Publishing LOD with a bent toward archivists



eye candy by Eric

This essay provides an overview of linked open data (LOD) with a bent towards archivists. It enumerates a few advantages the archival community has when it comes to linked data, as well as some distinct disadvantages. It demonstrates one way to expose EAD as linked data through the use of XSLT transformations and then through a rudimentary triple store/SPARQL endpoint combination. Enhancements to the linked data publication process are then discussed. The text of this essay in the form of a handout as well as a number of support files is can also be found at http://infomotions.com/sandbox/lodlamday/.

Review of RDF

The ultimate goal of LOD is to facilitate the discovery of new information and knowledge. To accomplish this goal, people are expected to make metadata describing their content available on the Web in one or more forms of RDF — Resource Description Framework. RDF is not so much a file format as a data structure. It is a collection of “assertions” in the form of “triples” akin to rudimentary “sentences” where the first part of the sentence is a “subject”, the second part is a “predicate”, and the third part is an “object”. Both the subjects and predicates are required to be Universal Resource Identifiers — URIs. (Think “URLs”.) The subject URI is intended to denote a person, place, or thing. The predicate URI is used to specify relationships between subjects and the objects. When verbalizing RDF assertions, it is usually helpful to prefix predicate URIs with a “is a” or “has a” phrase. For example, “This book ‘has a’ title of ‘Huckleberry Finn'” or “This university ‘has a’ home page of URL”. The objects of RDF assertions are ideally more URIs but they can also be “strings” or “literals” — words, phrases, numbers, dates, geo-spacial coordinates, etc. Finally, it is expected that the URIs of RDF assertions are shared across domains and RDF collections. By doing so, new assertions can be literally “linked” across the world of RDF in the hopes of establishing new relationships. By doing so new new information and new knowledge is brought to light.

Simple foray into publishing linked open data

Manifesting RDF from archival materials by hand is not an easy process because nobody is going to manually type the hundreds of triples necessary to adequately describe any given item. Fortunately, it is common for the description of archival materials to be manifested in the form of EAD files. Being a form of XML, valid EAD files must be well-formed and conform to a specific DTD or schema. This makes it easy to use XSLT to transform EAD files into various (“serialized”) forms of RDF such as XML/RDF, turtle, or JSON-LD. A few years ago such a stylesheet was written by Pete Johnston for the Archives Hub as a part of the Hub’s LOCAH project. The stylesheet outputs XML/RDF and it was written specifically for Archives Hub EAD files. It has been slightly modified here and incorporated into a Perl script. The Perl script reads the EAD files in a given directory and transforms them into both XML/RDF and HTML. The XML/RDF is intended to be read by computers. The HTML is intended to be read by people. By simply using something like the Perl script, an archive can easily participate in LOD. The results of these efforts can be seen in the local RDF and HTML directories. Nobody is saying the result is perfect nor complete, but it is more than a head start, and all of this is possible because the content of archives is often times described using EAD.

Triple stores and SPARQL endpoints

By definition, linked data (RDF) is structured data, and structured data lends itself very well to relational database applications. In the realm of linked data, these database applications are called “triple stores”. Database applications excel at the organization of data, but they are also designed to facilitate search. In the realm of relational databases, the standard query language is called SQL, and there is a similar query language for triples stores. It is called SPARQL. The term “SPARQL endpoints” is used denote a URL where SPARQL queries can be applied to a specific triple store.

4store is an open source triple store application which also supports SPARQL endpoints. Once compiled and installed, it is controlled and managed through a set of command-line applications. These applications support the sorts of things one expects with any other database application such as create database, import into database, search database, dump database, and destroy database. Two other commands turn on and turn off SPARQL endpoints.

For the purposes of LODLAM Training Day, a 4store triple store was created, filled with sample data, and made available as a SPARQL endpoint. If it has been turned on, then the following links ought to return useful information and demonstrating additional ways of publishing linked data:

Advantages and disadvantages

The previous sections demonstrate the ease at which archival metadata can be published as linked data. These demonstrations are not the the be-all nor end-all of linked data the publication process. Additional techniques could be employed. Exploiting content negotiation in response to a given URI is an excellent example. Supporting alternative RDF serializations is another example. It behooves the archivist to provide enhanced views of the linked data, which are sometimes called “graphs”. The linked data can be combined with the linked data of other publishers to implement even more interesting services, views, and graphs. All of these things are advanced techniques requiring the skills of additional people (graphic designers, usability experts, computer programmers, systems administrators, allocators of time and money, project managers, etc.). Despite this, given the tools outlined above, it is not too difficult to publish linked data now and today. Such are the advantages.

On the other hand, there are at least two distinct disadvantages. The most significant derives from the inherent nature of archival material. Archival material is almost always rare or unique. Because it is rare and unique, there are few (if any) previously established URIs for the people and things described in archival collections. This is unlike the world of librarianship, where the materials of libraries are often owned my multiple institutions. Union catalogs share authority lists denoting people and institutions. Shared URIs across domains is imperative for the idea of the Semantic Web to come to fruition. The archival community has no such collection of shared URIs. Maybe the community-wide implementation and exploitation of Encoded Archival Context for Corporate Bodies, Persons, and Families (EAC-CPF) can help resolve this problem. After all, it too is a form of XML which lends itself very will to XSLT transformation.

Second, and almost as importantly, the use of EAD is not really the best way manifest archival metadata for linked data publication. EADs are finding aids. They are essentially narrative essays describing collections as a whole. They tell stories. The controlled vocabularies articulated in the header do not necessarily apply to each of the items in the container list. For good reasons, the items in the container list are minimally described. Consequently, the resulting RDF statement come across rather thin and poorly linked to fuller descriptions. Moreover, different archivists put different emphases on different aspect of EAD description. This makes amalgamated collections of archival linked data difficult to navigate; the linked data requires cleaning and normalization. The solution to these problems might be to create and maintain archival collections in database applications, such as ArchivesSpace, and have linked data published from there. By doing so the linked data publication efforts of the archival community would be more standardized and somewhat centralized.

Summary

This essay has outlined the ease at which archival metadata in the form of EAD can be easily published as linked data. The result is far from perfect, but a huge step in the right direction. Publishing linked data is not an event, but rather an iterative process. There is always room for improvement. Starting today, publish your metadata as linked data.

Published by

Eric Lease Morgan

Artist- and Librarian-At-Large