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Some time after I wrote “UMR – Unified Metadata Resources“, I came across Chris Keene’s post “Linked data & RDF : draft notes for comment“, “just a list of links and notes” about Linked Data, RDF and the Semantic Web, put together to start collecting information about “a topic that will greatly impact on the Library / Information management world“.

While reading this post and working my way through the links on that page, I started realising that Linked Data is exactly what I tried to describe as One single web page as the single identifier of every book, author or subject. I did mention Semantic Web, URI’s and RDF, but the term “Linked Data” as a separate protocol had escaped me.

The concept of Linked Data was described by Tim Berners Lee, the inventor of the World Wide Web. Whereas the World Wide Web links documents (pages, files, images), which are basically resources about things, (“Information Resources” in Semantic Web terms), Linked Data (or the Semantic Web) links raw data and real life things (“Non-Information Resources”).

There are several definitions of Linked Data on the web, but here is my attempt to give a simple definition of it (loosely based on the definition in Structured Dynamics’ Linked Data FAQ):

I will try to illustrate the different aspects using some examples from the library world. The article is rather long, because of the nature of the subject, then again the individual sections are a bit short. But I do supply a lot of links for further reading.

Data is relationships
The important thing is that “data is relationships“, as Tim Berners Lee says in his recent presentation for TED.
Before going into relationships between things, I have to point out the important distinction between abstract concepts and real life things, which are “manifestations” of the concepts. In Object modeling these are called “classes” (abstract concepts, types of things) and “objects” (real life things, or “instances” of “classes“).

Examples:

• the class book can have the instances/objects “Cloud Atlas“, “Moby Dick“, etc.
• the class person can have the instances/objects “David Mitchell“, “Herman Melville“, etc.

In the Semantic Web/RDF model the concept of triples is used to describe a relationship between two things: subject – predicate – object, meaning: a thing has a relation to another thing, in the broadest sense:

• a book (subject) is written by (predicate) a person (object)

You can also reverse this relationship:

• a person (subject) is the author of (predicate) a book (object)

Triple

The person in question is only an author because of his or her relationship to the book. The same person can also be a mother of three children, an employee of a library, and a speaker at a conference.
Moreover, and this is important: there can be more than one relationship between the same two classes or types of things. A book (subject) can also be about (predicate) a person (object). In this case the person is a “subject” of the book, that can be described by a “keyword”, “subject heading”, or whatever term is used. A special case would be a book, written by someone about himself (an autobiography).

The problem with most legacy systems, and library catalogues as an example of these, is that a record for let’s say a book contains one or more fields for the author (or at best a link to an entry in an authority file or thesaurus), and separately one or more fields for subjects. This way it is not possible to see books written by an author and books about the same author in one view, without using all kinds of workarounds, link resolvers or mash-ups.
Using two different relationships that link to the same thing would provide for an actual view or representation of the real world situation.

Another important option of Linked Data/RDF: a certain thing can have as a property a link to a concept (or “class”) , describing the nature of the thing: “object Cloud Atlas” has type “book“; “object David Mitchell” has type “person“; “object Cloud Atlas” is written by “object David Mitchell“.

And of course, the property/relationship/predicate can also link to a concept describing the nature of the link.

Anywhere on the web

ERD

So far so good. But you may argue that this relationship theory is not very new. Absolutely right, but up until now this data-relationship concept has mainly been used with a view to the inside, focused on the area of the specific information system in question, because of the nature and the limitations of the available technology and infrastructure.

The “triple” model is of course exactly the same as the long standing methodology of Entity Relationship Diagrams (ERD), with which relationships between entities (=”classes“) are described. An ERD is typically used to generate a database that contains data in a specific information system. But ERD’s could just as well be used to describe Linked Data on the web.

Information systems, such as library catalogs, have been, and still are, for the greatest part closed containers of data, or “silos” without connections between them, as Tim Berners Lee also mentions in his TED presentation.

Lots of these silo systems are accessible with web interfaces, but this does not mean that items in these closed systems with dedicated web front ends can be linked to items in other databases or web pages. Of course these systems can have API‘s that allow system developers to create scripts to get related information from other systems and incorporate that external information in the search results of the calling system. This is what is being done in web 2.0 with so-called mash-ups.
But in this situation you need developers who know how to make scripts using specific scripting languages for all the different proprietary API’s that are being supported for all the individual systems.
If Linked Data was a global standard and all open and closed systems and websites supported RDF, then all these links would be available automatically to RDF enabled browser and client software, using SPARQL, the RDF Query Language.

• Linked Data/RDF can be regarded as a universal API.

The good thing about Linked Data is, that it is possible to use Linked Data mechanisms to link to legacy data in silo databases. You just need to provide an RDF wrapper for the legacy system, like has been done with the Library of Congress Subject Headings.

Some examples of available tools for exposing legacy data as RDF:

• Triplify – a web applications plugin that converts relational database structures into RDF triples
• D2R Server – a tool for publishing relational databases on the Semantic Web
• wp-RDFa – a wordpress plugin that adds some RDF information about Author and Title to WordPress blog posts

Of course, RDF that is generated like this will very probably only expose objects to link TO, not links to RDF objects external to the system.

Also, Linked Data can be used within legacy systems, for mixing legacy and RDF data, open and closed access data, etc. In this case we have RDF triples that have a subject URI from one data source and an object URI from another data source. In a situation with interlinked systems it would for instance be possible to see that the author of a specific book (data from a library catalog) is also speaking at a specific conference (data from a conference website). Objects linked together on the web using RDF triples are also known as an “RDF graph”. With RDF-aware client software it is possible to navigate through all the links to retrieve additional information about an object.

Linked Data

URI’s
URI’s (“Uniform Resource Identifiers”) are necessary for uniquely identifying and linking to resources on the web. A URI is basically a string that identifies a thing or resource on the web. All “Information Resources”, or WWW pages, documents, etc. have a URI, which is commonly known as a URL (Uniform Resource Locator).

With Linked Data we are looking at identifying “Non-information Resources” or “real world objects” (people, concepts, things, even imaginary things), not web pages that contain information about these real world objects. But it is a little more complicated than that. In order to honour the requirement that a thing and its relations can be interpreted and used by humans and software, we need at least 3 different representations of one resource (see: How to publish Linked Data on the web):

• Resource identifier URI (identifies the real world object, the concept, as such)
• RDF document URI (a document readable for semantic web applications, containing the real world object’s RDF data and relationships with other objects)
• HTML document URI (a document readable for humans, with information about the real world object)

Redirection

For instance, there could be a Resource Identifier URI for a book called “Cloud Atlas“. The web resource at that URI can redirect an RDF enabled browser to the RDF document URI, which contains RDF data describing the book and its properties and relationships. A normal HTML web browser would be redirected to the HTML document URI, for instance a web page about the book at the publisher’s website.

There are several methods of redirecting browsers and application to the required representation of the resource. See Cool URIs for the Semantic Web for technical details.

There are also RDF enabled browsers that transform RDF into web pages readable by humans, like the FireFox addon “Tabulator“, or the web based Disco and Marbles browsers, both hosted at the Free University Berlin.

RDF, vocabularies, ontologies
RDF or Resource Description Framework, is, like the name suggests, just a framework. It uses XML (or a simpler non-XML method N3) to describe resources by means of relationships. RDF can be implemented in vocabularies or ontologies, which are sets of RDF classes describing objects and relationships for a given field.
Basically, anybody can create an RDF vocabulary by publishing an RDF document defining the classes and properties of the vocabulary, at a URI on the web. The vocabulary can then be used in a resource by referring to the namespace (the URI) and the classes in that RDF document.

A nice and useful feature of RDF is that more than one vocabularies can be mixed and used in one resource.
Also, a vocabulary itself can reference other vocabularies and thereby inherit well established classes and properties from other RDF documents.
Another very useful feature of RDF is that objects can be linked to similar object resources describing the same real world thing. This way confusion about which object we are talking about, can be avoided.

A couple of existing and well used RDF vocabularies/ontologies:

• RDF – the base RDF vocabulary
• RDFS (for RDF Schema)
• DC (for Dublin Core)
• FOAF (for FOAF- Friend of a Friend) – online identities and social networks
• SKOS (for SKOS – Simple Knowledge Organisation System) – thesauri, classification schemes, subject heading systems and taxonomies
• OWL (for OWL -Ontology Web Language)

(By the way,  the links in the first column (to the RDF files themselves) may act as an illustration of the redirection mechanism described before. Some of them may link to either the RDF file with the vocabulary definition itself, or to a page about the vocabulary, depending on the type of browser you use: rdf-aware or not.)

A special case is:

• RDFa – a sort of microformat without a vocabulary of its own, which relies on other vocabularies for turning XHTML page attributes into RDF

Example
A shortened example for “Cloud Atlas” by David Mitchell from the RDF BookMashup at the Free University Berlin, which uses a number of different vocabularies:

A partial view on this RDF file with the Marbles browser:

RDF browser view

See also the same example in the Disco RDF browser.

Library implementations
It seems obvious that Linked Data can be very useful in providing a generic infrastructure for linking data, metadata and objects, available in numerous types of data stores, in the online library world. With such a networked online data structure, it would be fairly easy to create all kinds of discovery interfaces for bibliographic data and objects. Moreover, it would also be possible to link to non-bibliographic data that might interest the users of these interfaces.

A brief and incomplete list of some library related Linked Data projects, some of which already mentioned above:

• RDF BookMashup – Integration of Web 2.0 data sources like Amazon, Google or Yahoo into the Semantic Web.
• Library of Congress Authorities – Exposing LoC Autorities and Vocabularies to the web using URI’s
• DBPedia – Exposing structured data from WikiPedia to the web
• LIBRIS – Linked Data interface to Swedish LIBRIS Union catalog
• Scriblio+Wordpress+Triplify – “A social, semantic OPAC Union Catalogue”

And what about MARC, AACR2 and RDA? Is there a role for them in the Linked Data environment? RDA is supposed to be the successor of AACR2 as a content standard that can be used with MARC, but also with other encoding standards like MODS or Dublin Core.
The RDA Entity Relationship Diagram, that incorporates FRBR as well, can of course easily be implemented as an RDF vocabulary, that could be used to create a universal Linked Data library network. It really does not matter what kind of internal data format the connected systems use.