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Datalink usage?

• Question arises from duplicate granules providing alternative formats in EPN-TAP services. EPN-TAP v2 requires one granule / format in use (e.g. OMEGA_cubes service with both IDL and netcdf formats). Associated granules share the same obs_id (and other parameters) and have different access_format, so that different formats can be sorted out by a TAP query. This is nonetheless considered an issue for Aladin, and datalink is suggested to solve this.
• However, grabbing a given entry in a datalink table associated to a granule is not easy in general. An alternative format may be provided through, e.g.:
  semantics="#auxiliary",
  contentType="application/x-votable+xml"
  The point is: how do a portal or tool will grab files from this table for 20 selected granules?
• First, there does not seem to be an automated procedure to retrieve a bunch of files from a datalink table in TOCAT or Aladin at the moment (TBC).
• The current semantic vocabulary is too limited to identify an alternative format. This may improve in the short term with the addition of "alternate", "sibling" or whatever new name (but remember than other types of files can be listed in this datalink table, such as raw data files…)
• However, access would still require to pick-up a given mime-type from this table. 

Altogether, the benefit of datalink to access alternative file formats is unclear: this still requires a service-specific tweak and a multiparameter query; plus a (new?) mechanism in tools.