.g. DASK, ref ornladios/ADIOS2#3651)
• openpmd-pipe: modularize into visitor pattern
• openpmd-pipe: reuse in new CLI tools such as openpmd-coarsen (fields, particles #1390)

Mid-Term

• Support for joined arrays in backends other than ADIOS2
• async for iteration in s.read_iterations() for Python
• More flexible reads in C++17 #1372 and Python
• MPI-wise logging of IO actions
• Performance optimization: Long-running simulations (many iterations, reading and writing)
• Specify default attributes not upon construction, but upon closing, clean up the logic for specifying defaults, constructors and destructors of our object model
  • Context: crashing simulations
  • If in read, a standard attribute is missing, then warn and add a reasonable default (e.g., a 3-value axisLabel for a 3D-mesh)
• Support for PIConGPU-style dataset-specific JSON/TOML configuration
  https://picongpu.readthedocs.io/en/0.6.0/usage/plugins/openPMD.html#cfg-file, also for iteration-specific configuration, e.g. for InitialBufferSize per file
• Maybe Flag for writing attributes only from rank 0
• Python docstrings #1328
• Maybe SoA <-> AoS flexibility (affects the standard)
  • Probably requires struct-type fields
• Project structure: Separate MPI headers from serial headers
  With this change: Provide openPMD-api via Linux package managers
• Compression and plugins in HDF5
• HDF5 hardlinks + maybe as a fallback softlinks openPMD/openPMD-standard#283

Long-Term / Ideas

• Synchronous mode: Avoid UB for store and load chunks

  • In both C++ and Python, we would like to avoid that the user can interact with allocated but non-flushed (UB) data.
  • For this, we could rename storeChunk / loadChunk to ...Async(), which returns a std::future (C++) or asyncio.Future (Python).
    • We need to keep track which of these in-flight objects we created and will set them to valid on series.flush().
    • If a future is awaited before flush was called, we throw a runtime exception, which allows to recover in interactive use. Futures also allow us to check if the futures are valid w/o having to catch exceptions.
  • The existing APIs would be sync.

• Maybe Use ADIOS2 group feature in reading (-> faster parsing)

• Maybe Chunk distribution algorithms

• Maybe Async I/O (especially Python)