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OAI Protocol for Metadata Harvesting
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====================================
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The Open Archives Initiative Protocol for Metadata Harvesting (`OAI-PMH`_) was first
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developed in the late 1990's as a standard for harvesting metadata from
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distributed metadata/data repositories. The current version of the OAI-PMH
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standard is 2.0 as of June 2002, with minor updates in December 2008.
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.. _OAI-PMH: http://www.openarchives.org/pmh/
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The OAI-PMH standard uses the Hypertext Transport Protocol (HTTP) as a
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transport layer and specifies six query methods (called verbs) that must be
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supported by an OAI-PMH compliant data provider (also referred to as a
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repository). These methods are:
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1. ``GetRecord`` – retrieves zero or one complete metadata record from a repository;
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2. ``Identify`` – retrieves information about a repository;
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3. ``ListIdentifiers`` – retrieves zero or more metadata record “headers” (not the complete metadata record) from a repository;
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4. ``ListMetadataFormats`` – retrieves a list of available metadata record formats supported by a repository;
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5. ``ListRecords`` – retrieves zero or more complete metadata records from a respository; and
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6. ``ListSets`` – retrieves the set structure from a repository.
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The OAI-PMH compliant data provider must accept requests from both HTTP GET
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and HTTP POST request methods. Responses from the data provider must be
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returned as an XML-encoded (version 1.0) stream. Error handling must be
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supported by the data provider and return the correct error response code
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back to the harvester. Detailed specifications and examples of all six verbs
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may be viewed in Section 4 of the `OAI-PMH standards document`_.
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.. _OAI-PMH standards document: http://www.openarchives.org/OAI/openarchivesprotocol.html
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EML and Dublin Core
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-------------------
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The OAI-PMH requires that unqualified Dublin Core metadata be supported as a
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minimum. Although EML generally provides more fine-grained metadata than Dublin
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Core, the two metadata standards do share many of the same (or similar) content
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elements. Transformations from EML to Dublin Core performed by Metacat OAI-PMH
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produce *simple* or *unqualified* Dublin Core, which is associated with the reserved
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metadataPrefix symbol ``oai_dc`` in the OAI-PMH.
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The following table summarizes the element mappings of the EML to Dublin Core
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crosswalk performed by Metacat OAI-PMH, including notes specific to each
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element mapping.
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| EML Element | DC Element | Notes |
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+=======================================+=============+=================================================================================================================================================+
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| Title | title | |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Creator | creator | Use only the creator's name (givenName and surName elements); could be an organization name |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| keyword | subject | One subject element per keyword element |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| abstract | description | Must extract text formatting tags |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| publisher | publisher | Use only the publisher's name (givenName and surName elements); could be an organization name |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| associatedParty | contributor | Use only the party's name (givenName and surName); could be an organization name |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| pubDate | date | One-to-one mapping |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| dataset, citation, protocol, software | type | Type value is determined by the type of EML document rather than by a specific field value |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| physical | format | Use a mime type as the Format value? For example, if EML has <textFormat> element within <physical>, then use 'text/plain' as the Format value? |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| (1) packageId; | identifier | packageId can be used as the value of one identifier element; |
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| (2) URL to the EML document | | a second identifier element can hold a URL to the EML document |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| dataSource | source | Use the document URL of the referenced data source? |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Citation | relation | Use the document URL of the referenced citation? |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| geographicCoverage | coverage | Add separate coverage elements for geographic description and geographic bounding coordinates. |
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| | | For bounding coordinates, use minimal labeling, for example: |
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| | | 81.505000 W, 81.495000 W, |
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| | | 31.170000 N, 31.163000 N |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| taxonomicCoverage | coverage | Use only genus/species binomials; place each binomial in a separate coverage element |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| temporalCoverage | coverage | Include begin date and end date when available. For example: |
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| | | 1915-01-01 to 2004-12-31 |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| intellectualRights | rights | Must extract text formatting tags |
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+---------------------------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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Metacat OAI-PMH includes a set of XSLT stylesheets used for converting specific
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versions of EML to their Dublin Core equivalents.
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Metacat OAI-PMH Service Interfaces
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----------------------------------
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Metacat includes support for two OAI-PMH service interfaces: a data provider
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(or repository) service interface and a harvester service interface.
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Data Provider
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~~~~~~~~~~~~~
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The Metacat OAI-PMH Data Provider service interface supports all six OAI-PMH
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methods (GetRecord, Identify, ListIdentifiers, ListMetadataFormats, ListRecords,
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and ListSets) as defined in the OAI-PMH Version 2 Specification through a
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standard HTTP URL that accepts both HTTP GET and HTTP POST requests.
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The Metacat OAI-PMH Data Provider service was implemented using the Online
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Computer Library Center (OCLC) OAICat Open Source Software as the basis for
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its implementation, with customizations added to facilitate integration with
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Metacat.
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Users of the Metacat OAI-PMH Data Provider should be aware of the following issues:
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* 'Deleted' Status – OAI-PMH repositories can optionally flag records with
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a 'deleted' status, indicating that a record in the metadata format
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specified by the metadataPrefix is no longer available. Since Metacat does
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not provide a mechanism for retrieving a list of deleted documents, the use
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of the 'deleted' status is not supported in this implementation of the
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OAI-PMH Data Provider. This represents a possible future enhancement.
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* Sets – OAI-PMH repositories can optionally support set hierarchies. Since it
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has not been determined how set hierarchies should be structured in
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Metacat, this implementation of the OAI-PMH repository does not support
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set hierarchies. This represents a possible future enhancement.
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* Datestamp Granularity – When expressing datestamps for repository documents,
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OAI-PMH allows two levels of granularity: day granularity and seconds
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granularity. Since the Metacat database stores the value of its
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``xml_documents.date_updated`` field in day granularity, it is the level
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that is supported by the Metacat OAI-PMH Data Provider.
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Metacat OAI-PMH Harvester
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The Metacat OAI-PMH Harvester service interface utilizes OAI-PMH methods to
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request metadata or related information from an OAI-PMH-compliant data provider
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using a standard HTTP URL in either an HTTP-GET or HTTP-POST request.
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The Metacat OAI-PMH Harvester client was implemented using OCLC's
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OAIHarvester2 open source code as its base implementation, with customizations
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as needed to support integration with Metacat.
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Users of the Metacat OAI-PMH Harvester should be aware of the following issues:
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* Handling of 'Deleted' status – The Metacat OAI-PMH Harvester program does
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check to see whether a 'deleted' status is flagged for a harvested document,
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and if it is, the document is correspondingly deleted from the Metacat repository.
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* Datestamp Granularity – When expressing datestamps for repository documents,
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OAI-PMH allows two levels of granularity – day granularity and seconds
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granularity. Since the Metacat database stores the value of its
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``xml_documents.last_updated`` field in day granularity, it is also the
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level that is supported by both the Metacat OAI-PMH Data Provider and the
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Metacat OAI-PMH Harvester. This has implications when Metacat OAI-PMH
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Harvester (MOH) interacts with data providers such as the Dryad repository,
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which stores its documents with seconds granularity. For example, consider
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the following sequence of events:
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1. On January 1, 2010, MOH harvests a document from the Dryad repository
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with datestamp '2010-01-01T10:00:00Z', and stores its local copy with
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datestamp '2010-01-01'.
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2. Later that same day, the Dryad repository updates the document to a
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newer revision, with a new datestamp such as '2010-01-01T20:00:0Z'.
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3. On the following day, MOH runs another harvest. It determines that it
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has a local copy of the document with datestamp '2010-01-01' and does
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not re-harvest the document, despite the fact that its local copy is not
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the latest revision.
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Configuring and Running Metacat OAI-PMH
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---------------------------------------
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Metacat OAI-PMH Data Provider Servlet
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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To configure and enable the Data Provider servlet:
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1. Stop Tomcat and edit the Metacat properties (``metacat.properties``) file in
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the Metacat context directory inside the Tomcat application directory.
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The Metacat context directory is the name of the application (usually ``knb``):
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::
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<tomcat_app_dir>/<context_dir>/WEB-INF/metacat.properties
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2. Change the following properties appropriately:
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::
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``oaipmh.repositoryIdentifier`` – A string that identifies this repository
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``Identify.adminEmail`` – The email address of the repository administrator
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3. Edit the deployment descriptor (``web.xml``) file, also in the WEB-INF
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directory. Uncomment the servlet-name and servlet-mapping entries for the
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DataProvider servlet by removing the surrounding “<!--“ and “-->” strings:
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::
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<servlet>
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<servlet-name>DataProvider</servlet-name>
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<description>Processes OAI verbs for Metacat OAI-PMH Data Provider (MODP)</description>
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<servlet-class>edu.ucsb.nceas.metacat.oaipmh.provider.server.OAIHandler</servlet-class>
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<load-on-startup>4</load-on-startup>
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</servlet>
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<servlet-mapping>
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<servlet-name>DataProvider</servlet-name>
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<url-pattern>/dataProvider</url-pattern>
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</servlet-mapping>
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4. Save the ``metacat.properties`` and ``web.xml`` files and start Tomcat.
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The following table describes the complete set of ``metacat.properties``
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settings that are used by the DataProvider servlet.
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Property Name | Sample Value | Description |
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+========================================+============================================================================+=================================================================================================================================================+
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| oaipmh.maxListSize | 5 | Maximum number of records returned by each call to the ListIdentifiers and ListRecords verbs. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| oaipmh.repositoryIdentifier | metacat.lternet.edu | An identifier string for the respository. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| AbstractCatalog.oaiCatalogClassName | edu.ucsb.nceas.metacat.oaipmh.provider.server.catalog.MetacatCatalog | The Java class that implements the AbstractCatalog interface. This class determines which records exist in the repository and their datestamps. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| AbstractCatalog.recordFactoryClassName | edu.ucsb.nceas.metacat.oaipmh.provider.server.catalog.MetacatRecordFactory | The Java class that extends the RecordFactory class. This class creates OAI-PMH metadata records. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| AbstractCatalog.secondsToLive | 3600 | The lifetime, in seconds, of the resumptionToken. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| AbstractCatalog.granularity | YYYY-MM-DD or | Granularity of datestamps. Either ‘days granularity’ or ‘seconds granularity’ values can be used. |
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| | YYYY-MM-DDThh:mm:ssZ | |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Identify.repositoryName | Metacat OAI-PMH Data Provider | A name for the repository. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Identify.earliestDatestamp | 2000-01-01T00:00:00Z | Earliest datestamp supported by this repository |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Identify.deletedRecord | yes or no | Use ‘yes’ if the repository indicates the status of deleted records; use ‘no’ if it doesn’t. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Identify.adminEmail | mailto:tech_support@someplace.org | Email address of the repository administrator. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Crosswalks.oai_dc | edu.ucsb.nceas.metacat.oaipmh.provider.server.crosswalk.Eml2oai_dc | Java class that controls the EML 2.x.y to oai_dc (Dublin Core) crosswalk. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Crosswalks.eml2.0.0 | edu.ucsb.nceas.metacat.oaipmh.provider.server.crosswalk.Eml200 | Java class that furnishes EML 2.0.0 metadata. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Crosswalks.eml2.0.1 | edu.ucsb.nceas.metacat.oaipmh.provider.server.crosswalk.Eml201 | Java class that furnishes EML 2.0.1 metadata. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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| Crosswalks.eml2.1.0 | edu.ucsb.nceas.metacat.oaipmh.provider.server.crosswalk.Eml210 | Java class that furnishes EML 2.1.0 metadata. |
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+----------------------------------------+----------------------------------------------------------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
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Sample URLs
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...........
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Sample URLs that demonstrate use of the Metacat OAI-PMH Data Provider follow:
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| OAI-PMH Verb | Description | URL |
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+=====================+==============================================================+==========================================================================================================================================+
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| GetRecord | Get an EML 2.0.1 record using its LSID identifier | http://<your_context_url>/dataProvider?verb=GetRecord&metadataPrefix=eml-2.0.1&identifier=urn:lsid:knb.ecoinformatics.org:knb-ltergce:26 |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| GetRecord | Get an oai_dc (Dublin Core) record using its LSID identifier | http://<your_context_url>/dataProvider?verb=GetRecord&metadataPrefix=oai_dc&identifier=urn:lsid:knb.ecoinformatics.org:knb-lter-gce:26 |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| Identify | Identify this data provider | http://<your_context_url>/dataProvider?verb=Identify |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| ListIdentifiers | List all EML 2.1.0 identifiers in the repository | http://<your_context_url>/dataProvider?verb=ListIdentifiers&metadataPrefix=eml-2.1.0 |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| ListIdentifiers | List all oai_dc (Dublin Core) identifiers in the | http://<your_context_url>/dataProvider?verb=ListIdentifiers&metadataPrefix=oai_dc&from=2006-01-01&until=2010-01-01 |
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| | repository between a range of dates | |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| ListMetadataFormats | List metadata formats supported by this repository | http://<your_context_url>/dataProvider?verb=ListMetadataFormats |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| ListRecords | List all EML 2.0.0 records in the repository | http://<your_context_url>/dataProvider?verb=ListRecords&metadataPrefix=eml-2.0.0 |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| ListRecords | List all oai_dc (Dublin Core) records in the repository | http://<your_context_url>/dataProvider?verb=ListRecords&metadataPrefix=oai_dc |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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| ListSets | List sets supported by this repository | http://<your_context_url>/dataProvider?verb=ListSets |
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+---------------------+--------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------+
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Metacat OAI-PMH Harvester
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The Metacat OAI-PMH Harvester (MOH) is executed as a command-line program::
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sh runHarvester.sh -dn <distinguishedName> \
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-password <password> \
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-metadataPrefix <prefix> \
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[-from <fromDate>] \
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[-until <untilDate>] \
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[-setSpec <setName>] \
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<baseURL>
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The following example illustrates how the Metacat OAI-PMH Harvester is run from the command line:
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1. Open a system command window or terminal window.
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2. Set the METACAT_HOME environment variable to the value of the Metacat
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installation directory. Some examples follow:
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On Windows:
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::
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set METACAT_HOME=C:\somePath\metacat
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On Linux/Unix (bash shell):
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::
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export METACAT_HOME=/home/somePath/metacat
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3. cd to the following directory:
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On Windows:
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::
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cd %METACAT_HOME%\lib\oaipmh
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On Linux/Unix:
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::
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cd $METACAT_HOME/lib/oaipmh
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4. Run the appropriate Metacat OAI-PMH Harvester shell script, as determined by the operating system:
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On Windows:
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::
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runHarvester.bat \
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-dn uid=jdoe,o=myorg,dc=ecoinformatics,dc=org \
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-password some_password \
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-metadataPrefix oai_dc \
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http://baseurl.repository.org/knb/dataProvider
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On Linux/Unix:
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::
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sh runHarvester.sh \
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-dn uid=jdoe,o=myorg,dc=ecoinformatics,dc=org \
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-password some_password \
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-metadataPrefix oai_dc \
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http://baseurl.repository.org/knb/dataProvider
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Command line options and parameters are described in the following table:
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+-----------------------------+----------------------------------------------------+-----------------------------------------------------------------------------------------------------+
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| Command Option or Parameter | Example | Description |
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+=============================+====================================================+=====================================================================================================+
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| -dn | ``-dn uid=dryad,o=LTER,dc=ecoinformatics,dc=org`` | Full distinguished name of the LDAP account used when harvesting documents into Metacat. (Required) |
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+-----------------------------+----------------------------------------------------+-----------------------------------------------------------------------------------------------------+
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| -password | ``-password some_password`` | Password of the LDAP account used when harvesting documents into Metacat. (Required) |
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+-----------------------------+----------------------------------------------------+-----------------------------------------------------------------------------------------------------+
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| -metadataPrefix | ``-metadataPrefix oai_dc`` | The type of documents being harvested from the remote repository. (Required) |
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+-----------------------------+----------------------------------------------------+-----------------------------------------------------------------------------------------------------+
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| -from | ``-from 2000-01-01`` | The lower limit of the datestamp for harvested documents. (Optional) |
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+-----------------------------+----------------------------------------------------+-----------------------------------------------------------------------------------------------------+
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| -until | ``-until 2010-12-31`` | The upper limit of the datestamp for harvested documents. (Optional) |
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+-----------------------------+----------------------------------------------------+-----------------------------------------------------------------------------------------------------+
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| -setSpec | ``-setSpec someSet`` | Harvest documents belonging to this set. (Optional) |
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+-----------------------------+----------------------------------------------------+-----------------------------------------------------------------------------------------------------+
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349
|
| base_url | ``http://baseurl.repository.org/knb/dataProvider`` | Base URL of the remote repository |
|
350
|
+-----------------------------+----------------------------------------------------+-----------------------------------------------------------------------------------------------------+
|
351
|
|
352
|
|
353
|
OAI-PMH Error Codes
|
354
|
-------------------
|
355
|
|
356
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
357
|
| Error Code | Description | Applicable Verbs |
|
358
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
359
|
| badArgument | The request includes illegal arguments, is missing required arguments, | all verbs |
|
360
|
| | includes a repeated argument, or values for arguments have an illegal syntax. | |
|
361
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
362
|
| badResumptionToken | The value of the resumptionToken argument is invalid or expired. | ListIdentifiers |
|
363
|
| | | ListRecords |
|
364
|
| | | ListSets |
|
365
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
366
|
| badVerb | Value of the verb argument is not a legal OAI-PMH verb, the verb argument is | N/A |
|
367
|
| | missing, or the verb argument is repeated. | |
|
368
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
369
|
| cannotDisseminateFormat | The metadata format identified by the value given for the metadataPrefix | GetRecord |
|
370
|
| | argument is not supported by the item or by the repository. | ListIdentifiers |
|
371
|
| | | ListRecords |
|
372
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
373
|
| idDoesNotExist | The value of the identifier argument is unknown or illegal in this repository. | GetRecord |
|
374
|
| | | ListMetadataFormats |
|
375
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
376
|
| noRecordsMatch | The combination of the values of the from, until, set and metadataPrefix | ListIdentifiers |
|
377
|
| | arguments results in an empty list. | ListRecords |
|
378
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
379
|
| noMetadataFormats | There are no metadata formats available for the specified item. | ListMetadataFormats |
|
380
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
381
|
| noSetHierarchy | The repository does not support sets. | ListSets |
|
382
|
| | | ListIdentifiers |
|
383
|
| | | ListRecords |
|
384
|
+-------------------------+--------------------------------------------------------------------------------+---------------------+
|
385
|
|
386
|
|