Posted on June 19th, 2009 8 commentsLinked Data and bibliographic metadata models
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):Linked Data is a methodology for providing relationships between things (data, concepts and documents) anywhere on the web, using URI’s for identifying, RDF for describing and HTTP for publishing these things and relationships, in a way that they can be interpreted and used by humans and software.
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“).
- 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)
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
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.
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)
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:
<?xml version=”1.0″ encoding=”UTF-8″ ?>
- RDFa – a sort of microformat without a vocabulary of its own, which relies on other vocabularies for turning XHTML page attributes into RDF
<dc:publisher>Random House Trade Paperbacks</dc:publisher>
<dc:title>Cloud Atlas: A Novel</dc:title>
<rdfs:label>Cloud Atlas: A Novel</rdfs:label>
<rdfs:label>RDF document about the book: Cloud Atlas: A Novel</rdfs:label>
<rdfs:label>Review number 1 about: Cloud Atlas: A Novel</rdfs:label>
<rdfs:label>RDF Book Mashup</rdfs:label>
A partial view on this RDF file with the Marbles browser:
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.
Posted on May 15th, 2009 22 comments
Why use a non-normalised metadata exchange format for suboptimal data storage?
This week I had a nice chat with André Keyzer of Groningen University library and Peter van Boheemen of Wageningen University Library who attended OCLC’s Amsterdam Mashathon 2009. As can be expected from library technology geeks, we got talking about bibliographic metadata formats, very exciting of course. The question came up: what on earth could be the reason for storing bibliographic metadata in exchange formats like MARC?
Exactly my idea! As a matter of fact I think I may have used the same words a couple of times in recent years, probably even at ELAG2008. The thing is: it really does not matter how you store bibliographic metadata in your database, as long as you can present and exchange the data in any format requested, be it MARC or Dublin Core or anything else.
Of course the importance of using internationally accepted standards is beyond doubt, but there clearly exists widespread misunderstanding of the functions of certain standards, like for instance MARC. MARC is NOT a data storage format. In my opinion MARC is not even an exchange format, but merely a presentation format.
With a background and experience in data modeling, database and systems design (among others), I was quite amazed about bibliographic metadata formats when I started working with library systems in libraries, not having a librarian training at all. Of course, MARC (“MAchine Readable Cataloging record“) was invented as a standard in order to facilitate exchange of library catalog records in a digital era.
But I think MARC was invented by old school cataloguers who did not have a clue about data normalisation at all. A MARC record, especially if it corresponds to an official set of cataloging rules like AARC2, is nothing more than a digitised printed catalog card.
In pre-computer times it made perfect sense to have a standardised uniform way of registering bibliographic metadata on a printed card in this way. The catalog card was simultaneously used as a medium for presenting AND storing metadata. This is where the confusion originates from!
But when the Library of Congress says “If a library were to develop a “home-grown” system that did not use MARC records, it would not be taking advantage of an industry-wide standard whose primary purpose is to foster communication of information” it is saying just plain nonsense.
Actually it is better NOT to use something like MARC for other purposes than exchanging, or better, presenting data. To illustrate this I will give two examples of MARC tags that have been annoying me since my first day as a library employee:
- 100 – Main Entry-Personal Name (NR) – subfield $a – Personal name (NR)
- 773 – Host Item Entry (R) – subfield $g – Relationship information (R)
100 – Main Entry-Personal Name
Besides storing an author’s name as a string in each individual bibliographic record instead of using a code, linking to a central authority table (“foreign key” in relational database terms), it is also a mistake to use a person’s name as one complete string in one field. Examples on the Library of Congress MARC website use forms like “Adams, Henry”, “Fowler, T. M.” and “Blackbeard, Author of”. To take only the simple first example, this author could also be registered as “Henry Adams”, “Adams, H.”, “H. Adams”. And don’t say that these forms are not according to the rules! They are out there! There is no way to match these variations as being actually one and the same.
In a normalised relational database, this subfield $a would be stored something like this (simplified!):
- First name=Henry
773 – Host Item Entry
Subfield $g of this MARC tag is used for storing citation information for a journal article, volume, issue, year, start page, end page, all in one string, like: “Vol. 2, no. 2 (Feb. 1976), p. 195-230“. Again I have seen this used in many different ways. In a normalised format this would look something like this, using only the actual values:
- Start page=195
- End page=230
In a presentation of this normalised data record extra text can be added like “Vol.” or “Volume“, “Issue” or “No.“, brackets, replacing codes by descriptions (Month 2 = Feb.) etc., according to the format required. So the stored values could be used to generate the text “Vol. 2, no. 2 (Feb. 1976), p. 195-230” on the fly, but also for instance “Volume 2, Issue 2, dated February 1976, pages 195-230“.
The strange thing with this bibliographic format aimed at exchanging metadata is that it actually makes metadata exchange terribly complicated, especially with these two tags Author and Host Item. I can illustrate this with describing the way this exchange is handled between two digital library tools we use at the Library of the University of Amsterdam, MetaLib and SFX , both from the same vendor, Ex Libris.
The metasearch tool MetaLib is using the described and preferred mechanism of on the fly conversion of received external metadata from any format to MARC for the purpose of presentation.
But if we want to use the retrieved record to link to for instance a full text article using the SFX link resolver, the generated MARC data is used as a source and the non-normalised data in the 100 and 773 MARC tags has to be converted to the OpenURL format, which is actually normalised (example in simple OpenUrl 0.1):
isbn=;issn=0927-3255;date=1976; volume=2;issue=2;spage=195;epage=230; aulast=Adams;aufirst=Henry;auinit=;
In order to do this all kinds of regular expressions and scripting functions are needed to extract the correct values from the MARC author and citation strings. Wouldn’t it be convenient, if the record in MetaLib would already have been in OpenURL or any other normalised format?
The point I am trying to make is of course that it does not matter how metadata is stored, as long as it is possible to get the data out of the database in any format appropriate for the occasion. The SRU/SRW protocol is particularly aimed at precisely this: getting data out of a database in the required format, like MARC, Dublin Core, or anything else. An SRU server is a piece of middleware that receives requests, gets the requested data, converts the data and then returns the data in the requested format.
Currently at the Library of the University of Amsterdam we are migrating our ILS which also involves converting our data from one bibliographic metadata format (PICA+) to another (MARC). This is extremely complicated, especially because of the non-normalised structure of both formats. And I must say that in my opinion PICA+ is even the better one.
Also all German and Austrian libraries are meant to migrate from the MAB format to MARC, which also seems to be a move away from a superior format.
All because of the need to adhere to international standards, but with the wrong solution.
Maybe the projected new standard for resource description and access RDA will be the solution, but that may take a while yet.