Regarding the fill value specifically, the standard metadata for a zarr array includes a fill_value key. There are also rules about how to encode fill values to deal with values that do not have a natural representation in JSON. This includes fill values for arrays with a structured dtype. If possible, I would suggest to use this feature of standard array metadata, rather than adding a separate _FillValue attribute. If not, please do let us know what's missing, that would be an important piece of information to carry forward when considering spec changes.

Regarding attributes in general, we haven't tried to standardise any method to encode values that do not have a natural JSON representation. Currently it is left to the application developer to decide their own method for encoding and decoding values as JSON, e.g., I believe xarray has some logic for encoding values in zarr attributes. There has also been some discussion of this at #354 and #156.

Ultimately it would be good to standardise some conventions (or at least define some best practices) for representing various common value types in JSON, such as typed arrays. I'm more than happy for the community to lead on that.

NCZarr - Netcdf Support for Zarr #41

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