Re: Adding FROM/FROM NAMED support to Fuseki?

[Andy Seaborne]

Hi Rob,

Re: FROM/FROM NAMED processing
With TDB, the dataset for the query still matters because FROM/FROM 
NAMED are used to select the graphs out of the dataset.  The TDB query 
engine still needs to know what the base dataset is. What's more the 
dataset is used when deciding the correct query execution factory.

So the Fuseki changes, if the store is TDB, is just not to check but 
still configure the server with a dataset.

(sorry - I haven't had the chance to try this out - if you do try, 
please let us know).

What might need adding, depending on your expected usage is translating 
protocol parameters default-graph-uri and named-graph-uri into FROM/FROM 
NAMED for query.

If an engine doesn't handle dataset descriptions, it should complain 
(although it might ignore it).

If you want to force FROM/FROM NAMED even though there is an underlying 
datasets, then maybe adding a wrapper query engine that takes queries, 
does whatever processing you want (security masking maybe) and passes 
the corrected query down to TDB.

> Also how would you suggest handling the second case where we have
> some FROM NAMED clauses but no FROM clauses i.e. the default graph
> remains unchanged. How exactly do we construct a dataset that
> expresses that?

Re: real default graph

In SPARQL, if there is a dataset description, then it's a complete 
description.  If there is a FROM NAMED but no FROM then the default is 
empty.

You can name the real one with <urn:x-arq:DefaultGraph>

        Andy

Hi All

Following up on a previous discussion by colleague Tim had
(http://mail-archives.apache.org/mod_mbox/incubator-jena-users/201102.mbox/%3c4d4db755.1060...@epimorphics.com%3E) 
with regards to adding FROM/FROM NAMED support to Fuseki.

We need this for our own work so are willing to do the coding and
submit it to the project as a patch we'd just appreciated some guidance 
on how best to implement this.

It is easy to find where to turn off the automatic error messages for
use of a FROM/FROM NAMED clause as that just requires commenting out a 
couple of lines of code in SPARQL_QueryDataset in the validateQuery() 
method.  What we are struggling to understand is how we can actually get 
the dataset to change, there is a decideDataset() method which would 
seem to be the obvious place to make changes to the dataset and this is 
what we have so far:

     @Override
     protected Dataset decideDataset(HttpActionQuery action, Query
query, String queryStringLog)
     {
        if (query.hasDatasetDescription())
        {
              if (query.getGraphURIs().size()>  0)
              {
                     return
DatasetFactory.create(query.getGraphURIs(), query.getNamedGraphURIs(), 
null, null);
              }
              else
              {
                     //TODO: How do we create a dataset that
preserves the known default graph and just adds the named graphs?
                     errorBadRequest("Query with FROM NAMED without
any FROM clauses not yet supported");
                     return null;
              }
        }
        else
        {
              return DatasetFactory.create(action.dsg) ;
        }
     }

Are we on the right lines here or is this not an appropriate
place/way to achieve this?

Also how would you suggest handling the second case where we have
some FROM NAMED clauses but no FROM clauses i.e. the default graph 
remains unchanged.  How exactly do we construct a dataset that expresses 
that?

Thanks,

Rob Vesse