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What is an entity?

From: Jacco van Ossenbruggen <Jacco.van.Ossenbruggen@cwi.nl>
Date: Tue, 24 Jul 2012 19:16:14 +0200
Message-ID: <500ED85E.5020003@cwi.nl>
To: public-prov-comments@w3.org
Dear WG,

Congrats on publishing your last call documents!

I've one comment, which is about the definition of what and entity is, 
and especially, what it is not, in the context of PROV.
The current definition is:

"An entity is a physical, digital, conceptual, or other kind of thing 
with some fixed aspects; entities may be real or imaginary."

I'm confused about the "some fixed aspects" part.  What does that mean?  
In general, this definition is quite liberal, while the rest of the text 
suggests that you assume a much more constrained definition of what an 
entity should be.

 From the remainder of the text, I derive that the "some fixed aspects" 
should at least include that
- an entity is something with a fixed, permanent ID
- an entity is something of which all (not some!) aspects that the 
provenance records make claims about are assumed to remain fixed

E.g. the 5.2.2 revision example suggest that
- if Alice makes a file which is later changed by Bob, and
- if both contributions are to be recorded in the PROV records, that
- than the two revisions of the file _need_ to be modelled by two 
different entities.

Your current entity definition leaves sufficient room for assuming that 
entities may change over time without changing ids.

I think that for people to decide how to use PROV in their applications, 
the crucial decision is to decide which local application objects should 
be mapped to PROV entities.  I do not think the current liberal 
definition is very helpful in this process. For example from your 
definition, a database table that is dynamically updated could be an 
entity, while I think the assumption of PROV is that in such 
applications, a specific snapshot of the table would be an entity.

What about this def:

"An entity is a physical, digital, conceptual, or other kind of thing 
with a fixed id and a fixed set of properties; entities may be real or 
imaginary, but are assumed not to evolve over time".

Hope this helps,

Jacco van Ossenbruggen
Received on Tuesday, 24 July 2012 17:16:43 UTC

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