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 –Relatively recent innovation –Central service provider gathers information from collection holders

Z 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 Resources Multiple Services Service provider -- search, browse, compare, etc. 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 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.

OAI PMH Interoperability through Metadata Exchange The Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) is a low-barrier mechanism for repository interoperability. Data Providers are repositories that expose structured metadata via OAI-PMH. Service Providers then make OAI-PMH service requests to harvest that metadata. OAI- PMH is a set of six verbs or services that are invoked within HTTP.

OAI - ORE Aggregations of Web Resources Open Archives Initiative Object Reuse and Exchange (OAI-ORE) defines standards for the description and exchange of aggregations of Web resources. These aggregations, sometimes called compound digital objects, may combine distributed resources with multiple media types including text, images, data, and video. The goal of these standards is to expose the rich content in these aggregations to applications that support authoring, deposit, exchange, visualization, reuse, and preservation. Although a motivating use case for the work is the changing nature of scholarship and scholarly communication, and the need for cyberinfrastructure to support that scholarship, the intent of the effort is to develop standards that generalize across all web-based information including the increasing popular social networks of “web 2.0”.

OAI-ORE example 1.The URI of the human start page. 2.The formats in which the document is available, i.e. PostScript, PDF, etc. These are effectively the constituents of the aggregation that is the arXiv document. For the remainder of this example we will consider this human start page, the splash page, as also a constituent of the aggregation 3.The title of the arXiv document. 4.The authors of the arXiv document. 5.The creation and last modification date of the arXiv document. 6.Identifiers of entities that are in some manner comparable to this arXiv document. For example, a version of this document was later published as an article in a peer-reviewed journal, and the Digital Object Identifier of that article is shown. 7.The versions of this document. 8.Links to other arXiv documents in the same collection (i.e., astro-ph). 9.Citations made by this arXiv document, and citations it received from other documents. The problem is that this URI does not really represent the resource, although this is the human readable landing page.

OAI - ORE ORE allows aggregation of related web pages to form a logical unit –The representation allows access to all of the components of a resource at once. /ore/1.0/primer.html#Exampl e

Our focus We will concentrate on OAI – PMH –Allowing us to know about other resources of interest to our societies –Allowing others to know about the resources we have available

Spot check What sort of resources are handled by your site? Are the resources well represented by the landing page? Do you have complex resources that need structural description as well as the usual Dublin Core fields? Spend a few minutes talking to someone not on your team about the resources you have and what it takes to describe them. Then switch and listen to the other person’s analysis of their resources. Report your conclusions

Older approaches - 1 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 Z39.50 Z39.50OAI Content (Objects)Distributed World ViewBibliographic Object Presentation Data provider Searching isDistributedCentralized Search done byData providerService provider Metadata searched is Up to dateStale Semantic MappingWhen searchingMetadata 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 - PMH components Service Providers and Data Providers Requests and Responses

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: : oai:etd.vt.edu:etd –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 question of time What time is it? –How do you represent this moment in time in a message that goes to people in several different places around the world? There is a standard for that. –Look up (Wikipedia will do) the ISO 8601 standard for unambiguous specification of time. –Write down what time it is right now (use minutes, but not seconds) Yes, the time will change during our discussion.

The OAI-PMH verbs Each requests a specific response from a data repository

Identify Function: Description of the archive Example: Parameters: none Errors/exceptions: –badArgument (there should not be any) Response format: Element Example Ordinality ‡ repositoryName My Archive 1 baseURL 1 protocolVersion earliestDatestamp deleteRecords no, transient, persistent 1 granularity YYYY-MM-DD, YYYY-MM-DDThh:mm:ssZ 1 admin + compression deflate, compress * description oai-identifier, eprints, friends, … * ‡ Ordinality: 1 = mandatory, 1 only; + = mandatory, 1 only; * = optional, 0 or more

T21:30:33Z OLAC Aggregator no YYYY-MM-DD identity -->... Actual response from Continued These expand

Continued First expansion oai OLACA.language- archives.org : oai:ethnologue.com:aaa

Open Language Archives Community Philadelphia, U.S.A. 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. Metadata may be used only subject to the access permissions given by the individual archives.

ListMetadataFormats Function: retrieve available metadata formats from archive Parameters: identifier (optional) Errors/exceptions: –badArgument –idDoesNotExist –noMetadataFormats

Response to archives.org/cgi-bin/ olaca3.pl?verb=ListMetadataFormatshttp:// archives.org/cgi-bin/ olaca3.pl?verb=ListMetadataFormats T21:38:46Z bin/olaca3.pl olac olac_display olac_dla oai_dc

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: abbrieviated form of ListRecords, retrieve only headers 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 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 Parameters: –Identifier (required) –metadataPrefix (required) Errors/exceptions: –badArgument –cannotDisseminateFormat –idDoesNotExist

Spot Check Use the site from which we retrieved some information and use the other PMH verbs there.

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. (Simple Object Access Protocol) 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.

SOAP is a lightweight protocol intended for exchanging structured information in a decentralized, distributed environment. SOAP uses XML technologies to define an extensible messaging framework, which provides a message construct that can be exchanged over a variety of underlying protocols. The framework has been designed to be independent of any particular programming model and other implementation specific semantics.

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

RESTful applications use HTTP requests to post data (create and/or update), read data (e.g., make queries), and delete data. Thus, REST uses HTTP for all four CRUD (Create/Read/Update/Delete) operations. REST is a lightweight alternative to mechanisms like RPC (Remote Procedure Calls) and Web Services (SOAP, WSDL, et al.). Later, we will see how much more simple REST is. Despite being simple, REST is fully-featured; there's basically nothing you can do in Web Services that can't be done with a RESTful architecture.

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 SOAP Version 1.2 Part 0: Primer /#L /#L1153 OAI For Beginners - The Open Archives Forum online tutorial: Z39.50 Resource Page: Z39.50 An Overview of Development and the Future (1995) Plus a few other sites as noted in the slides