HowTo OAI-PMH: Unterschied zwischen den Versionen

Version vom 16. August 2007, 13:18 Uhr

Einstieg in OAI-PMH für Geometadaten (deutsch)

Der Zweck der "Open Archives Initiative - Protocol for Metadata Harvesting" (OAI-PMH): Suchdienste (inkl. 'Metadaten-Kataloge' oder Google) sammeln (harvest) Metadaten von anderen sog. "(Meta) Data-Providern".

Vorteile und Nachteile dieses Protokoll

• Technische Vorteile: Es ist einfach, offen für neue Metadatenmodelle, und lose gekoppelt, da es auf dem Harvesting-Prinzip basiert.
• Nicht-technische Vorteile: Es ist verbreiteter alls alle Geo-Metadaten-Standards (da älter), bewährt (Version 2.0) und wird von Google gelesen (vgl. Google Webmaster Tools).
• Nachteile: Das Harvesting-Prinzip dupliziert Daten und erzeugt daher Redundanzprobleme. Es ist in OGC/ISO-Kreisen wenig bekannt, bzw. hat dort keine Lobby.

Demos und Einführung:

• "(Meta) Data Provider": http://www.geometa.info/gmdb/oai_pmh_data_provider/
• Ein bekannter Biblio.-Katalog: http://oaister.umdl.umich.edu/o/oaister/

Über OAI-PMH:

• Einführung (aus GIS-Blickwinkel): http://gis.hsr.ch/wiki/OAI-PMH
• Homepage mit Community: http://www.openarchives.org/

Hinweise zur Implementierung:

• Spezifikation: http://www.openarchives.org/OAI/openarchivesprotocol.html
• Weitere Infos: OAI-PMH#Specifications_and_links_for_implementers
• Tools, insbes. der Explorer/Validator: http://www.openarchives.org/pmh/tools/
• Die wichtigsten zu implementierenden Requests sind Identify, ListMetadataFormats und ListRecords.

GIS-spezifische Tipps und Regelungen:

• Es wird nur das (obligatorische) XML-Schema ('oai_dc') implementiert.
• metadaten-id: direkter Link zum entspr. Metadaten-Katalog-Eintrag (AGIS)
• dc:publisher: kann eine (AGIS) URL sein
• dc:type: hat Wert 'data'
• dc:relation: direkter Link zu Daten (falls frei/offen), sonst wie metadaten-id
• dc:coverage: enthält BBox (WGS84) z.B. 8.34928 47.067242 8.380961 47.084862

HowTo implement OAI-PMH

• 1. Join the community: Register to the 'OAI Implementers list' at the official OAI Homepage
• 2. Read the spec.: The Harvesting Protocol (version 2.0) specification together with Implementation Guidelines
• 3. Look at existing tools, rated above.
• 4. Implement functions: Most important operations (called 'verbs') are Identify, ListMetadataFormats, ListRecords and GetRecord, perhaps initially without incremental harvesting (resumption process).
• 5. Implement mapping: of your internal data model to DC model elements. See also the special meaning of certain elements like dc:coverage, as explained below.
• 6. Test it: with the The official OAI Repository Explorer.
• 7. Extend it: eventually with specific metadata models and respective XML formats.

Minimal OAI-PMH data provider implementation

Following are more specific guidelines for a minimal OAI-PMH implementation of a so called 'data provider' using only the mandatory 'unqualified' Dublin Core (DC):

• Only three operations (verbs) are needed: Identify, ListMetadataFormats and ListRecords.
• Following operations are not required (initially): ListIdentifiers, ListSets, GetRecord.
• No incremental harvesting (resumption process for ListXxx operations with more than 1000 records)
• No compression as defined in the OAI-PMH spec. (compression at lower http level still possible)
• Date granularity may be 'day' not seconds (YYYY-MM-DD)
• Keeping track of deleted records may not be supported (deletedRecord=no)
• The default data model supported is Dublin Core but this still needs some decisions and explanations (as shown below)

Dublin Core with considerations for GIS

Mandatory Dublin Core (DC) supported as data model is sufficient for a start but with specific semantics (e.g. coverage, relation) (see also example below):

• dc:description contains dct:abstract
• dc:coverage contains a lat/lon bounding box encoding as defined in http://georss.org/simple.html#Box (minx, miny, maxx, maxy: dc:coverage example for a Swiss Canton: 7.30676 47.0585 8.10371 47.5371)
• dc:date means in fact dct:modified
• dc:relation is filled in with dclite4g:onLineSrc. If dc:type='service' dct:hasPart can be derived from GetCapabilities.

OAI implementations

• Geometa-Editor
• GeoShop, Fa. infoGrips GmbH, Zurich, Switzerland (C, Java)
• GeoNetwork, open source, rel. 2.2 (GPL, Java)