Re: PROV-ISSUE-469 (zero-timespan-entity): Can entity's have zero timespan? [prov-dm-constraints]

I think this is a typo; we meant for the only strict orderings to be the two involving derivation.  

Easy to fix by removing "strictly" if you agree.

--James


On Aug 6, 2012, at 4:31 PM, Provenance Working Group Issue Tracker wrote:

> PROV-ISSUE-469 (zero-timespan-entity): Can entity's have zero timespan? [prov-dm-constraints]
> 
> http://www.w3.org/2011/prov/track/issues/469
> 
> Raised by: Stian Soiland-Reyes
> On product: prov-dm-constraints
> 
> wasInvalidatedBy strictly follows wasGeneratedBy - do we have WG
> consensus?  Activities can be zero-length, is that OK? Need justification for the difference.
> 
> Has this been discussed with the WG?
> 
> 
>> From Stian's review: http://lists.w3.org/Archives/Public/public-prov-wg/2012Aug/0021.html
> 
> 
>> Constraint 38 (generation-precedes-invalidation)
>> IF wasGeneratedBy(gen;e,_a1,_t1,_attrs1) and wasInvalidatedBy(inv;e,_a2,_t2,_attrs2) THEN gen strictly precedes inv.
> 
> Why is this relation in particular *strictly precedes*? This needs to
> be justified (beyond "we need some strictly in there so we can do our
> loop testing"). So an entity can't have zero lifetime, but an activity
> may?
> 
> An activity can use an entity at the same time as it was generated,
> but it can't invalidate it then? This implies some kind of minimal
> planck time on entities, which is probably OK for most applications of
> PROV.
> 
> 
> After some discussion with fellow geeks, I have however come go agree
> that an entity can't have an empty lifespan, to avoid problems and to
> ensure time moves forward. I think we need to formulate this by using
> the description of an entity as characterizing and fixing aspects for
> some duration. But currently the DM descriptions seem to imply the
> opposite, entities can have zero lifespan, but activities cannot!
> 
> http://www.w3.org/TR/prov-dm/#term-entity
>> An entity ◊ is a physical, digital, conceptual, or other kind of thing with some fixed aspects; entities may be real or imaginary.
>> An activity ◊ is something that occurs over a period of time and acts upon or with entities; it may include consuming, processing, transforming, modifying, relocating, using, or generating entities.
> 
> I do however see bigger use for a zero lifespan activity, because it
> can be used to describe transitions of entities. So can we add to
> Constraint 38 some kind of remark about why an entity must have a
> non-zero lifespan? Something like:
> 
> "Constraint 38 implies that an entity must have a non-zero lifespan by
> using 'strictly precedes', that is the entity cannot be invalidated at
> the same instant as it is generated. The reasoning for this is that a
> meaningful entity is a thing with some aspects fixed. For these
> aspects to be fixed, the entity must exist for some (possibly
> infinitesimal) time. Note that this requirement does not apply to
> activities."
> 
> 
> 
> 


-- 
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

Received on Monday, 6 August 2012 15:35:45 UTC