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Re: granularity/definition of a "service"

From: Monika Solanki <monika@dmu.ac.uk>
Date: Thu, 16 Sep 2004 15:46:14 +0100
Message-ID: <4149A736.5050906@dmu.ac.uk>
To: Pranam Kolari <kolari1@cs.umbc.edu>
CC: public-sws-ig@w3.org

Pranam Kolari wrote:

> If you are looking at a programmer kind of definition of certain 
> activities being services, then this is my guess: 

Actually I am after a more conceptual definition

> For a BPEL specified composition to be executable on a BPEL engine, 
> both the BPEL composition as well as
> an accompanying WSDL is required. The WSDL declares all endpoints 
> which can be invoked by external entities.
> This includes the BPEL composed service and other services which are 
> part of the composition .Note that the invocation
> of these other activities is contingent on the composition being 
> instantiated.
>
> Speicifically, the other services that WSDL declares:
> receive ( one-way operation)
> receive, reply ( two-way )
> pick, onMessage (one/two-way)
> event, messageHandlers (one/two-way)
> .....

So communication primitives can indeed be classified as services as per 
BPEL ?

>
> Note that "invoke" is not part of WSDL since it is used by the 
> composition to invoke external web services and is not
> a provided service by the composition. Ofcourse one could argue that 
> certain "invokes: could be one-way operations
> reporting to external entities.
>
> So in this sense, a subset of activities (combination of activities) 
> are services as they serve clients. 

I agree that a combination of activities can indeed represent a service, 
but would an individual activity like for example <receive> be 
classified as a service?

Thanks,

Monika

> Certain others which
> was also mentioned like  compensate,  flow etc. provide internal 
> service to the composition by doing certain set
> of things, but cannot be classified as services.
>
> -- Pranam
> _____________________________________________________________________
>
> Pranam Kolari                        
> Department of Computer Science
> University of Maryland, Baltimore County     Baltimore, MD 
> 21250               
>
> Contact:
> (Work) +1 410 455 3971 :---: (Home) +1 410 536 4772
> kolari1@cs.umbc.edu    :---: http://www.cs.umbc.edu/~kolari1
> _____________________________________________________________________
>
>
> Monika Solanki wrote:
>
>>
>> My guess is PSMs actually deal with internals as well rather than 
>> just interfaces as in Atomic processes, so probably the semantics are 
>> different there slightly.
>>
>> If you look at the range of the "Activity " construct in BPEL, you find
>> " <receive>, " <reply>, " <invoke>, " <assign>, " <throw>, " 
>> <terminate>, " <wait>
>> " <empty>," <sequence>," <switch>," <while>," <pick>," <flow>," 
>> <scope>," <compensate>
>>
>> these include everything from communication primitives to control 
>> constructs. A BPEL service would then essentially be a composition of 
>> several "Activity" constructs. So the issue now is that can 
>> "Activity" be considered analogous to an "Atomic " service. I don't 
>> think it can, but it would be interesting to have other opinins.
>>
>> Daniel Elenius wrote:
>>
>>> Interestingly, Problem-solving methods (PSMs), which are in many 
>>> ways similar to owl-s atomic processes, seem to have been thought to 
>>> have a finer granularity than "services". What they describe is 
>>> rather algorithms, and these could be parts of services I guess...
>>>
>>> /Daniel
>>>
>>> Monika Solanki wrote:
>>>
>>>> Hi All,
>>>>
>>>> Keeping aside the "web" part, I am interested in understanding the 
>>>> semantics of the word "service". Within the web service world, what 
>>>> is the lowest level of granularity a service can have. What would 
>>>> be the most appropriate definition for the basic building block 
>>>> from which bigger units can be composed. Services communicate via 
>>>> sending and receiving messages. Can communication primitives also 
>>>> be classified as services i.e in this context is it appropriate to 
>>>> consider the operation of sending and receiving as services 
>>>> themselves and can they be modelled as such ? The BPEL4WS specifies 
>>>> these and other control constructs as "Activity". So in this 
>>>> context is a "service" at a higher level of abstraction then an 
>>>> "Activity" or do they have equivalent semantics.
>>>>
>>>> -Monika
>>>> **>><<**>><<**>><<**>><<**>><<**>><<**>><<**
>>>> Monika Solanki
>>>> Software Technology Research Laboratory(STRL)
>>>> De Montfort University
>>>> Gateway building, G4.61
>>>> The Gateway
>>>> Leicester LE1 9BH, UK
>>>>
>>>> phone: +44 (0)116 250 6170 intern: 6170
>>>> email: monika@dmu.ac.uk
>>>> web: http://www.cse.dmu.ac.uk/~monika
>>>> **>><<**>><<**>><<**>><<**>><<**>><<**>><<**
>>>
>>>
>>>
>>>
>>>
>>>
>>
>
>
>
>

-- 
**>><<**>><<**>><<**>><<**>><<**>><<**>><<**
Monika Solanki
Software Technology Research Laboratory(STRL)
De Montfort University
Gateway building, G4.61
The Gateway
Leicester LE1 9BH, UK

phone: +44 (0)116 250 6170 intern: 6170
email: monika@dmu.ac.uk
web: http://www.cse.dmu.ac.uk/~monika
**>><<**>><<**>><<**>><<**>><<**>><<**>><<**
Received on Thursday, 16 September 2004 14:47:20 UTC

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