| content
| - %VOSWARNING%
%VOSNAV%
---+ Sponger Cartridges (RDF Mappers or RDFizers) for Virtuoso
This article explains how to develop and test a custom Sponger Cartridge.
---++ Concept
Sponger Cartridges provide a modular approach to RDF-oriented entity extraction and ontology mapping as part of a Linked Data production pipeline. Typical sources include (X)HTML pages, images, Office documents, and PDFs, among others.
Cartridges expose their functionality to service consumers via the Virtuoso Sponger ("Sponger"), a middleware layer that extracts and delivers RDF to other Virtuoso components such as the Web Crawler and the SPARQL Query Processor. In addition, it is directly exposed as a REST-style Web Service for exploitation by external applications and services.
A Cartridge is comprised of an initialization PL procedure (hook) and an entity extractor and mapper. The entity extractor and mapping component can be developed using PL, <code>C</code>, or any external language supported by Virtuoso via the Virtuoso Server Extensions APIs.
Once the cartridge has been developed, it is plugged into the Virtuoso Sponger by adding a record to the <code>DB.DBA.<nowiki>SYS_RDF_MAPPERS</nowiki></code> table.
---++ How Cartridges Work
---+++ SPARQL Query Processing
When a SPARQL query is dispatched to Virtuoso, it invokes the Sponger during the act of URI dereferencing; i.e., it actually crawls the Web, locates a resource via it's URI/IRI (using a variety of RDF discovery heuristics), and then processes the data that the URI exposes. If RDF is discovered, the cartridges play no role. On the other hand, if RDF isn't discovered the Sponger will look in the <code>DB.DBA.<nowiki>SYS_RDF_MAPPERS</nowiki></code> table (in <code>RM_ID</code> order), and for every matching URI or MIME type pattern (depending on <code>RM_TYPE</code> column value), it will
invoke the associated cartridge via the hook procedure. If the hook returns zero, the next cartridge will be tried. If the result is negative, the process stops, informing the SPARQL engine that nothing was retrieved. If the result is positive, the process stops, this time informing the SPARQL engine that RDF data was successfully retrieved.
---+++ Sponger Cartridge RDF Extractor PL hook Requirements
Every PL function used to plug a cartridge into the SPARQL engine must have the following parameter signature:
| <b><code>IN graph_iri VARCHAR</code></b> | the IRI of the local storage graph |
| <b><code>IN <nowiki>new_origin_uri</nowiki> VARCHAR</code></b> | the URI of the target information resource |
| <b><code>IN destination VARCHAR</code></b> | the IRI of the target graph |
| <b><code>INOUT content ANY</code></b> | the content of the information resource retrieved for dispatch to the Sponger |
| <b><code>INOUT async_queue ANY</code></b> | an asynchronous queue; can be used for background processing (if required) |
| <b><code>INOUT ping_service ANY</code></b> | the value in the <code>[SPARQL]</code> section of a Virtuoso instance (i.e., <code><nowiki>PingService</nowiki></code> INI parameter) which enables RDF triple propagation and notification to pinger services such as <code>[[http://pingthesemanticweb.com/][http://pingthesemanticweb.com/]]<code> |
| <b><code>INOUT api_key ANY</code></b> | a plain text ID, either a single key value or serialized vector of keys (basically the value of <code>RM_KEY</code> column of the <code>DB.DBA.<nowiki>SYS_RDF_MAPPERS</nowiki></code> table), which is used for handling of API keys for 3rd party Web Services |
Note: the names of the parameters are not important, but their order and presence are vital.
---++ Implementation
In the example script we implement a basic cartridge which maps a <code>text/plain</code> MIME type to an imaginary ontology, which extends the class <code>Document</code> from <code>FOAF</code> with properties '<code>txt:UniqueWords</code>' and '<code>txt:Chars</code>', where we specity the prefix '<code>txt:</code>' as '<code>urn:txt:v0.0:</code>'.
<verbatim>
USE DB;
CREATE PROCEDURE DB.DBA.RDF_LOAD_TXT_META
(
IN graph_iri VARCHAR,
IN new_origin_uri VARCHAR,
IN dest VARCHAR,
INOUT ret_body ANY,
INOUT aq ANY,
INOUT ps ANY,
INOUT ser_key ANY
)
{
DECLARE words,
chars INT;
DECLARE vtb,
arr,
subj,
ses,
str ANY;
DECLARE ses ANY;
-- if any error, we just say nothing can be done
DECLARE EXIT HANDLER FOR SQLSTATE '*'
{
return 0;
};
subj := COALESCE (dest, new_origin_uri);
vtb := vt_batch ();
chars := LENGTH (ret_body);
-- using the text index procedures, we get a list of words
vt_batch_feed (vtb, ret_body, 1);
arr := vt_batch_strings_array (vtb);
-- the list has 'word' and positions array, so we must divide by 2
words := LENGTH (arr) / 2;
ses := string_output ();
-- we compose a N3 literal
http (sprintf ('<%s> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://xmlns.com/foaf/0.1/Document> .\n', subj), ses);
http (sprintf ('<%s> <urn:txt:v0.0:UniqueWords> "%d" .\n', subj, words), ses);
http (sprintf ('<%s> <urn:txt:v0.0:Chars> "%d" .\n', subj, chars), ses);
str := string_output_string (ses);
-- we push the N3 text into the local store
DB.DBA.TTLP (str, new_origin_uri, subj);
RETURN 1;
}
;
DELETE FROM DB.DBA.SYS_RDF_MAPPERS
WHERE RM_HOOK = 'DB.DBA.RDF_LOAD_TXT_META';
INSERT SOFT DB.DBA.SYS_RDF_MAPPERS
( RM_PATTERN,
RM_TYPE,
RM_HOOK,
RM_KEY,
RM_DESCRIPTION
)
VALUES
( '(text/plain)',
'MIME',
'DB.DBA.RDF_LOAD_TXT_META',
NULL,
'Text Files (demo)'
)
;
-- here we set order to some large number so don't break existing mappers
UPDATE DB.DBA.SYS_RDF_MAPPERS
SET RM_ID = 2000
WHERE RM_HOOK = 'DB.DBA.RDF_LOAD_TXT_META';
</verbatim>
To test the cartridge, we just use the <code>/sparql</code> endpoint with option 'Retrieve remote RDF data for all missing source graphs' to execute:
<verbatim>
SELECT *
FROM <URL-of-a-txt-file>
WHERE { ?s ?p ?o }
</verbatim>
Note that the <code>SPARQL_SPONGE</code> role must be granted to any user account (by default, the "<code>SPARQL</code>" user) which will be issuing SPARQL queries that invoke Sponger cartridges, to enable persistence to local storage.
If the above is set correctly, then you can just hit [[http://demo.openlinksw.com/sparql?default-graph-uri=http%3A%2F%2Fdemo.openlinksw.com%2Ftutorial%2Fhosting%2Fho_s_30%2FWebCalendar%2Ftools%2Fsummary.txt&should-sponge=&query=select+*+%0D%0Awhere+%7B+%3Fs+%3Fp+%3Fo+%7D&format=text%2Fhtml&debug=on][this link]].
More complex examples can be found in the cartridges package implementation.
---++ Authentication in the Sponger
To enable user-defined authentication, there are additional parameters for the <code><strong>/proxy</strong></code> and <code><strong>/sparql</strong></code> endpoints, which should be sent by RDF browser or iSPARQL users --
* for <code><strong>/proxy</strong></code> endpoint:
<verbatim>
'login=<account name>'
</verbatim>
* for <code><strong>/sparql</strong></code> endpoint:
<verbatim>
get-login=<account name>
</verbatim>
---++ References
* [[VirtSpongerCartridgeRDFExtractor][RDF Cartridge Extractor Component]]
* [[RDFMappers][SPARQL Processor and RDFizer Cartridges]]
CategoryRDF CategoryVirtuoso
%VOSCOPY%
|
![[RDF Data]](/fct/images/sw-rdf-blue.png)
![[cxml]](/fct/images/cxml_doc.png)
![[csv]](/fct/images/csv_doc.png)
![[text]](/fct/images/ntriples_doc.png)
![[turtle]](/fct/images/n3turtle_doc.png)
![[ld+json]](/fct/images/jsonld_doc.png)
![[rdf+json]](/fct/images/json_doc.png)
![[rdf+xml]](/fct/images/xml_doc.png)
![[atom+xml]](/fct/images/atom_doc.png)
![[html]](/fct/images/html_doc.png)