Data query attributes=all/msd underspecified

[hoehrmann]

https://github.com/sdmx-twg/sdmx-rest/blob/99e92fd7a7f2c1a0c5dcfb2776f2fdae3cf2458e/doc/data.md specifies a parameter attributes that can take various values. I cannot make sense of the all (and implictly msd) value for it. I may be way off here, but for me problems include:

• As far as I know, DSD and MSD do not share a namespace. You can have a COMMENT attribute in each of them, and attributes=all would have an implementation report both of them, but since they have the same name, and some formats like SDMX-CSV support identification by name only, the result of such a query may have no valid representation.
• I have been unable to find a requirement in the SDMX-IM that implies a DSD is linked to at most 1 MSD. So it seems the attributes in a response to such a query might even come from many MSDs.
• Values for the MSD attributes do not come from the MSD but from metadata reports. As far as I can tell, there can be many reports that apply to a given DSD, and there is no indication how to choose from the many options for a given data query.
• Existing implementations apparently require proprietary extra steps for them to accept metadata uploads, https://sis-cc.gitlab.io/dotstatsuite-documentation/using-dlm/manage-data/upload-data/upload-data-and-metadata/#prerequisites for instance requires a METADATA annotation on the DSD, which solves part of the problem (there then is only a single MSD linked to a DSD), but independent implementers have no way of knowing this from the SDMX specifications.

Substantial clarification on how to implement this feature needs to be added, to allow for independent and interoperable implementations.

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

Edit: I have since found that a data structure can reference zero or one metadata structure. Then I am guessing the METADATA annotation method might be a historical workaround for the lack of this feature.