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Re: Choice construct questions

From: David Martin <martin@AI.SRI.COM>
Date: Mon, 25 Oct 2004 21:14:57 -0700
Message-ID: <417DCF41.5010109@ai.sri.com>
To: Evren Sirin <evren@cs.umd.edu>
CC: Daniel Elenius <daele@ida.liu.se>, public-sws-ig@w3.org

Daniel, Evren -

Thanks to both of you for your comments.  Given our time constraints, and given that 
no-one to my knowledge has made use of Choose or asked for any refinements, and 
given that the tech. overview was already modified to answer these questions, I 
think we should follow Evren's suggestion, and bring Process.owl into line with the 
tech. overview.

Assuming no more discussion, I will go ahead and do that, and I'll also make it use 
the components property (which means I also need to update the tech. overview in the 
small way).

Cheers,
David

Evren Sirin wrote:

> 
> Daniel,
> The comments in Process.owl are left from older versions of OWL-S and in 
> the most recent technical overview document Choice is simply defined as:
> 
>> Choice is a control construct whose key property is chooseFrom, whose 
>> value is a list of processes the execution of one of which constitutes 
>> execution of the Choice.
>>
>> [[ This wording is a significant scale-down from the original idea, 
>> which involved being able to choose an arbitrary number of the 
>> processes specified as chooseFrom, and then impose further constraints 
>> on the set chosen. We have decided to simplify because the machinery 
>> used to implement the original idea was not thought through carefully, 
>> and because there doesn't seem to be much demand for the complicated 
>> version. ]]
>>
> 
> Obviously, we need to revise the comments in Process.owl to be 
> compatible with this simplified definition. Actually, one thing we might 
> consider to simplify the definition even more is simply use 
> process:components instead of process:chooseFrom to specify the bag of 
> processes. That would make the definition more compatible with the rest 
> of control constructs, e.g. Sequence, Any-Order, Split all use 
> process:components property.
> 
> Evren
> 
> 
> 
> 
> 
> Daniel Elenius wrote:
> 
>>
>> I'm slightly confused regarding the Choice construct, especially the 
>> comments about it in Process.owl, and the
>> "chosen" property (definitions from Process.owl below).
>>
>> First of all, the rdf:comment. It mentions a "choose property", which 
>> "takes a choice bag and returns a chosen
>> bag". This does not make sense. There is nothing called "choose" in 
>> OWL-S, and properties don't have parameters
>> and return values.
>>
>> Second, the "chosen" property. I assume that this is supposed to point 
>> to what the service consumer actually chooses
>> to execute. But why do we need such a property? Its value will depend 
>> on actual execution, so the process chosen
>> by the user should be kept track of by the OWL-S execution engine, not 
>> the service instance. Unless it needs to
>> be referenced somewhere else in the service...
>>
>> ...which may be the case given the comments after the definition of 
>> the Choice class. It mentions being able to
>> express "choose n from m" etc using cardinality restrictions, and 
>> being able to describe how to execute the chosen
>> construct(s). A possible interpretation is: If we have e.g. a 
>> Sequence, where the first element is a Choice, and the
>> second is an Any-Order, the Any-Order can use the value of the 
>> "chosen" property as the value of its "components"
>> property. This would mean: First choose a number of the control 
>> constructs in the "chooseFrom" bag of the Choice, and
>> then execute the chosen constructs in Any-Order.
>>
>> Is this how it is intended to be used? Of course, it can't be used 
>> this way now, because chosen takes only a single
>> ControlConstruct, not a ControlConstructBag. And even if it did take a 
>> bag, bags are interpreted as unordered, so we
>> could not enforce a particular order in a Sequence on the chosen 
>> constructs. So if Choice is to be used in the way
>> I described, I would suggets the following:
>>
>> 1) Remove the range restriction on chosen.
>> 2) Create two subclasses of Choice, called SingleChoice and 
>> MultipleChoice.
>> 3) Add local range restrictions on chosen for these two subclasses, so 
>> that
>>   SingleChoice restricts chosen to ControlConstruct, and 
>> MultipleChoice restricts
>>   it to ControlConstructBag (or ControlConstructList, to maintain the 
>> order in which the service consumer chose
>>   the items).
>>
>> I still don't see how you can describe "choose n from m". There is no 
>> way to express that there can only
>> be n elements in a ControlConstructBag.
>>
>>
>> /Daniel
>>
>>
>>
>> <!--
>> Choice.
>>
>> -->
>>
>> <owl:Class rdf:ID="Choice">
>>  <rdfs:comment>
>>    Choice is the selection among a bag of Processes.  The choose
>>    property, takes a choice bag and returns a chosen bag. The cardinality
>>    of the bag can be specified through a restriction to get choose(n).
>>  </rdfs:comment>
>>  <rdfs:subClassOf rdf:resource="#ControlConstruct"/>
>>  <rdfs:subClassOf>
>>    <owl:Restriction>
>>      <owl:onProperty rdf:resource="#chooseFrom"/>
>>      <owl:cardinality 
>> rdf:datatype="&xsd;#nonNegativeInteger">1</owl:cardinality>
>>    </owl:Restriction>
>>  </rdfs:subClassOf>
>>  <rdfs:subClassOf>
>>    <owl:Restriction>
>>      <owl:onProperty rdf:resource="#chosen"/>
>>      <owl:cardinality 
>> rdf:datatype="&xsd;#nonNegativeInteger">1</owl:cardinality>
>>    </owl:Restriction>
>>  </rdfs:subClassOf>
>>
>> </owl:Class>
>>
>> <!-- Note that given Chosen and ChooseFrom, we can define both a control
>> construct such as sequence or any-order (ex. choose and do chosen in
>> sequence, or choose and do chosen in parallel) as well as a class that
>> restricts the size of the Process Bag that corresponds to the
>> "components" of the chosen and chooseFrom subprocesses using
>> cardinality, mincardinality, maxcardinality to get choose at least n
>> from m, choose n from m, and choose at most n from m, etc.  These 
>> extensions are left as an exercises to the reader :)
>> -->
>>
>> <owl:ObjectProperty rdf:ID="chooseFrom">
>>   <rdfs:domain rdf:resource="#Choice"/>
>>   <rdfs:range rdf:resource ="#ControlConstructBag"/>
>> </owl:ObjectProperty>
>>
>> <owl:ObjectProperty rdf:ID="chosen">
>>   <rdfs:domain rdf:resource="#Choice"/>
>>   <rdfs:range rdf:resource ="#ControlConstruct"/>
>> </owl:ObjectProperty>
>>
>> <!-- end choice -->
>>
>>
>>
>>
>>
> 
> 
Received on Tuesday, 26 October 2004 04:15:48 GMT

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