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December 16, 2011

Where we are with RDFa 1.1?

English: RDFa Content Editor

Image via Wikipedia

There has been a flurry of activities around RDFa 1.1 in the past few months. Although a number of blogs and news items have been published on the changes, all those have become “officialized” only the past few days with the publication of the latest drafts, as well as with the publication of RDFa 1.1 Lite. It may be worth looking back at the past few months to have a clearer idea on what happened. I make references to a number of other blogs that were published in the past few months; the interested readers should consult those for details.

The latest official drafts for RDFa 1.1 were published in Spring 2011. However, lot has happened since. First of all, the RDFWA Working Group, working on this specification, has received a significant amount of comments. Some of those were rooted in implementations and the difficulties encountered therein; some came from potential authors who asked for further simplifications. Also, the announcement of schema.org had an important effect: indeed, this initiative drew attention on the importance of structured data in Web pages, which also raised further questions on the usability of RDFa for that usage pattern This came to the fore even more forcefully at the workshop organized by the stakeholders of schema.org in Mountain View. A new task force on the relationships of RDFa and microdata has been set up at W3C; beyond looking at the relationship of these two syntaxes, that task force also raised a number of issues on RDFa 1.1. These issues have been, by and large, accepted and handled by the Working Group (and reflected in the new drafts).

What does this mean for the new drafts? The bottom line: there have been some fundamental changes in RDFa 1.1. For example, profiles, introduced in earlier releases of RDFa 1.1, have been removed due to implementation challenges; however, management of vocabularies have acquired an optional feature that helps vocabulary authors to “bind” their vocabularies to other vocabularies, without introducing an extra burden on authors (see another blog for more details). Another long-standing issue was whether RDFa should include a syntax for ordered lists; this has been done now (see the same blog for further details).

A more recent important change concerns the usage of @property and @rel. Although usage of these attributes for RDF savy authors was never a real problem (the former is for the creation of literal objects, whereas the latter is for URI references), they have proven to be a major obstacle for ‘lambda’ HTML authors. This issue came up quite forcefully at the schema.org workshop in Mountain View, too. After a long technical discussion in the group, the new version reduces the usage difference between the two significantly. Essentially, if, on the same element, @property is present together with, say, @href or @resource, and @rel or @rev is not present, a URI reference is generated as an object of the triple. I.e., when used on a, say, <link> or <a> element, @property  behaves exactly like @rel. It turns out that this usage pattern is so widespread that it covers most of the important use cases for authors. The new version of the RDFa 1.1 Primer (as well as the RDFa 1.1 Core, actually) has a number of examples that show these. There are also some other changes related to the behaviour of @typeof in relations to @property; please consult the specification for these.

The publication of RDFa 1.1 Lite was also a very important step. This defines a “sub-set” of the RDFa attributes that can serve as a guideline for HTML authors to express simple structured data in HTML without bothering about more complex features. This is the subset of RDFa that schema.org will “accept”,  as an alternative to the microdata, as a possible syntax for schema.org vocabularies. (There are some examples on how some schema.org example look like in RDFa 1.1 Lite on a different blog.) In some sense, RDFa 1.1 Lite can be considered like the equivalent of microdata, except that it leaves the door open for more complex vocabulary usage, mixture with different vocabularies, etc. (The HTML Task Force will publish soon a more detailed comparison of the different syntaxes.)

So here is, roughly, where we are today. The recent publications by the W3C RDFWA Working Group have, as I said, ”officialized” all the changes that were discussed since spring. The group decided not to publish a Last Call Working Draft, because the last few weeks’ of work on the HTML Task Force may reveal some new requirements; if not, the last round of publications will follow soon.

And what about implementations? Well, my “shadow” implementation of the RDFa distiller (which also includes a separate “validator” service) incorporates all the latest changes. I also added a new feature a few weeks ago, namely the possibility to serialize the output in JSON-LD (although this has become outdated a few days ago, due to some changes in JSON-LD…). I am not sure of the exact status of Gregg Kellogg’s RDF Distiller, but, knowing him, it is either already in line with the latest drafts or it is only a matter of a few days to be so. And there are surely more around that I do not know about.

This last series of publications have provided a nice closure for a busy RDFa year. I guess the only thing now is to wish everyone a Merry Christmas, a peaceful and happy Hanukkah, or other festivities you honor at this time of the year.  In any case, a very happy New Year!

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