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Re: Multiple types from different vocabularies (ACTION-7)

From: Jeni Tennison <jeni@jenitennison.com>
Date: Sat, 15 Oct 2011 20:57:36 +0100
Cc: public-html-data-tf@w3.org
Message-Id: <32609FA1-F6AB-400E-B769-63A3940BEEE7@jenitennison.com>
To: Martin Hepp <martin.hepp@ebusiness-unibw.org>

On 15 Oct 2011, at 19:49, Martin Hepp wrote:
> My take is that if schema.org defines the property "secondaryType" for http://schema.org/Thing, this will be sufficient for 99 % of the cases. After all, schema.org is the only major Microdata vocabulary out there (IMHO).

Guha's indicated that he's in favour of schema.org defining the http://schema.org/type. Are you happy with this as a way forward?

>> The first is that it would mean adding this property to existing vocabularies to make them microdata ready. You could argue that to use existing RDF vocabularies in microdata you have to do some extra work anyway (as Hixie has pointed out, you can't just port them because there are extra semantics you have to define for microdata use), so adding another property isn't a big deal, but some vocabularies may be very hard to change.
> I would be interested to learn what those are. IMO, the only really important thing you need is to define, in plain English, how URIs for properties should be formed 
> a)base URI + local name or 
> b) URI of the itemtype + local name of the property.
> This is at least the approach we take with GoodRelations. Did I overlook anything?

In recent mails, Hixie said [1]:

> Incidentally, note that you can't just take, say, an RDF vocabulary, or a Microformats vocabulary, and just use it in microdata directly. A microdata vocabulary has to define processing rules that are often not provided for RDF and Microformats vocabularies, and has to use the terms defined in the HTML specification to describe how the terms work. You can see examples of how to define vocabularies in the HTML standard:
>   http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#mdvocabs

and defined a microdata vocabulary as [2]:

> A vocabulary is a set of property names, the semantics of those property 
> names, the processing rules for properties that use those names, the error 
> handling for items that use these properties incorrectly, the meaning of 
> the itemid="" value in the context of this vocabulary, potentially the 
> "sub"vocabularies of other untyped items that are the values of properties 
> whose names are defined by the vocabulary, and the set of one or more 
> types that identify that vocabulary.

I've put these quotes as placeholders on the wiki page about vocabulary design [3] but we need to flesh that page out more. Going through the microdata spec [4] there are things like:

  * whether the types can be dereferenced
  * for each type, whether itemids are allowed and how they are interpreted
  * how items with the same itemid are handled
  * which properties are allowed on which types
  * how many and what values are allowed for each property

The first three are easy to answer for RDF-friendly vocabularies, I think. The spec doesn't mention anything about having to specify error handling behaviour for property values, but that should be added to the list if we're creating guidelines.

You know it would be really great to use GoodRelations as an example of an RDF vocabulary being adapted so that it can also be used with microdata, showing the steps that need to be taken by vocabulary authors to port existing RDF schemas. Could I persuade you to put together a description of what you've had to do on the wiki page at [3]? (We might perhaps extend that to show use with microformats-2 if we were feeling ambitious.)



[1] http://lists.w3.org/Archives/Public/public-html-data-tf/2011Oct/0085.html
[2] http://lists.w3.org/Archives/Public/public-html-data-tf/2011Oct/0100.html
[3] http://www.w3.org/wiki/HTML_Data_Vocabularies
[4] http://dev.w3.org/html5/md/Overview.html
Jeni Tennison
Received on Saturday, 15 October 2011 19:58:00 UTC

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