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THREDDS OPeNDAP

Prerequiste:

  • Sun Java 1.6
  • Apache Tomcat 5.5 or above (Latest version of Tomcat 6.0 recommended)
  • THREDDS (Installation guide can be found here)

OPeNDAP on THREDDS

  • An integrated server provides OPeNDAP access to any CDM dataset. OPeNDAP is a widely used, subsetting data access method extending the HTTP protocol.
  • OPeNDAP is a client-server protocol for scientific data access. The package includes:
    • a C++ client and server,
    • Java client and server libraries
  • The current version, 2.0, is a NASA ESE standard. The OPeNDAP team are working on a new 4.0 protocol spec.

Configuring OPeNDAP.

  • OPeNDAP is configured in the threddsConfig.xml configuration file. This is done by adding an Opendap element as follows:
         <Opendap>
            <ascLimit>50</ascLimit>
            <binLimit>500</binLimit>
            <serverVersion>opendap/3.7</serverVersion>
         </Opendap>
         
    This controls the OPeNDAP data service. Because its easy for a user to inadvertently request very large amounts of data, the TDS limits the size of the data response. In our experience legitimate requests ask for subset sizes that are well below the defaults.

  • The Opendap tag accepts the following elements:
    • ascLimit : to allow, change this to true. You must also add a WCS service in your catalogs, for the gridded datasets you want to server through WCS.
    • binLimit : serve remote datasets with the WCS protocol.
    • serverVersion : the working directory for files to be downloaded (choosing a cache directory)

Configuring THREDDS catalog for OPeNDAP.

  • Datasets are configured to have a OPeNDAP access method in the TDS catalog configuration files (catalog.xml) similar to how other services are configured. The service element's serviceType and base attribute values must be as follows:
         <service name="openDAP" serviceType="OPENDAP" base="/thredds/dodsC/" />
         
  • The dataset to be served must reference this service (or a containing compound service) by the service name:
         <dataset ID="sample" name="Sample Data" urlPath="sample.nc">  <serviceName>openDAP</serviceName></dataset>
         

OPeNDAP Server Help

  • To access most of the features of this OPeNDAP server, append one of the following a eight suffixes to a URL: .das, .dds, .dods, .ddx, .blob, .info, .ver or .help. Using these suffixes, you can ask this server for:
    • das - Dataset Attribute Structure (DAS)
    • dds - Dataset Descriptor Structure (DDS)
    • dods - DataDDS object (A constrained DDS populated with data)
    • ddx - XML version of the DDS/DAS
    • blob - Serialized binary data content for requested data set, with the constraint expression applied.
    • info - info object (attributes, types and other information)
    • html - html form for this dataset
    • ver - return the version number of the server
    • help - help information (this text)

  • Suggestion: If you're typing this URL into a WWW browser and would like information about the dataset, use the `.info' extension.
  • If you'd like to see a data values, use the `.html' extension and submit a query using the customized form.

Debugging

  • The logs can be found at:
    • $CATALINA_HOME/logs
    • $CATALINA_HOME/content/thredds/logs

Related links:

Tested on:

  • apacsrv6

Acronyms

  • CDM - Common Data Model, similar to NetCDF but it has a richer set of low level data types and includes structures, sequences and grids.
  • DAS - Data Attribute Structure A description of the "metadata" attributes for an OPeNDAP dataset. See DODS.
  • DDS - Data Descriptor Structure A description of the data types and sizes of data in an OPeNDAP dataset. See DODS.
  • DODS - Distributed Oceanographic Data System The ancestor to OPeNDAP. See unidata.ucar.edu/packages/dods for information.
  • DTD - Document Type Definition This is the set of definitions that make up an XML specification.
  • OPeNDAP - Open Source Project for Network Data Access Protocol A network protocol for transmitting data across the internet. Though appropriate for many kinds of data, the system was designed with scientific data in mind.
  • THREDDS - Thematic Realtime Environmental Data Distributed Services A project of Unidata to create ways to establish useful collections of earth science data.

Topic revision: r3 - 15 Oct 2010, UnknownUser
 

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