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Re: The exact meaning of a 'global identifier' (itemid)

From: Laura Dawson <Laura.Dawson@bowker.com>
Date: Mon, 9 Jun 2014 19:08:45 +0000
To: Markus Lanthaler <markus.lanthaler@gmx.net>, 'W3C Web Schemas Task Force' <public-vocabs@w3.org>
Message-ID: <CFBB7CE6.7251E%laura.dawson@bowker.com>
Coming up through the world of describing books (physical and digital, as
objects and as concepts), the one rule that never let me down is ³Let the
identifier identify; let the metadata [or, in this case, ontology]
describe.² An ISBN, for example, is just a number - fairly meaningless,
definitely not dereference-able. But it is nonetheless a critical
identifier even as we move to different ways of describing books, for the
simple reason that it was developed to say, ³This book is not that book.²
The more you can decouple descriptions from identifiers, the more
stability and flexibility your schema will have.

On 6/9/14, 2:57 PM, "Markus Lanthaler" <markus.lanthaler@gmx.net> wrote:

>On Monday, June 09, 2014 8:24 PM, Jarno van Driel wrote:
>> I would like to know if the exact meaning of a Microdata global
>> identifier (itemid) has been documented?
>I'm not sure what exactly you mean by "meaning" but it is just the
>identifier that can be used to reference that specific item (or entity).
>It's similar to a primary key in a database but globally valid as it is a
>> Now I've tried to look for info about @itemid on schema.org but can't
>> find anything. The only info out there I could find just says it
>> should be documented by the vocabulary itself.
>That sentence says that what the identifier stands for, is described by a
>vocabulary. If I just give you the identifier
>http://example.com/an/entity it wouldn't tell you much. You need to look
>at the properties, types etc. it is marked up with... and those are
>defined by a vocabulary. In this case here, schema.org.
>> What is schema.org's perspective on this:
>Assuming that document was retrieved from http://example.com/document it
>tells you that
>> <body>
>>   <div itemid="CorporationPerson" itemscope
>>        itemtype="http://schema.org/Corporation">
>There's exists "item" of type http://schema.org/Corporation
>(Organization: A business corporation) that is identified with the URL
>>       <a itemprop="url"
>>   </div>
>And it has a property http://schema.org/url (URL of the item) whose value
>is http://www.example.org.
>>   <div itemid="Product" itemscope itemtype="http://schema.org/Product">
>There's also a second item, http://example.com/Product, which is of type
>schema.org/Product... I think you get the idea.
>Markus Lanthaler
Received on Monday, 9 June 2014 19:09:19 UTC

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