Re: VALUES not passed to SERVICE in federated queries

[Andy Seaborne]

On 26/11/2024 08:56, Martynas Jusevičius wrote:
Hi,

I have two Fuseki 4.6.1 instances in a Docker network (fuseki-admin
and fuseki-end-user) that federate SPARQL queries between each other.

If you want to provide detailed control of SERVICE, then there is

https://jena.apache.org/documentation/query/service_enhancer.html



When I execute this query on fuseki-admin:

PREFIX  rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX  acl:  <http://www.w3.org/ns/auth/acl#>
PREFIX  sioc: <http://rdfs.org/sioc/ns#>

SELECT  *
FROM <urn:x-arq:UnionGraph>
WHERE
   { VALUES ( ?this ?Container ) {
       ( <https://localhost:4443/whateverest/> <https://localhost:4443/> )
     }
     SERVICE <http://fuseki-end-user:3030/ds/>
       { GRAPH ?Container
           { ?Container  a  ?Type }
       }
   }

then I can see the ?Container binding was injected into the query
string executed on fuseki-end-user:

Query = SELECT  * WHERE   { GRAPH <https://localhost:4443/>       {
<https://localhost:4443/>                   a  ?Type       }   }

However when I add FILTER NOT EXISTS to the federated part

PREFIX  rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX  acl:  <http://www.w3.org/ns/auth/acl#>
PREFIX  sioc: <http://rdfs.org/sioc/ns#>

SELECT  *
FROM <urn:x-arq:UnionGraph>
WHERE
   { VALUES (?this ?Container) { (<https://localhost:4443/whateverest/>
<https://localhost:4443/>) }
     SERVICE <http://fuseki-end-user:3030/ds/>
       {  GRAPH ?Container
               {
                 ?Container  a  ?Type
               }
             FILTER NOT EXISTS { GRAPH ?this
                                   { ?this
sioc:has_parent|sioc:has_container  ?Container }
                               }
       }
   }

Without the service enhancer, there is no rewrite and it is a has join:

(join
   VALUES
   SERVICE
)

otherwise it can become a denial of service vector!

With control such as correlated joins:
https://jena.apache.org/documentation/query/service_enhancer.html#correlated-joins

you may be able to get the effect you want.

The ?this case is changing the meaning of the query -- ?this is not 
in-scope in the NOT EXISTS so it is undefined.

    Andy


I see a single request in fuseki-end-user but none of the variables
are bound to anything:

Query =  Query = SELECT  * WHERE   { GRAPH ?Container       {
?Container  a  ?Type }     FILTER NOT EXISTS { GRAPH ?this
               { ?this
<http://rdfs.org/sioc/ns#has_parent>|<http://rdfs.org/sioc/ns#has_container>
?Container }

This looks plain wrong because without bindings it eliminates too many
(positive) results, and the final result on fuseki-end-user comes out
empty.

Is this a bug or am I misunderstanding something about VALUES and/or SERVICE?

Martynas