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RE: status of xsd:duration in OWL (and RIF and SPARQL) - ACTION-164: RDF WG

From: Evain, Jean-Pierre <evain@ebu.ch>
Date: Tue, 8 May 2012 09:25:21 +0200
To: 'Ivan Herman' <ivan@w3.org>
CC: 'Michael Schneider' <schneid@fzi.de>, Bijan Parsia <bparsia@cs.man.ac.uk>, Ian Horrocks <ian.horrocks@cs.ox.ac.uk>, "public-owl-wg@w3.org" <public-owl-wg@w3.org>, "Public-Rif-Wg (E-mail)" <public-rif-wg@w3.org>, "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, Sandro Hawke <sandro@w3.org>
Message-ID: <7D1656F54141C042A1B2556AE5237D600116304E6137@GVAMAIL.gva.ebu.ch>
Hello Ivan,

I have quite a pretty simple basic requirement, which consist of expressing people know in this new format.

It is true that most probably reasoning and inferences will not provide significant added values based from time and duration (hence considerations different from what seem to have justified in the past the rejection of time and date). However, this is important descriptive information that must be represented properly.

The audiovisual community makes an heavy use of time and duration. There are three main formats: timecodes (being revised as they don't fulfil requirements of new digital formats), a number of edit units (which take as a reference fraction of time the equivalent of a frame for video or sample for audio) and the equivalent of normal play time and duration. The latter is missing in RDF/OWL.

My point is that in order to get people acquainted with something new (RDF/OWL is not new but is eventually getting a lot of traction and interest), it is easier if they recognise themselves. Asking a video editor to define a start time of T0+52" as ????-??-??T00:00:52 (I am using question marks to let you try figure by yourself what you'd like to put there inc. restrictions on possible values and knowing that date is irrelevant to a timestamp like this). And how would you express a duration of 30" (with months and seconds?)?

I understand the point which is being made being what is useful for reasoning or not. But does that mean that all other information is garbage? I am sure nobody thinks so. Should be use heterogeneous representation formats for what is for reasoning and what is not: this wouldn't help wider adoption of RDF/OWL-2.

Furthermore, I need to check how duration validates in RDF. I remember however I recently had problem with date and time.


-----Original Message-----
From: Ivan Herman [mailto:ivan@w3.org] 
Sent: mardi, 8. mai 2012 08:49
To: Evain, Jean-Pierre
Cc: 'Michael Schneider'; Bijan Parsia; Ian Horrocks; public-owl-wg@w3.org; Public-Rif-Wg (E-mail); Peter F. Patel-Schneider; Sandro Hawke
Subject: Re: status of xsd:duration in OWL (and RIF and SPARQL) - ACTION-164: RDF WG

On May 8, 2012, at 08:26 , Evain, Jean-Pierre wrote:

> Michael,
> Thanks for the documented explanations.
> However, that looks like very convoluted. Anytime, those who want to solve the issues that you mention can use dateTime.
> But, I am trying to bring RDF and OWL in audiovisual production and I need to express things as simple as:
> - segment video "y" start at time "t" for a duration "d"
> I do not see why I should use any date information in this, it is not relevant. Also because dateTime is a frequent source of errors.
> But maybe it is a bad idea altogether to bring RDF and OWL in audiovisual production?


without taking side on whether duration should or should not be part of OWL 2 for reasoning (I do not have enough technical baggage to comment on that): if you use duration today, it is valid RDF (even if RDF2004 still refers to duration as a SHOULD NOT be used, but that might change now with XSD1.1). Not being part of OWL 2 means that you do not get OWL 2 datatype reasoning on those literal values; I presume an OWL 2 reasoner would treat those literal values pretty much as strings. So the question is what you expect from your tooling v.a.v. that particular data. 


> Jean-Pierre
> -----Original Message-----
> From: Michael Schneider [mailto:schneid@fzi.de] 
> Sent: mardi, 8. mai 2012 01:42
> To: Bijan Parsia
> Cc: Ian Horrocks; Ivan Herman; public-owl-wg@w3.org; Public-Rif-Wg (E-mail); Peter F. Patel-Schneider; Sandro Hawke; Evain, Jean-Pierre
> Subject: Re: status of xsd:duration in OWL (and RIF and SPARQL) - ACTION-164: RDF WG
> Am 07.05.2012 12:42, schrieb Bijan Parsia:
>> On 7 May 2012, at 11:29, Michael Schneider wrote:
>>> Am 07.05.2012 00:19, schrieb Ian Horrocks:
>>>> Hi Bijan (et al),
>>>> According to my understanding, we agreed to keep the WG alive so that we could fix any OWL 2 problems caused by changes to XSD 1.1 and update the OWL 2 Rec to reference the XSD 1.1 Rec. It was also foreseen that we could take advantage of this update to fix any editorial errata in the OWL 2 Rec.
>>>> While I agree that the dividing line between editorial errata and substantive changes is not 100% clear, it does seem pretty obvious to me that adding support for a new datatype goes beyond the spirit of this agreement.
>>> I agree!
>> But, you know, who the heck cares about the spirit of some agreement?
> Oops, sorry for having been so short on words - that's not what people 
> generally expect from me and, certainly, I have more to say than simply 
> agreeing to Ian. :-)
> Fine, not talking about procedural stuff here, and also not mentioning 
> (ok, I do) that I believe that it's not so easy to bring a working group 
> happily back to work after 2 1/2 years, there is still the question 
> whether these three datatypes are technically appropriate for inclusion 
> in OWL at all (whether in OWL 2, or OWL 2.1, or whatever). So here is 
> the situation as I recall it:
> First to say, it's not that these datatypes were simply forgotten to be 
> considered, as some people seem to believe. Rather, at least two of them 
> were discussed and then excluded deliberately. Below is my understanding 
> of why and when this happened.
> NOTE: In the remainder, whenever I refer to the XSD datatype spec, I 
> refer to the Candidate Recommendation as of 30 April 2009, which was the 
> one to which the OWL 2 and RIF specs currently refer to.
> 1) xsd:time
> IIRC, the OWL WG had adopted a certain design principle for the 
> time-related datatypes of OWL 2, which was the "timeOnTimeline" 
> property, as defined in the XSD datatype spec [1]: each data value in 
> the value space of a datatype must be exactly one point on the 
> everlasting time line.
> This is certainly fulfilled by xsd:dateTimeStamp, which is included in 
> OWL 2. In xsd:time, however, each data value specifies infinitely many 
> times on the time line: "12:00:00 o'clock" happens every day! In fact, 
> The XSD spec at that time said [2]:
>     time represents instants of time that recur
>     at the same point in each calendar day, or
>     that occur in some arbitrary calendar day.
> For OWL 2 reasoners (well, actually for the OWL 2 semantics), such a 
> definition is a problem, because they have to decide for two given times 
> whether the two times are equal or not and, if not, whether one time 
> value is smaller or larger than the other one. Comparisons of this sort 
> become particularly troublesome, if one tries to compare, say, a 
> xsd:time value with a xsd:dateTimeStamp value: the dateTimeStamp value 
> is a fixed point on the time line, but any xsd:time value will refer to 
> time points both before and after that fixed time point.
> The corresponding OWL WG issue was ISSUE-126 [3]. The description of the 
> issue swiftly mentions the problems of the time-related datatypes. There 
> was a long discussion on this issue, as you can see from the list of 
> references below the issue description; I guess, these mails contain 
> more discussion on the xsd:time datatype problem, but I haven't read 
> them all. The eventually accepted proposal (which only mentions 
> xsd:dateTime but not any of the other time-related datatypes anymore) to 
> resolve the issue was [4], with resolution at F2F3 [5] and 
> implementation by ACTION-177 [6].
> 2) xsd:date
> Here, the situation was different compared to xsd:time, as data values 
> in the value space of xsd:date do not represent time points, but time 
> /intervals/, as stated in the old version of the spec [7]:
>     date represents top-open intervals of exactly
>     one day in length on the timelines of dateTime,
>     beginning on the beginning moment of each day,
>     up to but not including the beginning moment
>     of the next day.
> This, of course, is also not compatible with the 
> single-point-on-timeline design criteria. Consequently, xsd:date was 
> excluded by the resolution of ISSUE-126 as well.
> 3) xsd:duration
> Unfortunately, I wasn't able to find any discussion on xsd:duration in 
> the OWL WG mailing list. The reason might be that this datatype was 
> already excluded in the XSD datatype map of RDF [8]:
>     The other built-in XML Schema datatypes are
>     unsuitable for various reasons, and SHOULD NOT
>     be used: xsd:duration does not have a well-defined
>     value space
> In retrospect, this first came to a surprise to me, so let's see how the 
> value space of xsd:duration was defined in the old CR of XSD 1.1 [9] 
> (long after the RDF rec, which still referred to XSD 1.0!):
>     Duration values can be modelled as two-property
>     tuples. Each value consists of an integer number
>     of months and a decimal number of seconds.
> Hrmpf, that appears to me a strange definition: why months? The length 
> of a month is not well-defined, it may be anything between 28 and 31 
> days. So it would be easy to give two xsd:duration literals, basically 
> consisting of month-second tuples, for which it becomes non-well-defined 
> whether they are equal or, if not, which of them is smaller or larger. 
> In fact, the text continues by:
>     duration is partially ordered.
> Sorry, but... I'm not clear where this idea of using months as part of 
> the value space definition came from. Anyhow, for OWL 2 reasoners, 
> adopting xsd:duration would make things troublesome, as some OWL 2 
> ontologies would not have a well-defined semantics then.
>    * * *
> Now, so far for the situation at the time of OWL 2 recommendation. All 
> three datatypes, according to their definitions in the XSD candidate 
> recommendation at the time of OWL 2 and RIF spec, provided problems for 
> OWL 2, be it with respect to the one-point-on-timeline design criteria 
> (xsd:time and xsd:date), or with regard to well-definedness of the (two) 
> OWL 2 semantics (xsd:time and xsd:duration). I leave it to the 
> proponents of these datatypes to explain what has changed in the XSD 
> spec since the recommendation of OWL 2 to allow for these datatypes to 
> be included now, such that (a) the design decisions made for OWL 2 are 
> retained and (b) without rendering the semantics of OWL 2 non-well-defined.
> Best,
> Michael
> References:
> [1] 
> <http://www.w3.org/TR/2009/CR-xmlschema11-2-20090430/#vp-dt-timeOnTimeline>
> [2] <http://www.w3.org/TR/2009/CR-xmlschema11-2-20090430/#time>
> [3] <http://lists.w3.org/Archives/Public/public-owl-wg/2008Jun/0138.html>
> [4] <http://lists.w3.org/Archives/Public/public-owl-wg/2008Jul/0433.html>
> [5] <http://www.w3.org/2007/OWL/meeting/2008-07-29#resolution_1>
> [6] <http://www.w3.org/2007/OWL/tracker/actions/177>
> [7] <http://www.w3.org/TR/2009/CR-xmlschema11-2-20090430/#date>
> [8] <http://www.w3.org/TR/2004/REC-rdf-mt-20040210/#XSDtable>
> [9] <http://www.w3.org/TR/2009/CR-xmlschema11-2-20090430/#duration>
> -- 
> .........................................................
> Dipl.-Inform. Michael Schneider
> Research Scientist, IPE / WIM
> FZI Forschungszentrum Informatik
> Haid-und-Neu-Str. 10-14
> 76131 Karlsruhe, Germany
> Tel.: +49 721 9654-726
> Fax: +49 721 9654-727
> michael.schneider@fzi.de
> www.fzi.de
> .........................................................
> Forschungszentrum Informatik (FZI) an der Universität Karlsruhe
> Stiftung des bürgerlichen Rechts
> Stiftung Az: 14-0563.1 Regierungspräsidium Karlsruhe
> Vorstand: Dipl. Wi.-Ing. Michael Flor, Prof. Dr. Ralf Reussner,
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> Vorsitzender des Kuratoriums: Ministerialdirigent Günther Leßnerkraus
> .........................................................
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Ivan Herman, W3C Semantic Web Activity Lead
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