# Federated recursive query **URL:** \ **Category:** Feature Request\ **Created:** [November 20, 2019, 10:38am UTC](https://community.stardog.com/t/federated-recursive-query/2105 "2019-11-20T10:38:23Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![lavhal](https://avatars.discourse-cdn.com/v4/letter/l/f9ae1b/32.png) [@lavhal](https://community.stardog.com/u/lavhal)\ **Post date:** [November 20, 2019, 10:38am UTC](https://community.stardog.com/t/federated-recursive-query/2105/1 "2019-11-20T10:38:23Z")
Hi, I am trying to run the following query for combining data from Stardog with data from another SPARQL Endpoint: ``` Prefix ex: PREFIX rdfs: PREFIX rdf: SELECT * WHERE { ?subj ex:category "StatusCat"; rdfs:label ?status. SERVICE { ?uri rdfs:label ?status. } }} ``` Basically, this SPARQL Endpoint is built on top of a normal JSON Web API where conversion of the JSON response to RDF triples is done on the fly. So, this service accepts as a parameter the variable ?status, therefore the query should be executed in an iterative way, i.e. for each value of the ?status variable there should be initiated a call to ``. When I run directly: ``` Prefix ex: PREFIX rdfs: PREFIX rdf: SELECT * WHERE { SERVICE { ?uri rdfs:label "status1". } } ``` it works as expected, but this is only for one value. I also tried without success the following ways: ``` Prefix ex: PREFIX rdfs: PREFIX rdf: SELECT * WHERE { SERVICE { ?uri rdfs:label ?status. { select ?status where{ ?subj ex:category "StatusCat"; rdfs:label ?status. } } } } ``` and ``` Prefix ex: PREFIX rdfs: PREFIX rdf: SELECT * WHERE { ?subj ex:category "StatusCat". SERVICE { ?uri rdfs:label ?status. { ?subj rdfs:label ?status. } } } ``` The issue seems to be the same or similar to the one posted here: [https://community.stardog.com/t/federated-query-with-inline-data-returns-empty-result/487/2](https://community.stardog.com/t/federated-query-with-inline-data-returns-empty-result/487/2) Maybe I am doing something wrong. Best, Lav ---
**Author:** ![jess](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.stardog.com/jess/32/10_2.png) [@jess](https://community.stardog.com/u/jess)\ **Post date:** [November 20, 2019, 1:54pm UTC](https://community.stardog.com/t/federated-recursive-query/2105/2 "2019-11-20T13:54:10Z")
Hey Lav, Are you implementing Stardog's Java-based Service API? There's a method on the query called `getRequiredInputBindings` which lets you specify which input bindings are required. In this case you should return the `?status` variable. Jess ---
**Author:** ![lavhal](https://avatars.discourse-cdn.com/v4/letter/l/f9ae1b/32.png) [@lavhal](https://community.stardog.com/u/lavhal)\ **Post date:** [November 20, 2019, 2:25pm UTC](https://community.stardog.com/t/federated-recursive-query/2105/3 "2019-11-20T14:25:53Z")
Hi Jess, thanks for your reply. No, I am using another solution, which basically uses RDF to describe the WEB API Service and map their required parameter(s) directly to desired RDF properties, in this case "rdfs:label". So whatever value is next to rdfs:label (as a literal value) it is passed to the WEB API service, which after the invocation returns a JSON response. Next, this response is converted to triples using a SPARQL construct query, which allows defining a customized RDF graph in the way that I need/want. ---
**Author:** ![jess](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.stardog.com/jess/32/10_2.png) [@jess](https://community.stardog.com/u/jess)\ **Post date:** [November 21, 2019, 8:25pm UTC](https://community.stardog.com/t/federated-recursive-query/2105/4 "2019-11-21T20:25:52Z")
Lav, We do this type of optimization in certain situations but we would send a batch of `?status` bindings to minimize the number of queries. The batch would be sent as a `VALUES` list which I'm guessing might not play well with your SPARQL interpreter on the remote end. Jess ---
**Author:** ![lavhal](https://avatars.discourse-cdn.com/v4/letter/l/f9ae1b/32.png) [@lavhal](https://community.stardog.com/u/lavhal)\ **Post date:** [November 22, 2019, 9:21am UTC](https://community.stardog.com/t/federated-recursive-query/2105/5 "2019-11-22T09:21:48Z")
Jess, I already tried with `VALUES` list, but unfortunately (as you also said) that doesn't work for this scenario, since the WEB API accepts only one value per time. There is a possibility to write code some work on top of the the SPARQL interpreter but that wouldn't be a clean solution that we are looking for. Anyway, could you post here the optimization that you mentioned using VALUES list? -\> I would like to compare with the way how I am doing it, maybe there is something that can help temporarily. ---
**Author:** ![jess](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.stardog.com/jess/32/10_2.png) [@jess](https://community.stardog.com/u/jess)\ **Post date:** [November 25, 2019, 5:52pm UTC](https://community.stardog.com/t/federated-recursive-query/2105/6 "2019-11-25T17:52:25Z")
There's no straightforward way to force this at the moment. It's possible we'll add a hint for this in the not too distant future. Given a query that includes a SERVICE pattern and another arbitrary solution-producing pattern, we would implement it as follows. Query: ```auto { ?x a :LocalThing SERVICE <...> { ?x :someAttribute ?y } } ``` The naive approach here would send `select ?x ?y where { ?x :someAttribute ?y }` to the remote endpoint and join the results with the solutions produced by the local pattern. When the above mentioned optimization is applied, we send bindings of `?x` to the remote endpoint to avoid returning all possible bindings of `?x`. If the local database contains `:X1 a :LocalThing` and `:X2 a :LocalThing`, we might send the following query: ```auto select ?x ?y { ?x :someAttribute ?y VALUES (?x) { (:X1) (:X2) } } ``` If there were only one binding of `?x` we could _potentially_ replace instances of the variable with the constant. It's conceivable that the hint would allow specifying the batch size which could be set to 1 in your case leading to the behavior you need. Jess ---
**Author:** ![lavhal](https://avatars.discourse-cdn.com/v4/letter/l/f9ae1b/32.png) [@lavhal](https://community.stardog.com/u/lavhal)\ **Post date:** [November 26, 2019, 7:32am UTC](https://community.stardog.com/t/federated-recursive-query/2105/7 "2019-11-26T07:32:37Z")
Hi Jess, thank you for your explanation. I see and can imagine that this kind of issue is a bit out of traditional query-matching-binding way of the SPARQL and a bit tricky to implement. In the meantime, I will have to think for a workaround solution. Best, Lav ---
**Author:** ![zachary.whitley](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.stardog.com/zachary.whitley/32/94_2.png) [@zachary.whitley](https://community.stardog.com/u/zachary.whitley)\ **Post date:** [November 26, 2019, 3:48pm UTC](https://community.stardog.com/t/federated-recursive-query/2105/8 "2019-11-26T15:48:17Z")
Can you modify your service to extract values expression and iterate on the service side? It would be a lot more performant than a network round trip for each iteration. ---
**Author:** ![lavhal](https://avatars.discourse-cdn.com/v4/letter/l/f9ae1b/32.png) [@lavhal](https://community.stardog.com/u/lavhal)\ **Post date:** [November 27, 2019, 7:56am UTC](https://community.stardog.com/t/federated-recursive-query/2105/9 "2019-11-27T07:56:13Z")
Yes, this was one idea that I already wrote here: [Federated recursive query - #5 by lavhal](https://community.stardog.com/t/federated-recursive-query/2105/5) . So, in principle it is possible, but that would be not a clean solution and very constrained to each use case that we would have. ---
**Author:** ![zachary.whitley](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.stardog.com/zachary.whitley/32/94_2.png) [@zachary.whitley](https://community.stardog.com/u/zachary.whitley)\ **Post date:** [November 27, 2019, 1:49pm UTC](https://community.stardog.com/t/federated-recursive-query/2105/10 "2019-11-27T13:49:36Z")
You can use the example [here](https://github.com/stardog-union/stardog-examples/tree/develop/examples/service) to get started on your masterpiece. ![image](https://canada1.discourse-cdn.com/flex030/uploads/stardog/original/1X/e5f5ffaac2558b91b23b9d8279d2ac221c8829a9.jpeg) ---
**Author:** ![lavhal](https://avatars.discourse-cdn.com/v4/letter/l/f9ae1b/32.png) [@lavhal](https://community.stardog.com/u/lavhal)\ **Post date:** [November 28, 2019, 3:48pm UTC](https://community.stardog.com/t/federated-recursive-query/2105/11 "2019-11-28T15:48:04Z")
Thank you, I will have a look.