14. OAI Protocol for Metadata Harvesting

The Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) was first developed in the late 1990’s as a standard for harvesting metadata from distributed metadata/data repositories. The current version of the OAI-PMH standard is 2.0 as of June 2002, with minor updates in December 2008.

The OAI-PMH standard uses the Hypertext Transport Protocol (HTTP) as a transport layer and specifies six query methods (called verbs) that must be supported by an OAI-PMH compliant data provider (also referred to as a repository). These methods are:

  1. GetRecord - retrieves zero or one complete metadata record from a repository;
  2. Identify - retrieves information about a repository;
  3. ListIdentifiers - retrieves zero or more metadata record “headers” (not the complete metadata record) from a repository;
  4. ListMetadataFormats - retrieves a list of available metadata record formats supported by a repository;
  5. ListRecords - retrieves zero or more complete metadata records from a repository; and
  6. ListSets - retrieves the set structure from a repository.

The OAI-PMH compliant data provider must accept requests from both HTTP GET and HTTP POST request methods. Responses from the data provider must be returned as an XML-encoded (version 1.0) stream. Error handling must be supported by the data provider and return the correct error response code back to the harvester. Detailed specifications and examples of all six verbs may be viewed in Section 4 of the OAI-PMH standards document.

14.1. EML and Dublin Core

The OAI-PMH requires that unqualified Dublin Core metadata be supported as a minimum. Although EML generally provides more fine-grained metadata than Dublin Core, the two metadata standards do share many of the same (or similar) content elements. Transformations from EML to Dublin Core performed by Metacat OAI-PMH produce simple or unqualified Dublin Core, which is associated with the reserved metadataPrefix symbol oai_dc in the OAI-PMH.

The following table summarizes the element mappings of the EML to Dublin Core crosswalk performed by Metacat OAI-PMH, including notes specific to each element mapping.

EML Element DC Element Notes
Title title  
Creator creator Use only the creator’s name (givenName and surName elements); could be an organization name
keyword subject One subject element per keyword element
abstract description Must extract text formatting tags
publisher publisher Use only the publisher’s name (givenName and surName elements); could be an organization name
associatedParty contributor Use only the party’s name (givenName and surName); could be an organization name
pubDate date One-to-one mapping
dataset, citation, protocol, software type Type value is determined by the type of EML document rather than by a specific field value
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?
  1. packageId;
  2. URL to the EML document
identifier packageId can be used as the value of one identifier element; a second identifier element can hold a URL to the EML document
dataSource source Use the document URL of the referenced data source?
Citation relation Use the document URL of the referenced citation?
geographicCoverage coverage Add separate coverage elements for geographic description and geographic bounding coordinates. For bounding coordinates, use minimal labeling, for example: 81.505000 W, 81.495000 W, 31.170000 N, 31.163000 N
taxonomicCoverage coverage Use only genus/species binomials; place each binomial in a separate coverage element
temporalCoverage coverage Include begin date and end date when available. For example: 1915-01-01 to 2004-12-31
intellectualRights rights Must extract text formatting tags

Metacat OAI-PMH includes a set of XSLT stylesheets used for converting specific versions of EML to their Dublin Core equivalents.

14.2. Metacat OAI-PMH Service Interfaces

Metacat includes support for two OAI-PMH service interfaces: a data provider (or repository) service interface and a harvester service interface.

14.2.1. Data Provider

The Metacat OAI-PMH Data Provider service interface supports all six OAI-PMH methods (GetRecord, Identify, ListIdentifiers, ListMetadataFormats, ListRecords, and ListSets) as defined in the OAI-PMH Version 2 Specification through a standard HTTP URL that accepts both HTTP GET and HTTP POST requests.

The Metacat OAI-PMH Data Provider service was implemented using the Online Computer Library Center (OCLC) OAICat Open Source Software as the basis for its implementation, with customizations added to facilitate integration with Metacat.

Users of the Metacat OAI-PMH Data Provider should be aware of the following issues:

  • ‘Deleted’ Status - OAI-PMH repositories can optionally flag records with a ‘deleted’ status, indicating that a record in the metadata format specified by the metadataPrefix is no longer available. Since Metacat does not provide a mechanism for retrieving a list of deleted documents, the use of the ‘deleted’ status is not supported in this implementation of the OAI-PMH Data Provider. This represents a possible future enhancement.
  • Sets - OAI-PMH repositories can optionally support set hierarchies. Since it has not been determined how set hierarchies should be structured in Metacat, this implementation of the OAI-PMH repository does not support set hierarchies. This represents a possible future enhancement.
  • Datestamp Granularity - When expressing datestamps for repository documents, OAI-PMH allows two levels of granularity: day granularity and seconds granularity. Since the Metacat database stores the value of its xml_documents.date_updated field in day granularity, it is the level that is supported by the Metacat OAI-PMH Data Provider.

14.2.2. Metacat OAI-PMH Harvester

The Metacat OAI-PMH Harvester service interface utilizes OAI-PMH methods to request metadata or related information from an OAI-PMH-compliant data provider using a standard HTTP URL in either an HTTP-GET or HTTP-POST request.

The Metacat OAI-PMH Harvester client was implemented using OCLC’s OAIHarvester2 open source code as its base implementation, with customizations as needed to support integration with Metacat.

Users of the Metacat OAI-PMH Harvester should be aware of the following issues:

  • Handling of ‘Deleted’ status - The Metacat OAI-PMH Harvester program does check to see whether a ‘deleted’ status is flagged for a harvested document, and if it is, the document is correspondingly deleted from the Metacat repository.
  • Datestamp Granularity - When expressing datestamps for repository documents, OAI-PMH allows two levels of granularity - day granularity and seconds granularity. Since the Metacat database stores the value of its xml_documents.last_updated field in day granularity, it is also the level that is supported by both the Metacat OAI-PMH Data Provider and the Metacat OAI-PMH Harvester. This has implications when Metacat OAI-PMH Harvester (MOH) interacts with data providers such as the Dryad repository, which stores its documents with seconds granularity. For example, consider the following sequence of events:
    1. On January 1, 2010, MOH harvests a document from the Dryad repository with datestamp ‘2010-01-01T10:00:00Z’, and stores its local copy with datestamp ‘2010-01-01’.
    2. Later that same day, the Dryad repository updates the document to a newer revision, with a new datestamp such as ‘2010-01-01T20:00:0Z’.
    3. On the following day, MOH runs another harvest. It determines that it has a local copy of the document with datestamp ‘2010-01-01’ and does not re-harvest the document, despite the fact that its local copy is not the latest revision.

14.3. Configuring and Running Metacat OAI-PMH

14.3.1. Metacat OAI-PMH Data Provider Servlet

To configure and enable the Data Provider servlet:

  1. Stop Tomcat and edit the Metacat properties (metacat.properties) file in the Metacat context directory inside the Tomcat application directory. The Metacat context directory is the name of the application (usually knb):

    <tomcat_app_dir>/<context_dir>/WEB-INF/metacat.properties
  2. Change the following properties appropriately:

    ``oaipmh.repositoryIdentifier`` - A string that identifies this repository
    ``Identify.adminEmail`` - The email address of the repository administrator
  3. Edit the deployment descriptor (web.xml) file, also in the WEB-INF directory. Uncomment the servlet-name and servlet-mapping entries for the DataProvider servlet by removing the surroundin “<!–” and “–>” strings:

    <servlet>
      <servlet-name>DataProvider</servlet-name>
      <description>Processes OAI verbs for Metacat OAI-PMH Data Provider (MODP)</description>
      <servlet-class>edu.ucsb.nceas.metacat.oaipmh.provider.server.OAIHandler</servlet-class>
      <load-on-startup>4</load-on-startup>
    </servlet>
    <servlet-mapping>
      <servlet-name>DataProvider</servlet-name>
      <url-pattern>/dataProvider</url-pattern>
    </servlet-mapping>
  4. Save the metacat.properties and web.xml files and start Tomcat.

The following table describes the complete set of metacat.properties settings that are used by the DataProvider servlet.

Property Name Sample Value Description
oaipmh.maxListSize 5 Maximum number of records returned by each call to the ListIdentifiers and ListRecords verbs.
oaipmh.repositoryIdentifier metacat.lternet.edu An identifier string for the respository.
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.
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.
AbstractCatalog.secondsToLive 3600 The lifetime, in seconds, of the resumptionToken.
AbstractCatalog.granularity YYYY-MM-DD or YYYY-MM-DDThh:mm:ssZ Granularity of datestamps. Either “days granularity” or “seconds granularity” values can be used.
Identify.repositoryName Metacat OAI-PMH Data Provider A name for the repository.
Identify.earliestDatestamp 2000-01-01T00:00:00Z Earliest datestamp supported by this repository
Identify.deletedRecord yes or no Use “yes” if the repository indicates the status of deleted records; use “no” if it doesn’t.
Identify.adminEmail mailto:tech_support@someplace.org Email address of the repository administrator.
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.
Crosswalks.eml2.0.0 edu.ucsb.nceas.metacat.oaipmh.provider.server.crosswalk.Eml200 Java class that furnishes EML 2.0.0 metadata.
Crosswalks.eml2.0.1 edu.ucsb.nceas.metacat.oaipmh.provider.server.crosswalk.Eml201 Java class that furnishes EML 2.0.1 metadata.
Crosswalks.eml2.1.0 edu.ucsb.nceas.metacat.oaipmh.provider.server.crosswalk.Eml210 Java class that furnishes EML 2.1.0 metadata.

14.3.1.1. Sample URLs

Sample URLs that demonstrate use of the Metacat OAI-PMH Data Provider follow:

OAI-PMH Verb Description URL
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
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
Identify Identify this data provider http://<your_context_url>/dataProvider?verb=Identify
ListIdentifiers List all EML 2.1.0 identifiers in the repository http://<your_context_url>/dataProvider?verb=ListIdentifiers&metadataPrefix=eml-2.1.0
ListIdentifiers List all oai_dc (Dublin Core) identifiers in the repository between a range of dates http://<your_context_url>/dataProvider?verb=ListIdentifiers&metadataPrefix=oai_dc&from=2006-01-01&until=2010-01-01
ListMetadataFormats List metadata formats supported by this repository http://<your_context_url>/dataProvider?verb=ListMetadataFormats
ListRecords List all EML 2.0.0 records in the repository http://<your_context_url>/dataProvider?verb=ListRecords&metadataPrefix=eml-2.0.0
ListRecords List all oai_dc (Dublin Core) records in the repository http://<your_context_url>/dataProvider?verb=ListRecords&metadataPrefix=oai_dc
ListSets List sets supported by this repository http://<your_context_url>/dataProvider?verb=ListSets

14.3.2. Metacat OAI-PMH Harvester

The Metacat OAI-PMH Harvester (MOH) is executed as a command-line program:

sh runHarvester.sh -dn <distinguishedName> \
                   -password <password> \
                   -metadataPrefix <prefix> \
                   [-from <fromDate>] \
                   [-until <untilDate>] \
                   [-setSpec <setName>] \
                   <baseURL>

The following example illustrates how the Metacat OAI-PMH Harvester is run from the command line:

  1. Open a system command window or terminal window.

  2. Set the METACAT_HOME environment variable to the value of the Metacat installation directory. Some examples follow:

    On Windows:

    set METACAT_HOME=C:\somePath\metacat

    On Linux/Unix (bash shell):

    export METACAT_HOME=/home/somePath/metacat
  3. cd to the following directory:

    On Windows:

    cd %METACAT_HOME%\lib\oaipmh

    On Linux/Unix:

    cd $METACAT_HOME/lib/oaipmh
  4. Run the appropriate Metacat OAI-PMH Harvester shell script, as determined by the operating system:

    On Windows:

    runHarvester.bat \
        -dn uid=jdoe,o=myorg,dc=ecoinformatics,dc=org \
        -password some_password \
        -metadataPrefix oai_dc \
        http://baseurl.repository.org/metacat/dataProvider

    On Linux/Unix:

    sh runHarvester.sh \
        -dn uid=jdoe,o=myorg,dc=ecoinformatics,dc=org \
        -password some_password \
        -metadataPrefix oai_dc \
        http://baseurl.repository.org/metacat/dataProvider

Command line options and parameters are described in the following table:

Command Option or Parameter Example Description
-dn -dn uid=dryad,o=LTER,dc=ecoinformatics,dc=org Full distinguished name of the LDAP account used when harvesting documents into Metacat. (Required)
-password -password some_password Password of the LDAP account used when harvesting documents into Metacat. (Required)
-metadataPrefix -metadataPrefix oai_dc The type of documents being harvested from the remote repository. (Required)
-from -from 2000-01-01 The lower limit of the datestamp for harvested documents. (Optional)
-until -until 2010-12-31 The upper limit of the datestamp for harvested documents. (Optional)
-setSpec -setSpec someSet Harvest documents belonging to this set. (Optional)
base_url http://baseurl.repository.org/metacat/dataProvider Base URL of the remote repository

14.4. OAI-PMH Error Codes

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