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

From: Markus Lanthaler <markus.lanthaler@gmx.net>
Date: Mon, 9 Jun 2014 21:50:31 +0200
To: "'W3C Web Schemas Task Force'" <public-vocabs@w3.org>
Message-ID: <059901cf841c$12d16fe0$38744fa0$@gmx.net>
On Monday, June 09, 2014 9:29 PM, Jarno van Driel wrote:
> But what about the two entities with the same @itemid? 
> And what about fragment identifiers (#), how does schema.org look at
> those?

The value of itemid will always be resolved against the base URL. So, if your document lives at http://example.com/document the "global identifier" will be http://example.com/document#fragment 

> The Microdata spec says "...It is up to such specifications to define
> whether multiple items with the same global identifier (whether on the
> same page or on different pages) are allowed to exist, and what the
> processing rules for that vocabulary are with respect to handling the
> case of multiple items with the same ID."

Well, that says it all, doesn't it? From a Microdata perspective, it is undefined. Schema.org states [1] that the datamodel is derived from RDF Schema. I interpret that as "schema.org is based on RDF's data model". In RDF, "multiple items with the same global identifier" do not exist, they are all the same "item" (resource) [2]. So, if you use the same itemid for different items, all properties would be merged so that you end up with a single item. Obviously I don't know for sure what search engines do internally with such data but I assume that's what they do.. especially since RDFa and JSON-LD are supported as well and they work exactly this way.

[1] http://schema.org/docs/datamodel.html
[2] http://www.w3.org/TR/rdf11-concepts/

Markus Lanthaler
Received on Monday, 9 June 2014 19:51:01 UTC

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