Semantic Generation Of WCS Catalogs

From OPeNDAP Documentation
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Introduction

A WCS service must maintain a catalog of Coverages that will be used to generate the wcs:Capabilities and wcs:Coverages documents. In the Hyrax WCS service the implementation of this catalog is identified at server start-up from the DispatchHandler's configuration element in the olfs.xml file. This page documents the work on the development and usage of a catalog implementation that utilizes semantic web technologies to generate the WCS catalog content from existing (and probably supplemented) metadata from within the existing OPeNDAP data framework.

Technologies

The semantic activities rely on a number of software technologies that may or may not be familiar to the reader. Here is a brief overview of the core concepts.

RDF

The Resource Description Framework (RDF) is a W3C standard for describing Web resources, such as the title, author, modification date, content, and copyright information of a Web page. RDF documents can be written in XML (RDF/XML). RDF is intended to be read by machines, not by people (which means that reading it with your eyes is a pain)

RDF is comprised of a simple statement syntax call a triple that could be expressed in english as: The property of the resource is the the property's value. (ex: The color of the flower is red.)

All of the working bits of the WCS catalog are stored as RDF statements.

There is much more to know about RDF. These pages can provide a starting point:

OWL

The OWL Web Ontology Language is designed for use by applications that need to process the content of information instead of just presenting information to humans. OWL facilitates greater machine interpretability of Web content than that supported by XML, RDF, and RDF Schema (RDF-S) by providing additional vocabulary along with a formal semantics. OWL has three increasingly-expressive sublanguages: OWL Lite, OWL DL, and OWL Full. [1] OWL adds vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. "exactly one"), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.

For our work OWL is used to express ontologies of various metadata conventions/standards found within the DAP community, and to define crosswalks between these ontologies that allow us to "migrate" metadata from one convention to another.

As with RDF, there is much more to know about OWL. These pages can provide a starting point:

SWRL, SeQRL, & SPARQL

Implementations

Sesame RDF Datastore

Swift OWLIM

Ontologies

(My understanding is that once tuned the ontologies will be fairly static. We should make sure that they are checked in to the subversion repository http://scm.opendap.org/svn in the (as yet to be determined) appropriate spot and the discuss and link to them from here.)

Processing Scheme

Currently the processing steps are described on Benno's test page: http://iri.columbia.edu/~benno/opendaptest/

As this project moves forward and the RDF processing activities are integrated into the OLFS/Hyrax application we will document them here.

Catalog Implementation

The specifics of the catalog implementation should be discussed here. Design/architecture diagrams, components etc.


WCS DispatchHandler Configuration

The Hyrax WCS service is made of a collection 4 implementations of the opendap.coreServlet.DispatchHandler interface.

  • HTTP GET: opendap.wcs.v1_1_2.DispatchHandler
  • HTTP POST (For posting unadorned XML WCS requests): opendap.wcs.v1_1_2.PostHandler
  • HTTP POST (For posting SOAP envelopes containg WCS requests): opendap.wcs.v1_1_2.SoapHandler
  • HTTP POST (For handling WCS requests posted from an HTML form): opendap.wcs.v1_1_2.FormHandler

The service is enabled by creating an entry for each service component in the olfs.xml file.

The primary DispatchHandler is opendap.wcs.v1_1_2.DispatchHandler. It is required for the WCS service and it's Handler declaration contains the configuration information for the service. In it's Handler declaration the location of the ServiceIdentification, ServiceProvider, and OperationsMetadata documents are identified, along with a WCS catalog implementation to be used by the service.


To "turn on" the service Handler declarations must appear in the olfs.xml opendap.wcs.v1_1_2.DispatchHandler Configuration


Example Configuration

 <?xml version="1.0" encoding="UTF-8"?>
 <OLFSConfig>
   <DispatchHandlers>
       <HttpGetHandlers>
           <Handler className="opendap.bes.BESManager">
               <BES>
                   <prefix>/</prefix>
                   <host>localhost</host>
                   <port>10002</port>
                   <ClientPool maximum="10" />
               </BES>
           </Handler>

           <Handler className="opendap.coreServlet.BotBlocker">
              <IpMatch>65\.55\.[012]?\d?\d\.[012]?\d?\d</IpMatch>
            </Handler>  
                       

           <Handler className="opendap.wcs.v1_1_2.DispatchHandler">
               <prefix>WCS</prefix>
               <ServiceIdentification>/absolute/path/to/the/document/ServiceIdentification.xml</ServiceIdentification>
               <ServiceProvider>/absolute/path/to/the/document/ServiceProvider.xml</ServiceProvider>
               <OperationsMetadata>/absolute/path/to/the/document/OperationsMetadata.xml</OperationsMetadata>

               <WcsCatalog className="opendap.wcs.v1_1_2.RdfOwlCatalog">
                   <RDFServer>/absolute/path/to/the/directory/containing/the/wcs:CoverageDescription/documents/coverages/dir</RDFServer>
               </WcsCatalog>

           </Handler>
 
           <Handler className="opendap.threddsHandler.StaticCatalogDispatch">
               <prefix>thredds</prefix>
               <useMemoryCache>true</useMemoryCache>
           </Handler>
           <Handler className="opendap.bes.DapDispatchHandler" />
           <Handler className="opendap.bes.DirectoryDispatchHandler" />
           <Handler className="opendap.coreServlet.SpecialRequestDispatchHandler" />
           <Handler className="opendap.bes.VersionDispatchHandler" />
           <Handler className="opendap.bes.FileDispatchHandler" >
               <AllowDirectDataSourceAccess />
           </Handler>
           <Handler className="opendap.bes.BESThreddsDispatchHandler" />
       </HttpGetHandlers>


       <HttpPostHandlers>

           <Handler className="opendap.wcs.v1_1_2.PostHandler" >
               <prefix>WCS/post</prefix>
           </Handler>
           <Handler className="opendap.wcs.v1_1_2.SoapHandler" >
               <prefix>WCS/soap</prefix>
           </Handler>
           <Handler className="opendap.wcs.v1_1_2.FormHandler" >
               <prefix>WCS/form</prefix>
           </Handler>
 

           <Handler className="opendap.coreServlet.SOAPRequestDispatcher" >
               <OpendapSoapDispatchHandler>opendap.bes.SoapDispatchHandler</OpendapSoapDispatchHandler>
           </Handler>
       </HttpPostHandlers>
   </DispatchHandlers>
</OLFSConfig>
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