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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 repository; 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 surroundin "<!--" 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/metacat/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/metacat/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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| base_url                    | ``http://baseurl.repository.org/metacat/dataProvider`` | Base URL of the remote repository                                                                   |
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+-----------------------------+--------------------------------------------------------+-----------------------------------------------------------------------------------------------------+
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OAI-PMH Error Codes
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-------------------
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| Error Code              | Description                                                                    | Applicable Verbs    |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| badArgument             | The request includes illegal arguments, is missing required arguments,         | all verbs           |
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|                         | includes a repeated argument, or values for arguments have an illegal syntax.  |                     |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| badResumptionToken      | The value of the resumptionToken argument is invalid or expired.               | ListIdentifiers     |
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|                         |                                                                                | ListRecords         |
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|                         |                                                                                | ListSets            |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| badVerb                 | Value of the verb argument is not a legal OAI-PMH verb, the verb argument is   | N/A                 |
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|                         | missing, or the verb argument is repeated.                                     |                     |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| cannotDisseminateFormat | The metadata format identified by the value given for the metadataPrefix       | GetRecord           |
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|                         | argument is not supported by the item or by the repository.                    | ListIdentifiers     |
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|                         |                                                                                | ListRecords         |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| idDoesNotExist          | The value of the identifier argument is unknown or illegal in this repository. | GetRecord           |
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|                         |                                                                                | ListMetadataFormats |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| noRecordsMatch          | The combination of the values of the from, until, set and metadataPrefix       | ListIdentifiers     |
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|                         | arguments results in an empty list.                                            | ListRecords         |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| noMetadataFormats       | There are no metadata formats available for the specified item.                | ListMetadataFormats |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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| noSetHierarchy          | The repository does not support sets.                                          | ListSets            |
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|                         |                                                                                | ListIdentifiers     |
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|                         |                                                                                | ListRecords         |
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+-------------------------+--------------------------------------------------------------------------------+---------------------+
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