Allow user to store non-openPMD information

[RemiLehe]

Today I had a request from a prospective user of openPMD: can the user store other datasets in the HDF5-openPMD files, which are not meant to be read by the openPMD parser / viewer ? (for instance because they don't fall either in the category mesh or or in the category particle)

From my point of view, the best way to do this is to store this data outside of the basePath (in this case the parser will not try to read it).

@ax3l : Does that make sense ? If yes, I think it would be good to add it as a side-remark in the standard.

[ax3l]

Absolutely, they can store it everywhere as long as it does not collide with reserved attributes. The standard is not exclusive and only defines a minimal required markup :) That is also the way how new extensions (might) develop: from best practices of existing additional attributes & records. Regarding other data sets (records): if they are stored in the base path + particle/mesh path they need to fulfill the requirements of those locations. Else just add them to an other non-reserved location inside the base path and it will be fine since we don't search outside for standardized formatting.

[ax3l]

Does that make sense ? If yes, I think it would be good to add it as a side-remark in the standard.

Make absolutely sense and good point and we should make this more clear in the standard.
I marked it as a revision change (patch level) which means it can be added in e.g., a 1.0.1 release (as you already marked!).

[RemiLehe]

Ok, great! I'll do a corresponding PR in the next few days.

[ax3l]

An other and additional/orthogonal approach to allow non-openPMD information inside basePath too: if one wants to avoid parsing of additional records, in detail

• directories
• data sets

(since additional attributes do not harm the parsers), we could also provide a prefix that is ignored by the parsers. Lets say their names must start with "+".
But maybe that is messy.

Would allow to experiment with new additional information, e.g., irregular mesh geometries, inside basePath.

Currently, only attributes can be freely added at any place (well, it's recommended to name them comment). Groups and data sets are restricted inside those paths.

[ax3l]

implemented in 1.0.1