[sc34wg3] Towards TMDM 3.0

Lars Marius Garshol larsga at garshol.priv.no
Tue Feb 24 04:43:21 EST 2009

* Rani Pinchuk
> You say that an item identifiers can be used to refer to topics, but
> actually, if I understood correctly, according to you, item identifier
> can be used to refer indirectly to a subject.

Yes. Given the close correspondence between topics and subjects this  
is true of any form of topic identifier.

> So, all topics having the same item identifier must be a proxy to  
> the same subject (so they should be merged).


> That is, all topics in different topic maps, having the same item
> identifier must be a proxy to the same subject.


> Why do we need this? What this kind of identifiers give us, that  
> PSIs do
> not give?

The reason these identifiers exist at all is in order to handle topic  
IDs and topicRefs in XTM. Firstly, the specs need to be able to  
resolve topicRefs and make sure they match up. Secondly, in most cases  
this is the only form of identity a topic has, and so in order to  
ensure that merging works these identifiers have to be preserved.

The [item identifiers] property is separate from the [subject  
identifiers] property simply because item identifiers are lower-grade  
identifiers than subject identifiers, and we do not want to mix the  
two types of identifiers up.

> I thought that an item identifier allows us to refer to a specific  
> topic
> and not to a group of topics. After all, it might be useful to be able
> to refer somehow to one of the topics that are about to be merged.

Defining an identifier that is guaranteed to be unique across all  
topic maps is not going to be easy, and in any case it's hard to see  
any clear benefit in it. After all, identifying the topic map + giving  
a topic identifier will do the same thing today.

--Lars M.

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