WCS Service Prototype
Introduction
The WCS service prototype is written as a DispatchHandler implementation for the OLFS (essentially a plug-in).
One of the challenges encountered when implementing a WCS service for Hyrax is catalog generation: How do we generate a list of coverages from existing holdings in Hyrax?
The initial plan was to use semantic web technology to crosswalk from existing (Climate Forcast, aka cf) metadata into the WCS metadata space. This effort is ongoing but the software technology involved has proved to be bleeding edge and somewhat difficult to make progress with. In order to move forward with the other aspects of WCS, we have implemented a configuration based catalog. In this catalog every wcs:CoverageDescription is essentially hand written and placed in a file in a designated directory. The directory is identified in the DispatchHandler's configuration object and at system start-up each file in the Coverages directory is ingested (with the assumption that the file contains a single wcs:CoverageDescription) and added to the catalog.
Although there are many drawbacks to this approach it is something that has a few positive qualities:
- It works.
- Although it's possible for a data provider to have large numbers of wcs:Coverages in their holdings, it is not practical:
- Large numbers of wcs:Coverages will cause the wcs:Capabilities document to grow to such a size as to be indigestible by clients.
- In practice many of these coverages would be appropriately aggregated into a singe larger coverage.
WCS Configuration
WCS is made up of three commands:
- wcs:GetCapabilities
- wcs:DescribeCoverage
- wcs;GetCoverage
The responses for both the wcs:DescribeCoverage and wcs:GetCoverage requests can be generated directly from the WCS "catalog". The response for a wcs:GetCapaibilities request requires localization information that can only be determined by the individual(s) that are installing the service.
The wcs:Capabilites document contains 4 sections:
- Service Identification (ows:ServiceIdentification)
- Service Provider (ows:ServiceProvider)
- Operations Metadata (ows:OperationsMetadata)
- Contents (wcs:Contents)
For any given installation of a WCS service the first three sections must be carefully localized. Their contents cannot realistically be ascertained by an automation. Thus, they are really part of the WCS configuration activity that must be performed by the service installer. The last section, Contents, can be automatically generated from the catalog of Coverages.
In this WCS implementation the ows:ServiceIdentification, ows:ServiceProvider and ows:OperationsMetadata sections are each stored as a separate XML file that is loaded by the server at startup.
Service Identification
The Service Identification metadata as described in section 7.4.4 of OGC document OGC 06-121r3.
Localization should include (but may not be limited to) updating the ows:Keytwords, ows:Fees and ows:AccessConstraints
Example Document
<?xml version="1.0" encoding="UTF-8"?>
<!-- ************************************************************ -->
<!-- * SERVICE IDENTIFICATION SECTION * -->
<!-- ************************************************************ -->
<ows:ServiceIdentification xmlns="http://www.opengis.net/wcs/1.1"
xmlns:ows="http://www.opengis.net/ows/1.1"
xmlns:xlink="http://www.w3.org/1999/xlink" >
<ows:Title>OPeNDAP Hyrax Prototype WCS Service</ows:Title>
<ows:Abstract>A prototype WCS server intended to provide WCS 1.1 (1.1.2) access to DAP datasets</ows:Abstract>
<ows:Keywords>
<ows:Keyword>Web Coverage Service</ows:Keyword>
<ows:Keyword>OPeNDAP</ows:Keyword>
<ows:Keyword>nectcdf</ows:Keyword>
<ows:Keyword>DAP</ows:Keyword>
</ows:Keywords>
<ows:ServiceType>WCS</ows:ServiceType>
<ows:ServiceTypeVersion>1.1.2</ows:ServiceTypeVersion>
<ows:Fees>NONE</ows:Fees>
<ows:AccessConstraints>NONE</ows:AccessConstraints>
</ows:ServiceIdentification>
Service Provider
The ows:ServiceProvider section should be localized to identify the local operator of the system. Typically this would be the name and contact information of the individual or group responsible for installing and maintaing the WCS service.
<?xml version="1.0" encoding="UTF-8"?>
<!-- ************************************************************ -->
<!-- * SERVICE PROVIDER SECTION * -->
<!-- ************************************************************ -->
<ows:ServiceProvider xmlns="http://www.opengis.net/wcs/1.1"
xmlns:ows="http://www.opengis.net/ows/1.1"
xmlns:xlink="http://www.w3.org/1999/xlink">
<ows:ProviderName>OPeNDAP Inc.</ows:ProviderName>
<ows:ProviderSite xlink:href="http://www.opendap.org"/>
<ows:ServiceContact>
<ows:IndividualName>Nathan D. Potter</ows:IndividualName>
<ows:PositionName>Senior Software Engineer</ows:PositionName>
<ows:ContactInfo>
<ows:Phone>
<ows:Voice>123-456-7890</ows:Voice>
<ows:Facsimile>234-567-8901</ows:Facsimile>
</ows:Phone>
<ows:Address>
<ows:DeliveryPoint>165 Dean Knauss Dr</ows:DeliveryPoint>
<ows:City>Narragansett</ows:City>
<ows:AdministrativeArea>Rhode Island</ows:AdministrativeArea>
<ows:PostalCode>02882</ows:PostalCode>
<ows:Country>USA</ows:Country>
<ows:ElectronicMailAddress>support[a.t]opendap[d.o.t]org</ows:ElectronicMailAddress>
</ows:Address>
<ows:OnlineResource xlink:href="http://ndp.opendap.org:8080/opendap/"/>
<ows:HoursOfService>24x7x365</ows:HoursOfService>
<ows:ContactInstructions>email</ows:ContactInstructions>
</ows:ContactInfo>
<ows:Role>Developer</ows:Role>
</ows:ServiceContact>
</ows:ServiceProvider>
Operations Metadata
<?xml version="1.0" encoding="UTF-8"?>
<!-- ************************************************************ -->
<!-- * OPERATIONS METADATA * -->
<!-- ************************************************************ -->
<ows:OperationsMetadata xmlns="http://www.opengis.net/wcs/1.1"
xmlns:ows="http://www.opengis.net/ows/1.1"
xmlns:xlink="http://www.w3.org/1999/xlink">
<ows:Operation name="GetCapabilities">
<ows:DCP>
<ows:HTTP>
<ows:Get xlink:href="http://ndp.opendap.org:8080/opendap/WCS?"/>
<ows:Post xlink:href="http://ndp.opendap.org:8080/opendap/WCS/post">
<ows:Constraint name="PostEncoding">
<ows:AllowedValues>
<ows:Value>XML</ows:Value>
</ows:AllowedValues>
</ows:Constraint>
</ows:Post>
<ows:Post xlink:href="http://ndp.opendap.org:8080/opendap/WCS/soap">
<ows:Constraint name="PostEncoding">
<ows:AllowedValues>
<ows:Value>SOAP</ows:Value>
</ows:AllowedValues>
</ows:Constraint>
</ows:Post>
</ows:HTTP>
</ows:DCP>
</ows:Operation>
<ows:Operation name="GetCoverage">
<ows:DCP>
<ows:HTTP>
<ows:Get xlink:href="http://ndp.opendap.org:8080/opendap/WCS?"/>
<ows:Post xlink:href="http://ndp.opendap.org:8080/opendap/WCS/post">
<ows:Constraint name="PostEncoding">
<ows:AllowedValues>
<ows:Value>XML</ows:Value>
</ows:AllowedValues>
</ows:Constraint>
</ows:Post>
<ows:Post xlink:href="http://ndp.opendap.org:8080/opendap/WCS/soap">
<ows:Constraint name="PostEncoding">
<ows:AllowedValues>
<ows:Value>SOAP</ows:Value>
</ows:AllowedValues>
</ows:Constraint>
</ows:Post>
</ows:HTTP>
</ows:DCP>
<ows:Parameter name="Format">
<ows:AllowedValues>
<ows:Value>application/x-netcdf-cf1.0</ows:Value>
<ows:Value>application/x-dap2.0</ows:Value>
</ows:AllowedValues>
</ows:Parameter>
</ows:Operation>
<ows:Operation name="DescribeCoverage">
<ows:DCP>
<ows:HTTP>
<ows:Get xlink:href="http://ndp.opendap.org:8080/opendap/WCS?"/>
<ows:Post xlink:href="http://ndp.opendap.org:8080/opendap/WCS/post">
<ows:Constraint name="PostEncoding">
<ows:AllowedValues>
<ows:Value>XML</ows:Value>
</ows:AllowedValues>
</ows:Constraint>
</ows:Post>
<ows:Post xlink:href="http://ndp.opendap.org:8080/opendap/WCS/soap">
<ows:Constraint name="PostEncoding">
<ows:AllowedValues>
<ows:Value>SOAP</ows:Value>
</ows:AllowedValues>
</ows:Constraint>
</ows:Post>
</ows:HTTP>
</ows:DCP>
<ows:Parameter name="Format">
<ows:AllowedValues>
<ows:Value>text/xml</ows:Value>
</ows:AllowedValues>
</ows:Parameter>
</ows:Operation>
</ows:OperationsMetadata>
WCS Coverages Catalog
opendap.wcs.v1_1_2.DispatchHandler Configuration
<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.LocalFileCatalog">
<CoveragesDirectory>/absolute/path/to/the/directory/coverages</CoveragesDirectory>
</WcsCatalog>
</Handler>
