add support for LDF (linked data fragments) endpoints

[ali1k]

need to deal with streaming query result

[mathib]

Hi Ali, I've been trying to get my LD-R app to work with the Linked Data Fragments client , but connecting them seemed not to work. This is the workflow I've tried:

1. launch LDF server.js (localhost:5000) with some dummy Turtle triples
2. launch ldf-client-http (localhost:9999) to create a SPARQL endpoint (http://localhost/9999/sparql) that fetches data from my LDF server on localhost:5000. I've tested with Postman, and I was able to send GET requests with a sparql query successfully. Results came back in JSON.
3. add the following line in the server.js config of the LD-R app:

        'http://localhost:9999/demo': {
            host: 'localhost', port: 9999, path: '/sparql', endpointType: 'virtuoso'
        },

4. Launch the LD-R app
5. open localhost:3000 in the browser (works) and navigate to browse the added dataset in LD-R (http://localhost:9999/demo). Here, I get the following 400 code response (bad request) from both LD-R and ldf-client-http. The LD-R console shows the following error:

body: 'ERROR: Query execution could not start.\n\nThe query is not yet supported\nUnsupported group type: graph\n'

It might be possible that the above approach was quite naive. There might be issues with the endpointType from LD-R, as ldf-client-http sparql endpoints is a new kind of endpoint for LD-R (meaning that some queries need to be adapted). On the other hand, I was not able to make a POST request using postman on the ldf-client-http instance, but this might just be me :)

Any ideas? Did you already played with ldf-client-http?

[ali1k]

The fact is that LDF client/server has a limited coverage of the SPARQL specification. As you clearly see in the error msg, it doesn't support aggregate functions such as GROUP BY. For that you need to rewrite most of the current LD-R SPARQL query templates.

[mathib]

Thanks for your response, Ali!

Can you point me to the LD-R code that contains these SPARQL query templates? I found these: https://github.com/ali1k/ld-r/tree/master/services/sparql, but there might be more?

But I also guess that the current LD-R UI is expecting SPARQL aggregating functionality, right? So changing the queries would not be enough, as there would need to be some kind of update regarding the UI (in case of connecting to a LDF sparql endpoint)?

[mathib]

There's also potentially an issue with queries containing IN operator:

body: 'ERROR: Query execution could not start.\n\nThe query is not yet supported\nUnsupported operator: in.\n'

[mathib]

Maybe it's more interesting to look to the Comunica project which includes a SPARQL interface. This somehow seems to be the successor of the LDF client.js and can also deal with LDF and other Linked Data sources. See LinkedDataFragments/Client.js#42

Sadly enough, I can't run Comunica on Windows, but it works for me on my old Mac