chaining federated (service) calls

[Matthew Nguyen]

Hey all, am playing with ontop to virtualize a number of our dbs at work.  So far it's been great.  We have a large engineering staff with thousands of data silos at work and so I'm investigating how we can federate out queries that may hit multiple endpoints in one query possibly using data from one endpoint to optimize queries on another.

I've done some poking around and it doesn't seem like you can pass along bounded values from one service call to another in sparql 1.1 w/o subqueries, is that the case?

In which case, that would require SERVICE support from ontop?

Are my assumptions/conclusions correct or is there another reasonable way?

[Benjamin Cogrel]

Hi Matthew,

I am happy to hear that you had a positive experience at work with Ontop. I am sorry for the late reply.

The federation can be approached in 2 ways:

  - At the SPARQL level. For that, you need to use a SPARQL federator above Ontop (like for instance FedX: https://rdf4j.org/documentation/programming/federation/), as Ontop does not support the SERVICE clause (https://ontop-vkg.org/guide/compliance.html#sparql-1-1). There have been some interest in bringing some basic support for the SERVICE clause (see  https://github.com/ontop/ontop/pull/365), but it is still in a very early stage.

  - At the SQL level. This is the level we are the most familiar with. In this setting, Ontop delegates the federation concerns to a system like Dremio or Denodo (see for instance our tutorial: https://ontop-vkg.org/tutorial/federation/).

Best,

Benjamin

Are my assumptions/conclusions correct or is there another reasonable way? --
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