Metadata Harvesting

Interoperable digital collections

Distributed libraries

• The reality in most digital libraries is that no one location has all the materials that may be of interest.

• It is often more efficient to allow a number of sites each to retain some of the materials.

• How can we assure clients that they will see all relevant resources, regardless of which library they search?

Two basic approaches

• One service provider with access to resources stored in multiple locations– Information about all the resources located at the

service provider. – Services (DL scenarios) use the information to

provide connections to resources at multiple locations

• Distributed services– Information kept with the resources– Services, local to each collection, interact with other

collection sites

Two protocols

• Z39.50 – Developed before the web– Protocol for communicating with collection

holders in order to provide services.

• Open Archives Initiative– Recent innovation– Central service provider gathers

information from collection holders

Z39.50 - briefly• Information Retrieval Service Definition and Protocol

Specifications for Library Applications• Initially developed over the OSI network standards• Protocol for information exchange

– Free the information seeker from the need to know the details of the target database configuration

• Each site provides services– Each service queries remote sites for needed information

• Information requests mapped to database queries at the collection site.

• Some inconsistency in the interpretation of queries.

Distributed ResourcesMultiple Services

Service provider -- search, browse, compare, etc.

Data provider

Data provider

Data provider

Data provider

Data provider

Approach 1 - One service provider gathers information about data and uses it to provide services

Distributed data and services

Approach 2: Each system is both a data repository and a service provider. Services query other data providers as needed.

Search, browse

Search, browse, compare

Service provider -- search, browse, compare, etc.

Data provider

Data provider

Data provider

Data provider

Data provider

Each server likely to have its own clients. Difference is whether the information exchange is periodic or ad hoc

Hybrid systems

Open Archives Initiative (OAI)

• Web-based– Uses HTTP to communicate between sites

• Centralized server– Services provided from a site that has

already gathered the information it needs for those services from a distributed collection of sites.

Z39.50

• Special purpose protocol (machine to machine, not web interface)

• Gathers information when it is requested, not on a scheduled basis.

OAI Compared to Z39.50Z39.50 OAI

Content (Objects) Distributed Distributed

World View Bibliographic Bibliographic

Object Presentation Data provider Data provider

Searching is Distributed Centralized

Search done by Data provider Service provider

Metadata searched is Up to date Stale

Semantic Mapping When searching Metadata delivery

Source: oai.grainger.uiuc.edu/FinalReport/JCDL_2003_OAI_Intro.ppt

Open Archives Initiative Protocol for Metadata Harvesting -- OAI-PMH

Repository

OAI

Harvester

OAI

HTTP req (OAI verb)

HTTP resp (XML)

OAI PMH defines an interface between the Harvester and any number of Repositories

Metadata Provider

Service Provider

Implemented as CGI, ASP, PHP, or other

Any system may serve as a harvester, repository, or both

OAI componentsService Providers

and

Data Providers

Requests and Responses

http://www.oaforum.org/tutorial/english/page3.htm#section3

Records• Metadata of a resource.• Three parts

– Header (required)• Identifier (required: 1 only)• Datestamp (required: 1 only)• setSpec elements (optional: 0, 1, or more)• Status attribute for deleted item

– Metadata (required)• XML encoded metadata with root tag, namespace• Repositories must support Dublin Core, other formats optional

– “About” statement (optional)• Right statements• Provenance statements

Identifiers

• Globally unique identifier

• Valid URI– Examples

• oai:<archiveId>:<recordId>• oai:etd.vt.edu:etd-1234567890

– Must resolve to one item• No duplicates• No reuse of previously used identifiers

Datestamps

• Date of last modification of a record– Used only for harvesting (meta metadata?)

• Mandatory for each item in the repository• Two levels of granularity possible

– YYYY-MM-DD– YYYY-MM-DDThh:mm:ssZ

• T … Z = Time zone -- must be GMT

• Allows harvesting incrementally -- get only what is new since last visit– Accessed by arguments from and until

The OAI-PMH verbs

• Each requests a specific response from a data repository

Identify• Function: Description of the archive• Example: http://www.language-archives.org/cgi-bin/olaca3.pl?verb=Identify• Parameters: none• Errors/exceptions:

– badArgument (there should not be any)• Response format:Element Example Ordinality ‡repositoryName My Archive 1baseURL http://archive.org/oai 1protocolVersion 2.0 1earliestDatestamp 1999-01-01 1deleteRecords no, transient, persistent 1granularity YYYY-MM-DD, YYYY-MM-DDThh:mm:ssZ 1adminEmail oai-admin@archive.org +compression deflate, compress *description oai-identifier, eprints, friends, … * ‡ Ordinality: 1 = mandatory, 1 only; + = mandatory, 1 only; * = optional, 0 or more

<OAI-PMH xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/ http://www.openarchives.org/OAI/2.0/OAI-PMH.xsd">

<responseDate>2006-10-17T01:37:44Z</responseDate><request

verb="Identify">http://www.language-archives.org/cgi-bin/olaca3.pl</request>

− <Identify><repositoryName>OLAC Aggregator</repositoryName><baseURL>http://www.language-archives.org/cgi-bin/olaca3.pl</baseURL><protocolVersion>2.0</protocolVersion><adminEmail>mailto:haejoong@ldc.upenn.edu</adminEmail><earliestDatestamp>2002-12-14</earliestDatestamp><deletedRecord>no</deletedRecord><granularity>YYYY-MM-DD</granularity>− <!-- maybe later <compression>identity</compression> -->

Actual response from

http://www.language-archives.org/cgi-bin/olaca3.pl?verb=Identify

Continued

− <description>− <oai-identifier xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai-identifier http://www.openarchives.org/OAI/2.0/oai-identifier.xsd"><scheme>oai</scheme><repositoryIdentifier>OLACA.language-archives.org</repositoryIdentifier><delimiter>:</delimiter><sampleIdentifier>oai:ethnologue.com:aaa</sampleIdentifier></oai-identifier></description>

Continued

− <description>− <olac-archive type="institutional" xsi:schemaLocation="http://www.language-archives.org/OLAC/1.0/olac-archive http://www.language-archives.org/OLAC/1.0/olac-archive.xsd"><archiveURL>http://www.language-archives.org:8082/dp9/</archiveURL><curator>Steven Bird & Gary Simons</curator><curatorTitle>Coordinators</curatorTitle><curatorEmail>mailto:olac-admin@language-archives.org</curatorEmail><institution>Open Language Archives Community</institution><institutionURL>http://www.language-archives.org/</institutionURL><shortLocation>Philadelphia, U.S.A.</shortLocation><location/>− <synopsis>This repository contains all records from OLAC-registered archives. It is intended to be used by services which do not want to harvest individual OLAC archives.</synopsis>− <access>Metadata may be used only subject to the access permissions given by the individual archives.</access></olac-archive></description></Identify></OAI-PMH>

ListMetadataFormats

• Function: retrieve available metadata formats from archive

• Example: archive.org/oai-script?verb=ListMetadataFormats&

• identifier=oai:HUBerlin.de:3000218

• Parameters: identifier (optional)• Errors/exceptions:

– badArgument– idDoesNotExist– noMetadataFormats

− <OAI-PMH xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/ http://www.openarchives.org/OAI/2.0/OAI-PMH.xsd"><responseDate>2006-10-17T01:58:06Z</responseDate><request verb="ListMetadataFormats">http://www.language-archives.org/cgi-bin/olaca3.pl</request>− <ListMetadataFormats>− <metadataFormat><metadataPrefix>olac</metadataPrefix><schema>http://www.language-archives.org/OLAC/1.0/olac.xsd</schema><metadataNamespace>http://www.language-archives.org/OLAC/1.0/</metadataNamespace></metadataFormat>− <metadataFormat><metadataPrefix>olac_display</metadataPrefix><schema>http://www.language-archives.org/OLAC/1.0/olac.xsd</schema><metadataNamespace>http://www.language-archives.org/OLAC/1.0/</metadataNamespace></metadataFormat>− <metadataFormat><metadataPrefix>oai_dc</metadataPrefix><schema>http://www.openarchives.org/OAI/2.0/oai_dc.xsd</schema><metadataNamespace>http://www.openarchives.org/OAI/2.0/oai_dc/</metadataNamespace></metadataFormat></ListMetadataFormats></OAI-PMH> Response to

http://www.language-archives.org/cgi-bin/ olaca3.pl?verb=ListMetadataFormats

ListSets

• Function: retrieve set structure of a repository

• Example: archive.org/oai-script?verb=ListSets

• Parameters: resumptionToken (exclusive)• Errors/exceptions:

– badArgument– badResumptionToken– noSetHierarchy

Sets are optional and are used to divide a repository into separate units that will be of interest to different harvesters.

ListIdentifiers• Function: abbieviated form of ListRecords, retrieve only headers• Example: archive.org/oai-script?verb=ListIdentifiers&metadataPrefix= oai_dc&from=2002-12-01

• Parameters:– from (optional)– until (optional)– metadataPrefix (required)– set (optional)– resumptionToken (exclusive)

• Errors/exceptions:– badArgument– badResumptionToken– cannotDisseminateFormat– noRecordsMatch– noSetHierarchy

ListRecords• Function: harvest records from a repository• Example: archive.org/oai-script?verb=ListRecords&

metadataPrefix=oai_dc&set=biology

• Parameters:– from (optional)– until (optional)– metadataPrefix (required) – set (optional)– resumptionToken (exclusive)

• Errors/exceptions:– badArgument– badResumptionToken– cannotDisseminateFormat– noRecordsMatch– noSetHierarchy

GetRecord

• Function: retrieve an individual metadata record from a repository

• Example:archive.org/oai-script?verb=GetRecord&identifier=oai:HUBerlin.de: 3000218

&metadataPrefix=oai_dc

• Parameters:– Identifier (required)– metadataPrefix (required)

• Errors/exceptions:– badArgument– cannotDisseminateFormat– idDoesNotExist

Interoperability

• The goal: communication, without human intervention, between information sources– Books that “talk to each other”

• Live links for references• Knowledge of how to find relevant resources

when needed• Ability to query other information locations

Protocols

• Precise rules for interactions between independent processes– Format of the messages

• Both structure and content

– Specified behavior in response to specific messages

• Many ways to accomplish the same result, but both sides must have the same understanding of the rules of engagement.

Protocol Types

• RPC model– Point to point– Completely open to definition by developer

• Verbs (methods)• Nouns (objects, resources)

– Useful to closed community or group who know about the availability of the resource.

SOAP

• Initial words of the acronym have been discontinued.

• Initially developed as part of the Microsoft .NET paradigm– Now in W3C committee

• Stateless, one-way message exchange paradigm

• XML encoded• Flexibility of RPC, but more constrained in the

way communication is formatted.

REST

• REpresentational State Transfer• An after-the-fact definition of the architecture of the

World Wide Web• The model is

– Client/server– Stateless– Cacheable– Layered

• Resource interface constrained– Restricted verbs– Restricted content types

REST and RPC

• RPC provides flexibility for any type of interaction between any type of resources

• REST provides consistency to allow interaction among resources without prior discovery of accepted actions and responses.

SOAP and REST

• Debate in the Web community about which is the better paradigm for application development

• REST -- restricted, but simple extension of existing Web processes

• SOAP -- added flexibility with cost in terms of bandwidth, security, complexity for development

References

• Giving SOAP a REST http://www.devx.com/DevX/Article/8155• SOAP Version 1.2 Part 0: Primer

http://www.w3.org/TR/2003/REC-soap12-part0-20030624/#L1153• OAI For Beginners - The Open Archives Forum online tutorial:

http://www.oaforum.org/tutorial/index.php• Z39.50 Resource Page:

http://www.niso.org/standards/resources/Z3950_Resources.html• Z39.50 An Overview of Development and the Future (1995)

http://www.cqs.washington.edu/~camel/z/z.html