Improvements to the OAIPMH Harvester

Date	2012-07-16
Contact(s)	Simon Pigot
Last edited	2012-07-16
Status	draft, being discussed
Assigned to release	2.9
Resources	Available
Ticket #	#XYZ

Overview

OAIPMH Harvester in GeoNetwork needs to be enhanced to support the following:

Metadata Object harvesting

In recent times GeoNetwork has moved from supporting a simple ISO19115/19139 metadata in the form of a 'record' to supporting a set of ISO19115/19139 metadata 'objects' with hierarchical relationships between the objects. The diagram below shows an example of some objects and relationships:

The mechanisms for handling these relationships are part of the ISO standard. They can be explicit in the form of an xlink that refers directly to the related metadata object or implicit by including the UUID of a related metadata object as content in an element. Here is an example of an explicit relationship between a metadata record and a fragment of contact information that it includes:

  <mcp:metadataContactInfo>
    <mcp:CI_Responsibility>
      <mcp:role>
        <gmd:CI_RoleCode codeList="..." codeListValue="custodian"/>
      </mcp:role>
      <mcp:party xlink:href="http://mygeonetwork.com/xml.metadata.get?uuid=urn:marine.csiro.au:marlin:person:28_person_organisation"/>
    </mcp:CI_Responsibility>
  </mcp:metadataContactInfo>

Here is an example of an implicit relationship where the UUID of the parent record in a parent-child relationship is held in the content of the parent identifier element:

    <gmd:parentIdentifier>
        <gco:CharacterString>urn:marine.csiro.au:project:187</gco:CharacterString>
    </gmd:parentIdentifier>

And another example of an implicit relationship where the UUID of the sibling record in a sibling relationship between a dataset metadata record and a project metadata record (uuid: urn:marine.csiro.au:marlin:project:187) is held as a code in an identifier element:

    <gmd:aggregateInformation>
       <gmd:MD_AggregateInformation>
            <gmd:aggregateDataSetIdentifier>
                ...
                <gmd:MD_Identifier>
                    <gmd:code>
                        <gco:CharacterString>urn:marine.csiro.au:marlin:project:187</gco:CharacterString>
                    </gmd:code>
                </gmd:MD_Identifier>
                ...
            </gmd:aggregateDataSetIdentifier>
            <gmd:associationType>
                <gmd:DS_AssociationTypeCode codeList="..." codeListValue="crossReference">crossReference</gmd:DS_AssociationTypeCode>
            </gmd:associationType>
            <gmd:initiativeType>
                <gmd:DS_InitiativeTypeCode codeList="..." codeListValue="project">project</gmd:DS_InitiativeTypeCode>
            </gmd:initiativeType>
        </gmd:MD_AggregateInformation>
    </gmd:aggregateInformation>

OAIPMH and most other harvesters are record based. ie. it is expected or assumed that a harvest will retrieve one or more metadata records. GeoNetwork's OAIPMH server returns records by resolving xlink references to metadata objects. The resolve process:

• finds the fragment of metadata referenced by the xlink (which could be local to the catalog or external to the catalog)
• copies the fragment of metadata into the record

Metadata objects that are implicitly referenced as UUIDs in the content are not resolved.

One of the goals of this proposal is to provide an alternative OAIPMH harvester service that:

• retrieves metadata records with unresolved references
• retrieves and adds all referenced metadata objects to the OAIPMH harvest results
• extends the current OAIPMH implementation: the default behaviour will be to return resolved metadata records. Referencing the alternative OAIPMH service will deliver all metadata objects in unresolved form.

The reason for implementing this extension is to enable conversions to formats that support metadata objects eg. ANDS RIF-CS.

Deleted Records

This is a feature of the OAIPMH standard that has not been implemented in the GeoNetwork OAIPMH server but has been implemented in the OAIPMH client. To implement this capability in the GeoNetwork OAIPMH server records (and metadata objects in general) that have been deleted from the catalog should remain searchable by date range and/or set but be marked as deleted if returned in the results to an OAIPMH client. The idea here is that the client can then determine which records have been deleted during an incremental harvest.

Proposal Type

• Type: Core Change
• App: GeoNetwork
• Module: Kernel, Harvester, Data Manager, Lucene Index

Detailed Proposal

The two components from the overview section of this proposal will be implemented as follows.

Metadata Object Harvesting

• Implement two-stage Lucene search that retrieves metadata objects
  • First stage returns metadata objects with unresolved references to other external metadata objects. This is not difficult as metadata objects are stored with unresolved references in the database. Note: links to internal relationships (eg. repeated metadata in the same record) will still need to be resolved.
  • Since !XLinks and parent-child + sibling relationships are or can be indexed in Lucene, a second query will be generated to collect all related metadata objects and add those to the search results.
• As RIF-CS is a metadata object standard, we can test conversion of the ISO metadata objects returned by the two stage search to the RIF-CS schema. To implement this we will need to adapt the RIF-CS XSLTs for iso19139.mcp, eml-gbif and iso19139.anzlic to handle:
  • metadata objects such as person and organisation contact information as parties
  • metadata records with a project hierarchy level as activities
  • metadata records with dataset as collections
  • xlinks and parent-child + sibling relationships as related parties, activities
• Implement the alternative oaipmh service that will use the two stage lucene search + modified RIF-CS XSLTs (this should be a straightforward change to the existing oaipmh server as the code can decide whether to use the normal lucene search for metadata records or the two-stage lucene search for metadata objects based on the name of the oaipmh server service). The alternative oaipmh server could be called geonetwork/srv/eng/oaipmhMetadataObjects

The estimated time to implement this feature, including testing is 10 weeks.

Deleted Records

OAIPMH server changes:

• Metadata records that are deleted from the GeoNetwork catalog will continue to be removed from the database table and archived as is currently implemented.
• Before removal, a metadata object will be added to a table of deleted records and indexed with status set to deleted. Normal searches on the catalog will not return these records as the default search terms will exclude records with this status (this will require a change to the default Lucene search terms).
• The OAIPMH server will advertise that it maintains deleted records in a persistent manner when answering the OAIPMH "Identify" request as per the OAI protocol document in the Links section.

The estimated time to implement this feature, including testing is 3 weeks.

Links

• Documents: ​http://www.openarchives.org/OAI/openarchivesprotocol.html#DeletedRecords
• Email discussions: ANDS agenda for meeting about OAIPMH improvements.
• Other wiki discussions:

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