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The recent OAI-ORE meeting was just up the road in Baltimore, so it was easy for a bunch of us from the Library of Congress to attend. I work on a team at LC that is specifically looking at the role that repositories play at the library; I’ve implemented OAI-PMH data providers and harvesters, and in the past couple of years I’ve gotten increasingly interested in semantic web technologies — so OAI-ORE is of particular interest to me. I’ve commented a bit about OAI-ORE on here before, but I figure it can’t hurt to follow in my coworker’s footsteps and summarize my thoughts after the meeting.

(BTW, above is an image of some constellations I nabbed off of wikipedia. I included it here because the repeated analogy (during the meeting) of OAI-ORE resource maps as constellations was really compelling — and quite poetic.)

The Vocabulary

It seems to me that the real innovation of the OAI-ORE effort is that it provides a lightweight RDF vocabulary for talking about aggregated resources on the web. Unfortunately I think that this kernel gets a little bit lost in the 6 specification documents that were released en masse a few months ago.

The ORE vocabulary essentially consists of three new resource types: ore:ResourceMap, ore:Aggregation, ore:AggregatedResource ; and 5 new properties to use with those types: ore:describes, ore:isDescribedBy, ore:aggregates, ore:isAggregatedBy, ore:analogousTo. In addition, the Vocabulary document
provides guidance on how to use a few terms from the DublinCore vocabulary: dc:creator, dc:rights, dcterms:modified, dcterms:created.

The vocabulary is small, so if I were them I would publish the vocabulary elements using hash URIs, instead of slash URIs. The reason for this is that you don’t have to jigger the web server to do a httpRange-14 style 303 correctly:

• http://www.openarchives.org/ore/0.2/terms#Aggregation
• http://www.openarchives.org/ore/0.2/terms#AggregatedResource
• http://www.openarchives.org/ore/0.2/terms#ResourceMap
• http://www.openarchives.org/ore/0.2/terms#describes
• http://www.openarchives.org/ore/0.2/terms#isDescribedBy
• http://www.openarchives.org/ore/0.2/terms#aggregates
• http://www.openarchives.org/ore/0.2/terms#isAggregatedBy
• http://www.openarchives.org/ore/0.2/terms#analogousTo

Also, I think ore:AggregatedResource is currently missing from the rdf/xml vocabulary, so it should be added. Also ore:isDescribedBy seems to be commented out.

There is a lot of redundancy between the Abstract Data Model and the Vocabulary documents–so I would recommend collapsing them down into a single, succinct document. This is in keeping with the DRY principle and will have the added benefit of making it easier for newbies to hit the ground running (not having to wade through multiple docs and mentally reconcile them). I could understand having a separate Abstract Data Model document if it were totally divorced from the web and semantic web technologies like RDF, but it’s not.

The Graph

The OAI-ORE effort seemed to be mostly driven by a desire to take harvesting agents the last mile to the actual repository resources themselves–enabling digital library objects (in addition to their metadata) to be harvested from repositories (using HTTP) ; and to be referenced from other contexts (say objects in other repositories). This desire was born out of real, hard won experience with harvesting metadata records, and marked a shift from metadata-centric harvesting to resource-centric harvesting.

In addition OAI-ORE marks a departure from predictable and mind-numbing arguments about SIP formats (METS, DIDL, FOXML, IEEE LOM, XFDU, etc). Yet as soon as we have our shiny new OAI-ORE vocabulary we have to learn yet-another-packaging-format, this time one built on top of Atom.

First, let me just say I’m a big fan of RFC 4287, in particular how it is used in the RESTful Atom Publishing Protocol (RFC 5023). I also think it makes sense to have an Atom serialization for OAI-ORE resource maps — assuming there is a GRDDL transform for turning it into RDF. But the workshop in Baltimore seemed to stress that the Atom serialization was the only way to do OAI-ORE, and didn’t emphasize that there are in fact lots of ways of representing RDF graphs on the web. For example GRDDL allows you to associate arbitrary XML with an XSLT transform to extract a RDF graph. And you could encode your RDF graph directly with RDFa, N3, Turtle, ntriples, or RDF/XML.

Perhaps there is a feeling that stressing the RDF graph too much will alienate some people who are more familiar with XML technologies. Or perhaps all these graph serialization choices could be perceived as being too overwhelming. But I think the opposite extreme of making it look like you can only use an overloaded Atom document as a means to publishing ORE resource maps is misguided, and will ultimately slow adoption. Why not encourage people to publish GRDDL transforms for METS, DIDL or mark up their “splash pages” with RDFa? This would bring the true value of the OAI-ORE work home–it’s not about yet-another-packaging format, it’s about what the various packaging formats have in common on the web.

Release Early, Release Often

In hindsight I think it would’ve been helpful for the OAI-ORE group to privately build consensus about the core OAI-ORE vocabulary (if necessary), then release that into the world wild web for discussion. Then once the kinks were worked out, and there was general understanding, moving on to issues such as discovery and serialization. As it stands the various documents were all dumped at the same time, and seem somewhat fragmented, and in places redundant. Clearly a lot of conversations have gone on that aren’t happening on the public discussion list.

I expressed interest in being part of the OAI-ORE and was politely turned down. I’m actually kind of glad really because I also don’t want to be part of some cabal of digital library practitioners. Maybe I should’ve titled this post “Sour Grapes” :-) Seriously though, the digital library needs good practical solutions and communities of users that encourage widespread adoption and tool support. We don’t need research-ware. Having secret discussions and occasional public events that feel more like lectures than meetings isn’t a good way to encourage adoption.

Anyhow, I hope that this isn’t all seen as being too harsh. Everyone’s a critic eh? All in all there is a lot in OAI-ORE to be proud of. The effort to integrate Web Architecture into Digital Library practices is most welcome indeed. Keep up the good work y’all.

The OAI-ORE meeting is coming up, and in general I’ve been really impressed with the alpha specs that have come out. It’s not clear that there’s an established vocabulary for talking about aggregated resources on the web, so the Data Model and Vocabulary documents were of particular interest to me.

One thing I didn’t quite understand, and which I think may have some significance for implementors, is some language in the Discovery document on the subject of URI conflation:

The Data Model document [ORE Model] explicitly prohibits a URI of a ReM (URI-R) ever returning anything other than a ReM. This allows multiple representations to be associated with URI-R, such as using content negotiation to return ReMs in different languages, character sets, or compression encodings. But it does not allow URI-R to return a human readable “splash page”, either by HTTP content negotiation or redirection. For example, clients MUST NOT merge with content negotiation the following URI pair that would correspond to a ReM and a “splash page” for an object:

If I’m understanding right this would prohibit using technologies like microformats, eRDF, RDFa and GRDDL in a “splash page” to represent the resource map. It seems odd to me that you can represent a resource map in Atom, but not in HTML.

To illustrate what this might look like I took a splash page off of arXiv (hope that was ok!) and marked it up with oai-ore RDFa.

Take a look. So all I did is modify the existing XHTML at arxiv.org, and I’ve been able to represent an ORE Resource Map. This seems like a relatively simple, and powerful way for existing repositories to make their aggregated resources available.

RDFa just entered Last Call, but there are already multiple implementations. Try out the GetN3 bookmarklet on the splash page, and you should see some triples come back. I ran them through the validator at w3c and got the following graph (kinda too big to include here inline).

This kind of issue seem to be at the heart of what Ian Davis refers to when he asks “Is the Semantic Web Destined to be a Shadow?“. Andy Powell and Pete Johnston have also been strong voices for integrating digital library repositories and the web–and they are also involved with the oai-ore effort. It feels like some of the oia-ore language could be loosened a bit to allow machine readable and human readable information to commingle a bit more.

In case you missed it the Object-Reuse-and-Exchange (ORE) folks are having a get together at Johns Hopkins University (Baltimore, MD) on March 3, 2008. It’s free to register, but space is limited. The Compound information objects whitepaper, May 2007 Technical Committee notes and the more recent Interoperability for the Discovery, Use, and Re-Use of Units of Scholarly Communication provide a good taste of what the beta ORE specs are likely to look like.

The ORE group isn’t small, and includes individuals from quite different organizations. So any consensus that can be garnered I think will be quite powerful. Personally I’ve been really pleased to see how much the ORE work is leaning on web architecture: notably resolvable HTTP URIs, content-negotiation, linked-data and named graphs. Also interesting in the recent announcement is that the initial specs will use RFC 4287 for encoding the data model. Who knows, perhaps the spec will rely on archive feeds as discussed recently on the code4lib discussion list.

I’m particularly interested to see what flavor of URIs are used to identify the compound objects:

The protocol-based URI of the Resource Map identifies an aggregation of resources (components of a compound object) and their boundary-type inter-relationships. While this URI is clearly not the identifier of the compound object itself, it does provide an access point to the Resource Map and its representations that list all the resources of the compound object. For many practical purposes, this protocol-based URI may be a handy mechanism to reference the compound object because of the tight dependency of the visibility of the compound object in web space on the Resource Map (i.e., in ORE terms, a compound object exists in web space if and only if there exists a Resource Map describing it).

We note, however, two subtle points regarding the use of the URI of the Resource Map to reference the compound object. First, doing so is inconsistent with the web architecture and URI guidelines that are explicit in their suggestion that a URI should identify a single resource. Strictly interpreted, then, the use of the URI of the Resource Map to identify both the Resource Map and the compound object that it describes is incorrect. Second, some existing information systems already use dedicated URIs for the identification of compound information objects “as a whole.” For example, many scholarly publishers use DOIs whereas the Fedora and aDORe repositories have adopted identifiers of the info URI scheme. These identifiers are explicitly distinct from the URI of the Resource Map. from: Interoperability for the Discovery, Use, and Re-Use of Units of Scholarly Communication

I understand the ORE group is intentionally not aligning themselves too closely with the semantic web community. However I think they need to consider whether compound information objects are WWW information resources or not:

By design a URI identifies one resource. We do not limit the scope of what might be a resource. The term “resource” is used in a general sense for whatever might be identified by a URI. It is conventional on the hypertext Web to describe Web pages, images, product catalogs, etc. as “resources”. The distinguishing characteristic of these resources is that all of their essential characteristics can be conveyed in a message. We identify this set as “information resources.” (from Architecture of the World Wide Web vol. 1).

I’m not totally convinced that the resource map can’t serve as a suitable representation for the compound information object–however for the sake of argument lets say I am. It seems to me that the URI for the compound information object identifies the concept of a particular compound information object, which lies in various pieces on the network. However this doesn’t preclude the use of HTTP URLs to identify the compound objects. Indeed the What HTTP URIs identify and Cool URIs for the Semantic Web provide specific guidance on how to serve up these non-information resources. Of course philosophical arguments around httpRange-14 have raged for a while. But the Linking Open Data project is using the hash URI and 303 redirect very effectively. There has even been some work on a sitemap extension to enable crawling. As a practical matter using URLs to identify compound information objects will encourage their use because they will naturally find their ways into publications, blogs, other compound objects. Using non-resolvable or quasi-resolvable info-uris or dois will mean people just won’t create the links–and when they do they will create links that can’t be easily verified and evolved over time with standard web tools. The OAI-ORE effort represents a giant leap forward for the digital library community into the web. Here’s to hoping they land safely–we need this stuff.