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Version 36 (modified by simonp, 12 years ago) ( diff )
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Using RDF as metadata storage

Date	2012-10-28
Contact(s)	Simon Pigot
Last edited
Status	draft, being discussed, in progress
Assigned to release	Not yet assigned to a release
Resources	Not allocated yet
Ticket #	#XYZ

Overview

GeoNetwork stores metadata records from different schemas as rows in a database table. To provide search, a metadata record is:

transformed into to a common XML index document via XSLT;
the common XML document is ingested by Lucene, which creates an index of the fields within the document;
the Lucene index and query format is used for searching

The essence of this proposal is to change this process as follows:

transform the metadata record into RDF (resource description framework) facts when it is ingested by GeoNetwork
store the RDF facts in an RDF store
use the RDF store and the SPARQL/GeoSPARQL query language for searching these facts

Why would we do this?

A metadata record is a collection of facts: RDF is purpose designed for representing facts and relationships between facts
RDF stores and the SPARQL/GEOSPARQL query language are designed to query facts and relationships between facts
Simplify the architecture of GeoNetwork: metadata would be stored and searched in the same persistence solution - the RDF store

Proposal Type

Type: Core Change
App: GeoNetwork
Module: Data Manager, Search,

Voting History

Not proposed for voting yet.

Motivations

RDF (Resource Description Framework) is a general method to decompose knowledge into simple facts that describe at most three pieces of information (a triple). In a metadata record, the triple would be: vocabulary name (the meaning of the fact), metadata element name, text content of metadata element.

A lot of resources in the GeoNetwork space have been devoted to managing ISO19115 metadata records, ISO19115 metadata objects and relationships between objects and records. In many ways, exploring RDF is an attempt to achieve the same outcomemeans that metadata records would be broken down into small facts that can be used and reused in any metadata record (not just an ISO metadata record) - history shows that no single metadata standard has ever gained ascendancy over all domains eg. Taxon Concept Schema and Darwin Core are popular for biological data, Dublin Core remains popular for (which?) some applications, SDMX is used for statistical metadata and so on - using a generic fact representation seems to be a more worthwhile and sustainable approach

Proposal

TBA

Issues

Object Identifiers: One of the stated key advantages of RDF is that objects are identified once and then reused. In the work done to date, I don't see how converting a record to RDF will identify the individual objects for reuse eg. if a piece of contact info is present in two different metadata records, then how is that object uniquely identified? Perhaps the object identifier could be derived from an md5sum on the content of the object?

Profile support in ISO19115 mapping: introduce additional rdf namespaces/concepts?

Relationship to DCAT proposal?

Speed of RDF triple stores versus Lucene? Free text search in Apache JENA RDF triple store/sparql queries is supported by using Lucene to help - see LARQ sub-project: http://jena.apache.org/documentation/larq/index.html

Spatial searching: At present we do mixed spatial and textual searches for OGC CSW query support by filtering Lucene searches with query results from spatial database via GeoTools. How would this work in SPARQL? OGC GeoSPARQL would be part of the approach here I suppose: http://code.google.com/p/geospatialweb/ How mature is the GeoSPARQL implementation for Apache JENA or OpenRDF-sesame? (see links section: these projects don't look like they are mainstream parts of RDF frameworks yet?). Also, we use GeoTools to parse the OGC filter language - how would this be converted to a GeoSPARQL query? (hmm).

Two RDF triple stores now used in GeoNetwork: OpenRDF/sesame and now Apache JENA?

Need to tie metadata permissions system used in GeoNetwork (in database) to record in RDF triple store.

Backwards Compatibility Issues

We have begun to use Lucene as a very fast persistence in place of the database (cf for example, search service q). Need to determine whether these queries can also be run quickly against the RDF triple store.

RDF mappings for other standards? Probably most popular standards will have projects in place or ongoing to do mappings to RDF (eg. dublin core), however some of the more substantial ISO efforts (ISO19110, ISO19135 and others) are less likely to have these so could be a fair body of work to do these (that said, some of the metadata standards have some concepts mapped to ISO19115 so could be reasonably straightforward to use that mapping to the RDF for ISO19115).

New libraries added

Apache JENA - used as RDF triple store in UWA patch (but OpenRDF-Sesame is already being used in GeoNetwork).

Risks

RDF and the semantic web has been the 'coming' technology for some time now. Cynically speaking, could this be another ebRIM? Somewhat mitigated by the maturity of OpenRDF-sesame and Apache JENA and the fact that GeoNetwork already relies upon OpenRDF-sesame for vocabulary support.

Participants

Simon Cox, CSIRO Australia - instigator and RDF mappings
Wahhaj Ali, Tianyi Chen, Cameron Fitzgerald, Joshua Hollick, Saxon Jensen, Rebecca Papadopoulos - University of Western Australia - developed patch for trunk that uses an RDF metadata store
Simon Pigot, CSIRO Australia and GeoNetwork PSC member