In the "VO first" redesign of the LSST data service APIs, we adopted a plan to prioritize the use of ObsTAP over SIAv2 for image metadata service, although with the intent to provide both in the end.

An "ObsTAP service", briefly, is simply the provision of a table (or view) in a TAP service, containing image metadata in ObsCore format (http://www.ivoa.net/documents/ObsCore/20170509/index.html), and with the explicitly prescribed table name ivoa.ObsCore.

That is, once you have a database, and a TAP service, you can have an ObsTAP service simply by serving the right data from that database.

It was, in particular, intended that we use ObsTAP internally, for several reasons, including:

• Once you have an ObsTAP service, there is a conceptually trivial way to implement SIAv2 as a layer on top of it, and there is existing code to do so (e.g., https://github.com/opencadc/dal/tree/master/cadc-sia).
• ObsTAP is more flexible and is permitted to return data about a wider range of data products than SIAv2.
• ObsTAP permits the use of ADQL queries to efficiently gather statistics about the available data (e.g., via SELECT DISTINCT queries) to allow an introspective generation of information to guide a user in exploring the space of data products.

Because of an oversight, this design choice was not reflected in the LSP Requirements, LDM-554. I would like to remedy that.

I would like to modify requirement DMS-API-REQ-0022 from

Specification: The API Aspect shall provide for retrieval of image and visit metadata via TAP ADQL queries.

to

Specification: The API Aspect shall provide for retrieval of image and visit metadata via TAP ADQL queries. In addition to the provision of tables exposing the CAOM2 data model (see DMS-API-REQ-0021), the API Aspect shall provide an ObsTAP service exposing an IVOA ObsCore-compliant, simplified data model for the image and visit metadata.

and add to DMS-API-REQ-0016 (SIA Service for Image Availability) the following text:

Discussion: The SIA service should present the same ObsCore view of the image and visit metadata that is provided by the ObsTAP service (see DMS-API-REQ-0022), subject to the restriction on dataproduct_type from Section 2.1.14 of the SIAv2 standard.