Talis Platform

Serving Linked Data

Posted on by petej

Back near the start of the project, I published a post outlining the processes involved in generating the Archives Hub RDF dataset and serving up “Linked Data” descriptions from that dataset; it’s perhaps best summarised in the following diagram from that post:

Diagram showing process of transforming EAD to RDF and exposing as Linked Data

In this post, I’ll say a little bit more about what is involved in the “Expose” operation up in the top right of the diagram.

Cool URIs for the Semantic Web

In an earlier post, I discussed the URI patterns we are using for the URIs of “things” described in our data (archival resources, concepts, people, places, and so on). One of the core requirements for exposing our RDF data as Linked Data is that, given one of these URIs, a user/consumer of that URI can use the HTTP protocol to “look up” that URI and obtain a description of the thing identified by that URI. So as providers of the data, our challenge is to enable our HTTP server to respond to such requests and provide such descriptions.

The W3C Note Cool URIs for the Semantic Web lists a number of possible “recipes” for achieving this while also paying attention to the principle of avoiding URI ambiguity i.e. of avoiding using a single URI to refer to more than one resource – and in particularly to maintaining a distinction between the URI of a “thing” and the URIs of documents describing that thing.

Document URI Patterns

Within the JISCExpo programme which funds LOCAH, projects generating Linked Data were encouraged to make use of the guidelines provided by the UK Cabinet Office in Designing URI Sets for the UK Public Sector.

Thse guidelines refer to the URIs used to identify “things” (somewhat tautologically, it seems to me!) as “Identifier URIs”, where they have the general pattern:

http://{domain}/id/{concept}/{reference}

where:

  • concept is a name for a resource type, like “person”;
  • reference is a name for an individual instance of that type or class

(The guidelines also allow for the option of using URIs with fragment identifiers (“Hash URIs”) as “Identifier URIs”.)

The document also recommends patterns for the URIs of the documents which provide information about these “things”, “Document URIs”:

http://{domain}/doc/{concept}/{reference}

These documents are, I think, what Berners-Lee calls Generic Resources. For each such document, multiple representations may be available, each in different formats, and each of those multiple “more specific” documents in a single concrete format may be available as a separate resource in its own right. So a third set of URIs, “Representation URIs,” name documents in a specific format, using the suggested pattern:

http://{domain}/doc/{concept}/{reference}/{doc.file-extension}

i.e. for each “thing URI”/”Identifier URI” in our data, like:

http://data.archiveshub.ac.uk/id/person/ncarules/skinnerbeverley1938-1999artist, which identifies a person, the artist Beverley Skinner;

there is a corresponding “Document URI” which identifies a (“generic”) document describing the thing:

http://data.archiveshub.ac.uk/doc/person/ncarules/skinnerbeverley1938-1999artist

and a set of “Representation URIs” each identifying a (“specific”) document in a particular format:

http://data.archiveshub.ac.uk/doc/person/ncarules/skinnerbeverley1938-1999artist.html, which identifies an HTML document;

http://data.archiveshub.ac.uk/doc/person/ncarules/skinnerbeverley1938-1999artist.rdf, which identifies an RDF/XML document;

http://data.archiveshub.ac.uk/doc/person/ncarules/skinnerbeverley1938-1999artist.turtle, which identifies a Turtle document;

http://data.archiveshub.ac.uk/doc/person/ncarules/skinnerbeverley1938-1999artist.json, which identifies a JSON document (more specifically one using Talis’ RDF/JSON conventions for serializing RDF)

(We’ve deviated slightly from the recommended pattern here in that we just add “.{extension}” to the “reference” string, rather than adding “/doc.{extension}”, but we’ve retained the basic approach of distinguishing generic document and documents in specific formats, which I think is the significant aspect of the recommendations.)

This set of URI patterns corresponds to those used in the “recipe” described in section 4.2 of the W3C Cool URIs note, “303 URIs forwarding to One Generic Document”.

The Talis Platform

It is perhaps worth emphasising here that in the LOCAH case a “description” of any one of the things in our model may contain data which originated in multiple EAD documents e.g. a description of a concept may contain links to multiple archival resources with which it is associated, or a description of a repository may contain links to multiple finding aids they have published, and so on. A description may also contain data which originated from a source other than the EAD documents: for example, we add some postcode data provided by the National Archives, and most of the links to external resources, such as people described by VIAF records, are generated by post-transformation processes.

This aggregated RDF data – the output of the EAD-to-RDF transformation process and this additional data – is stored in an instance of the Talis Platform store. Simplifying things slightly, the Platform store is a “database” specialised for the storage and retieval of RDF data. It is hosted by Talis, and made avalable as what in cloud computing terms is referred to as “Software as a Service” (SaaS). (Actually, a Platform store allows the storage of content other than RDF data too – see the discussion of the ContentBox and MetaBox features in the Talis documentation – but we are, currently at least, making use only of the MetaBox facilities).

Access to the store is provided through a Web API. Using the MetaBox API, data can be added/uploaded to the MetaBox using HTTP POST, updates can be applied through what Talis call “Changesets” (essentially “remove that set of triples” and “add this set of triples”) again using HTTP POST, and “bounded descriptions” of individual resources can be retrieved using HTTP GET. There are also “admin” functions like “give me a dump of the contents” and “clear the database”. In addition, the Platform provides a simple full-text search over literals (which returns result sets in RSS), a configurable faceted search, an “augment” function and a SPARQL endpoint.

A number of client software libraries for working with the Platform are available, developed either by Talis staff or by developers who have worked with the Platform.

Delivering Linked Data from the Platform

I’m going to focus here on retrieving data from the MetaBox, and more specifically retrieving the “bounded descriptions” of individual resources which which provide the basis for the “Linked Data” documents.

This process involves a small Web application which responds to HTTP GET requests for these URIs:

  • For an “Identifier URI”, the server responds with a 303 status code and a Location header redirecting the client to the “Document URI”
  • For a “Document URI”, the server derives the corresponding “Identifier URI”, queries the Platform store to obtain a description of the thing identified by that URI, and responds with a 200 status code, a document in a format selected according to the preferences specified by the client (i.e. following the principles of HTTP content negotiation), and a Content-Location header providing a “Representation URI” for a document in that format.
  • For a “Representation URI”, the server derives the corresponding “Identifier URI”, queries the Platform store to obtain a description of the thing identified by that URI, and responds with a 200 status code and a document in the format associated with that URI.

The first step above is handled using a simple Apache rewrite rule. For the latter two steps, we’ve made use of the Paget PHP library created by Ian Davis of Talis for working with the Platform (Paget itself makes use of another library, Moriarty, also created by Ian). I’m sure there are many other ways of achieving this; I chose Paget in part because my software development abilities are fairly limited, but having had a quick look at the documentation and one of Ian’s blog posts, I felt there was enough there to enable me to take an example and apply my basic and rather rusty PHP skills to tweak it to make it work – at least as a short-term path to getting something functional we could “put out there”, and then polish in the future if necessary.

The main challenge was that the default Paget behaviour seemed to be to use the approach described in section 4.3 of the Cool URIs document, “303 URIs forwarding to Different Documents”, where the server performs content negotiation on the request for the “Identifier URI” and redirects directly to a “Representation URI”, i.e. a GET for an “Identifier URI” like http://data.archiveshub.ac.uk/id/person/ncarules/skinnerbeverley1938-1999artist resulted in redirects to “Representation URIs” like http://data.archiveshub.ac.uk/id/person/ncarules/skinnerbeverley1938-1999artist.html or http://data.archiveshub.ac.uk/id/person/ncarules/skinnerbeverley1938-1999artist.rdf

If possible we wanted to use the alternative “recipe” described in the previous section, and after some tweaking we managed to get something that did the job. We also made some minor changes to provide a small amount of additional “document metadata”, e.g. the publisher of and license for the document. (I do recognise that the presentation of the HTML pages is currently pretty basic, and there is room for improvement!)

Finally, it’s maybe worth noting here that the Platform store itself doesn’t contain any information about the documents i.e. neither the Document URI nor the Representation URIs appear in RDF triples loaded to the store. So, in principle at least, we could add additional formats using additional Representation URIs simply by extending the PHP to handle the URIs and generate documents in those formats, without needing to extend the data in the store.

I’d started to write more here about extending what we’ve done to provide other ways of accessing the data, but having written quite a lot here already, I think that is probably best saved for a future post.

Some thoughts on architecture and workflows

Posted on by petej

This is an attempt to sketch out some of my/our initial thoughts on the approaches the project is considering to exposing data as Linked Data. I should emphasise that these are very much initial thoughts, and things may change as we progress.

The project is dealing with two main data sources, and at the moment two different approaches are being considered to those sources.

The first data source is the collection of archival finding aids describing the holdings of the archives of educational and research institutions in the UK, aggregated by the JISC Archives Hub service. This data takes the form of XML documents in the Encoded Archival Description (EAD) format, created by archivists in the various institutions, and submitted to the Hub.

Currently, the aggregated data is indexed using the Cheshire 3 application, and exposed as HTML pages on the archiveshub.ac.uk site for search and browse. (SRU and Z39.50 targets and an OAI-PMH repository are also available.)

To expose (probably a subset of) the Hub EAD finding aids as Linked Data, the workflow is expected to look something like that represented in Figure 1 below:

Diagram showing process of transforming EAD to RDF and exposing as Linked Data (1)
  1. Transform: EAD XML documents are transformed to an RDF format. We’ll write about our current thinking on this more in a subsequent post, as working out how best to represent the EAD data in RDF as the target for the transform is in itself a significant chunk of work (and an area I’m particularly interested in). This is likely to be something of an “iterative” process: we’ll start with a fairly basic transform that captures some subset of the content of the input documents, and perhaps refine things later to generate more data (and correct errors we’ll no doubt make in the first cut!)
  2. Enhance: RDF data from the previous step is “enhanced” and augmented. This step might include processes to (1) generally “clean up” the data (e.g. normalise some literals, identify internal co-references etc); (ii) add links to resources in other datasets; (iii) (maybe) pull in some useful data from other datasets, either data held by the Hub but not included in the EAD docs or data from other sources. Again this will probably be a process which we extend and refine over time.
  3. Upload: Load the RDF data from the previous step to an instance of the Talis Platform triple store, which Talis are kindly making available to the project.
  4. Expose: Expose a set of linked “bounded descriptions” from the triple store over HTTP, as documents in both human-readable and RDF formats, following the principles of the W3C TAG httpRange-14 resolution/Cool URIs for the Semantic Web. The use of the Platform also provides us with a SPARQL endpoint for the data – which we can make available to others to use – and which also means we can consider layering other Web interfaces over that endpoint. For example, I’d be interested in trying out the Linked Data API, which I talked about over on eFoundations a while ago.

It may be that that the second and third steps are reversed and we upload the data to the triple store and perform the “enhance” step on the data there, i.e. something closer to Figure 2:

Diagram showing process of transforming EAD to RDF and exposing as Linked Data (2)

Or indeed that a “hybrid” of the two is appropriate, and some “enhance” processes take place before upload and others take place afterwards.

We’ll also need to integrate some provision for “version control” and “provenance”/”attribution” (e.g. to track which data comes directly from the EAD sources, and which is added from elsewhere) into this process.

So for the Hub data, the plan is that the data is “exported” from the existing EAD dataset, and that the Platform triplestore provides the “back-end” for the app that serves up the “Linked Data” document views and provides a SPARQL endpoint.

The second data source of interest is the collection of bibliographic metadata aggregated into the Copac catalogue from the member libraries of Research Libraries UK and from other specialist libraries. This data is also held as XML in the MODS XML format. (Bethan Ruddock has a couple of posts on the Copac Development blog which describe the processes by which data is transferred from the contributor libraries to the Copac catalogue).

As for the case of the Archives Hub data, the first stage will be to design an appropriate RDF representation and an algorithm for transforming the MODS data to RDF (or to select – and maybe adapt, if necessary – an existing one).

In contrast to the case of the Hub I outlined above, the plan is to serve the RDF data from the existing Copac database, rather than upload it to a triplestore. This will probably require the development of a small additional application (or maybe just the configuration of an HTTP server) to service the new URIs coined for resources, to support content negotiation and redirect to URIs of appropriate pages.

One of the questions raised by this approach is how to handle the process I described above as “enhance”, and in particular how to accommodate the addition of new data – at a minimum, links to existing resources described in other Linked Data datasets – assuming that we aren’t going to be able to update the source MODS XML documents. For some cases, it may be trivial to incorporate this in the MODS-to-RDF transform (e.g., to generate links to languages described by lexvo.org). Another approach might be to generate simple “seeAlso” links to an additional set of documents (which could be simple static documents or could be served from an RDF store). Hmm. As you can probably tell, I’ve thought about this rather less than I’ve thought about the Hub case! Anyway, the suggested approach is sketched in Figure 3:

Diagram showing process of transforming MODS to RDF and exposing as Linked Data

Another constraint of this approach would be that although we can serve the set of linked documents, it doesn’t provide a SPARQL endpoint.

One of the expectations for the project is that it “explores and reports on the opportunities and barriers in making content structured and exposed”, and an assessment of the pros and cons of the different approaches to hosting the data should contribute to that report.