openPMD/openPMD-api

Parameter Read/Write: Data Policy

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#470 opened on Feb 16, 2019

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backendhelp wantedrefactoring

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Description

It would probably be useful if we added a trait / policy or runtime converter to the Parameter< Operation::READ_DATASET > (and write) data members of shared pointers in order to share more complex data objects.

For example returning to python a py::array, that the user might not handle, might result in it getting garbage collected before our flush() can be called, which in turn causes in invalid memory access in the backend. We could keep that py::array alive until flush() as in C++ by returning a shared_ptr< py::array >.

Overloads for RecordComponent::loadChunk<T> are already okay and well designed, but we cannot share yet an object that needs a more indirect data access pattern than a shared_ptr of raw memory, since we access it right away: https://github.com/openPMD/openPMD-api/blob/0.7.1-alpha/src/IO/HDF5/HDF5IOHandler.cpp#L1017

Contributor guide

Research direction
Investigate how Parameter<Operation::READ DATASET> is currently defined and used in the HDF5 backend, especially the shared ptr handling in IOTask.hpp and HDF5IOHandler.cpp. Understand why a Python py::array needs to be kept alive until flush. Explore how a trait or runtime converter could be added to allow sharing complex data objects, and review existing loadChunk overloads for patterns.
Tech stack
cpppython
Domain
apibackend
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitC++Python
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45