W3C home > Mailing lists > Public > public-vocabs@w3.org > June 2012

Re: additionalType property, vs extending Microdata syntax for multiple types

From: Martin Hepp <martin.hepp@ebusiness-unibw.org>
Date: Sat, 16 Jun 2012 23:25:13 +0200
Cc: Dan Brickley <danbri@danbri.org>, Ivan Herman <ivan@w3.org>, "public-vocabs@w3.org" <public-vocabs@w3.org>, Ramanathan Guha <guha@google.com>, Jeni Tennison <jeni@jenitennison.com>
Message-Id: <E578B2DA-069C-4FBE-8C15-77F0D6172659@ebusiness-unibw.org>
To: Peter Mika <pmika@yahoo-inc.com>
Hi Peter,
the idea behind the additionalType property is two-fold:

First, we wanted an approach that works as of now without requiring an update to the MD spec, but can easily be handled in case of a future standardization of this.

Second, it preserves the frame-based paradigm of schema.org (and MD vocabs, to a lesser extend), where you need a main type for an entity to know the set of and definition of suitable properties.

The current workaround is to use the full URI of rdf:type, which is kind of an ugly approach:

<div itemscope itemtype="http://schema.org/Product">

    <link itemprop="http://www.w3.org/1999/02/22-rdf-syntax-ns#type" href="http://www.productontology.org/id/Laser_printer" />
<!-- other schema.org properties go in here -->

So adding  "type" or "additionalType" property to http://schema.org/Thing seems like a quick, pragmatic yet future-prone approach to me.


On Jun 16, 2012, at 5:28 PM, Peter Mika wrote:

> Let me add my $0.02...
> My main concern is that schema.org is and should be about the schema, not the syntax. The reason of adding this property is to patch microdata, but if schema.org defines this term, it will be available for all syntaxes (microdata, RDFa, possibly soon OData). In both microdata and RDFa, we would end up with two ways of defining types (using typeof/itemtype vs. additionalType). Standard parsers however will only see one type (the one expressed using the standard mechanism).
> A minor concern is that the name additionalType implies that somehow one type is more important than the other.
> Cheers,
> Peter
> On 6/16/12 7:48 AM, Dan Brickley wrote:
>> On 16 June 2012 06:55, Ivan Herman<ivan@w3.org>  wrote:
>>> On Jun 15, 2012, at 20:26 , Dan Brickley wrote:
>>>> HTML5 Microdata, as defined in
>>>> http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#encoding-microdata
>>>> ... has only limited support for describing multiple types that
>>>> something belongs to. In particular it requires they are described
>>>> using a single schema.
>>>> For Good Relations integration (and other scenarios) people have asked
>>>> for a way of listing more types within schema.org markup.
>>>> * One model is to use RDFa 1.1 (Lite), where this is quite natural.
>>>> * Another is to add (as a workaround) a new property, e.g. called
>>>> 'type' or 'additionalType', to schema.org's vocab (Martin requests
>>>> this in http://www.w3.org/wiki/WebSchemas/GoodRelations )
>>>> * A 3rd is to stretch
>>>> http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#items
>>>> to allow different namespaces to be used.
>>> I think that the third option is not obvious. The issue is that, in microdata, two notions, namely the typing of an item and the vocabulary used in that item, are conflated. The vocabulary information (hence the interpretation of the terms in the item) is deduced from the itemtype value(s). Hence the restriction in the current spec to restrict multiple types to be in the same vocabulary. Technically, it would be possible to declare that the order in the attribute's value counts, ie, the first type value determines the vocabulary, but that would be terribly error-prone and I hence do not believe it would be a good idea.
>>> It would require a more substantial change in the microdata spec (essentially introducing the equivalent of RDFa's @vocab) to solve that properly. I do not see that happening.
>> Yes. I think it is well appreciated that RDFa 1.1 handles this better.
>>  I don't think anyone expects Microdata to change a lot more, and I
>> don't see anyone with the energy/interest to keep pushing for these
>> kinds of changes/improvements in Microdata. As you say, it wouldn't be
>> easy. The "main type comes first" design was all I could think of,
>> too.
>>> Which leaves us with the first or the second option. I am obviously biased here, but I think the first option is clearly better in this respect...
>> The question is then, what do we say for all those publishers who are
>> on board the Microdata train? A lot of people have worked hard to get
>> colleagues/stakeholds (and tools!) adopting Microdata, over the last
>> year. While RDFa is a good thing, we want to be careful to support
>> early adopters too, and have sensible advice for them (other than
>> 're-do everything with a new syntax'). Which is what makes
>> 'additionalType' attractive. But the concern there is ... if we go
>> that route, validators/checkers will need to understand the attribute
>> since it is almost an extension of the underlying syntax...
>> cheers,
>> Dan

martin hepp
e-business & web science research group
universitaet der bundeswehr muenchen

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Received on Saturday, 16 June 2012 21:25:50 UTC

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