[Mulgara-general] CRUD Operations in Java

[Alex Hall]

I'll take a stab at this one since I believe Paul is on vacation this
week...

Chuck Borromeo wrote:
> Hello,
>    I have deployed a mulgara 2.1.1 
> server and used the WebUI (http://localhost:xxxx/webui/) to load 
> some RDF files into a graph and run some SPARQL select statements.  I am also 
> using some Java code to perform SPARQL queries against the 
> endpoint.
>  
> I need to perform CRUD operations on 
> the mulgara server using Java.  These operations 
> include:
> - loading RDF 
> files
> - deleting 
> data
> - drop 
> graphs

All of these operations (load, delete, and drop) are supported by the
new REST interface to Mulgara.  More information is available on the
wiki at [http://mulgara.org/trac/wiki/RESTInterface].

> The deleting data commands will 
> delete a set of triples (instead of single statements).  For example, the RDF 
> files contain data from individual universities.  These RDF files get updated 
> periodically.  I would like to delete their data before I load the updated 
> information.  Ideally, I would like to perform a delete command like this: 
>  
> delete $s $p $o from 
> <graphXYZ> 
> where $s $p $o from <graphXYZ> 
>    where $s $p ‘University of Sinatra’
>    AND $s $p 
> $o;
>  
> So basically I want to delete all 
> the triples with an object of ‘University of Sinatra’.

Mulgara does not support SPARQL/Update, but TQL does support an
equivalent delete/select operation.  The syntax you're looking for is:

delete (select $s $p 'University of Sinatra'
        from <graphXYZ>
        where $s $p 'University of Sinatra')
from <graphXYZ>;

Note that, unlike SPARQL, TQL allows constant values in the SELECT list.

> What is the best approach to 
> accomplish this through Java?  Using the mulgara Java API?  Open a URL to the 
> iTQL endpoint and pass the delete command? 

Once you've written your TQL query, you may execute it any way you like
-- parse it in Java and execute it using the Connection API, or send it
to the TQL endpoint.  I don't know that there's necessarily an advantage
of doing it one way or the other, though most people seem to lean
towards using the TQL servlet over the Java API.

> The iTQL syntax appears to require a 
> subject, predicate, and object for the delete command to work.  Are there some 
> examples of deleting using iTQL syntax?

See above for an example.  More information about TQL, including
examples, is available at [http://mulgara.org/trac/wiki/TQLUserGuide].

> Is there something in 2.1.2 that makes 
> this easier?

The only new feature in 2.1.2 related to this is that multiple TQL
commands may now be sent to the TQL servlet, separated by ';'.  Multiple
commands will be wrapped in a single transaction and wound back if any
one of them fails.

Regards,
Alex