Unclear representation of metadata in data messages

[hoehrmann]

I take it from https://sdmx.org/wp-content/uploads/SDMX_3-0-0_Major_Changes_FINAL-1_0.pdf section 3.4 that »Reference metadata associated with datasets, data series or observations are now reported with the data.« at least in SDMX-ML. None of the current SDMX-JSON specifications actually explain how to encode e.g. the example in the aforementioned document in SDMX-JSON.

Going out of my way of guessing, it seems it would be possible to put MSD attributes alongside DSD attributes in $.data.structures[*] with a number of problems:

• If a DSD and a MSD both have an attribute like COMMENT there might be two objects with the same id under $.data.structures[*] which seems surprising and possibly invalid.
• There is no indication how to disambiguate them (possibly the MSD attribute might link to the MSD, but that seems a rather fragile method)
• MSD attributes do not have attribute relationships on their own, the relationship property description in the data message specification even explicitly refers to only »data structure definitions«, not MSDs. The property is required, however.
• If one uses the relationship property to attach a metadata attribute to individual measures, it seems to me that the SDMX-JSON data message does not have an equivalent representation in SDMX-ML (and there might be other cases like that)

(In doubt, please handle this as a public review comment on SDMX 3.1 once the comment period begins.)