W3C home > Mailing lists > Public > public-ws-chor@w3.org > December 2003

Re: Semantics and choreographies

From: Monica J. Martin <Monica.Martin@Sun.COM>
Date: Mon, 01 Dec 2003 08:03:35 -0700
To: Ugo Corda <UCorda@SeeBeyond.com>
Cc: Steve Ross-Talbot <steve@enigmatec.net>, public-ws-chor@w3.org, "farrukh.najmi" <Farrukh.Najmi@Sun.COM>
Message-id: <3FCB5847.2040407@sun.com>

Ugo Corda wrote:

>Hi Steve,
>
>The OASIS UDDI TC is currently discussing how to express semantics inside UDDI, and it might be useful background material for our discussions on WS-Chor semantics.
>
>There is certainly a direct connection in the part that you describe as "locating a choreography and checking that it matches the requirements of a requesting Web Service" (it roughly corresponds to finding a Web service in UDDI that provides some desired service capabilities).
>
>But there might be other relevant areas. For instance, one of the discussions in UDDI is whether the UDDI registry should just have references to RDF expressions and ontology statements and let other engines deal with the reasoning part, or instead have a reasoning engine included and draw inferences itself. I would imagine something similar could be discussed for choreography.
>
>In any case, you can find samples of these discussions on the UDDI mailing list. There is also a recent thread titled "UDDI and semantics" in the recently formed Semantic Web Services Interest Group (see http://lists.w3.org/Archives/Public/public-sws-ig/2003Nov/thread.html).
>  
>
mm1: I would suggest you also consider Reg/Rep as it provides support 
for web services, event notification, content management, etc. In fact 
there is an interesting demonstration set for XML 2003 in Philadelphia 
next week where Reg/Rep will participate (and integrate with ebMS, BPSS, 
CPPA, XACML, and the use of web services in an unusual demonstration in 
the health care space).  Contact Farrukh Najmi, Sun Microsystems. For 
v4, the Reg/Rep team is working on ontologicial support and has been 
gained the attention of many government or private players in that 
community. Farrukh can fill in the details. Thanks.

>Ugo
>
>  
>
>>-----Original Message-----
>>From: public-ws-chor-request@w3.org
>>[mailto:public-ws-chor-request@w3.org]On Behalf Of Steve Ross-Talbot
>>Sent: Tuesday, November 25, 2003 5:44 AM
>>To: public-ws-chor@w3.org
>>Subject: Semantics and choreographies
>>
>>
>>
>>What can semantics be used for in a choreography description language?
>>
>>To explore this question we need to define a few basic concepts and 
>>terms.
>>
>>I make the distinction between locating a service and a 
>>choreography. A 
>>service is a Web Service
>>that exhibits a WSDL interface.  A choreography is not a Web Service 
>>but a contract between two or more
>>services.
>>
>>In order for a service to be part of a choreography it must agree to 
>>abide by the contract and so exhibit the
>>required external observable behaviour.
>>
>>Choreographies are non-executable descriptions of observable 
>>behaviour. 
>>They are loosely bound to the
>>services that they describe which allows services to join and leave a 
>>choreography dynamically. A description
>>of a choreography fully elaborates the externally observable message 
>>exchanges between a collection of
>>services.
>>
>>Consider a service P that sends a message to service Q and 
>>waits for a 
>>response while Q sends a message
>>to service R and then continues doing something else but is 
>>now able to 
>>receive a response from R asynchronously,
>>which is required for Q to respond to P. This is exactly the 
>>observable 
>>behaviour that a CDL should describe. It includesl
>>all of the dependencies that are externally visible. In this case the 
>>response from R back to Q is a dependency for
>>P. So a CDL will describe the visible "interactions" that occur and 
>>their visible dependencies.
>>
>>A choreography may include certain system level interaction that are 
>>also externally observable. To a large extent
>>the external observable behaviour as at the level of a business model 
>>as opposed to a system model [aka MDA].
>>System model behaviour that is observable might be said to be 
>>transaction boundaries in either a conventional ACID based
>>approaches or long running transactions of some flavour. System model 
>>behaviour might include any observable exceptions
>>that are propagated by the participant services.
>>
>>Service location/selection
>>Semantics in a CDL are most likely to be used in support of a query 
>>that may be used to dynamically locate a choreography by a 
>>Web Service.
>>This is akin to a conversation (that may be in play already) in which 
>>someone enters a room listens for a while and decides they wish to 
>>participate
>>or not - in which case they might move to another room and 
>>listen in on 
>>another conversation and so on. The sort of thing we do at parties.
>>
>>This sort of conversation surfing that we do at parties is a good 
>>analogy for the way in which web services might come together to 
>>accomplish
>>some user-driven goal. They also surf for conversations that we call 
>>choreography descriptions. They evaluate the conversation, not by 
>>listening,
>>but by engaging in a short chat to identify if the conversation makes 
>>sense for them (i.e. do they understand it at all), that they are not 
>>going to get into trouble by entering (i.e. It all sounds too 
>>political 
>>for me to enter into this conversation) or that they do understand, 
>>feel comfortable with but decide that one of the individuals 
>>they don't 
>>like and so move on.
>>
>>For a Web Service it might look more like the following:
>>
>>	I am a Web Service called P.
>>	I would like to participate in a choreography to sell 
>>paint? (do I 
>>understand what they are talking about)
>>	I would like to participate in a choreography to sell 
>>paint such that 
>>the choreography is lockfree? (am I comfortable)
>>	I would like to participate in a choreography to sell 
>>paint such that 
>>the choreography supports a
>>	role called "paint mixer"? (do I have a way into this 
>>conversation)
>>	I would like to participate in a choreography to sell 
>>paint such that 
>>the choreography supports the an exchange
>>	between a paint broker role and a paint shipper role 
>>where the broker 
>>and shipper never directly communicate
>>	to me as a supplier? (I'll only join in if I don't have 
>>to talk to ...)
>>	I would like to participate in a choreography to sell 
>>paint such that 
>>the choreography supports a transaction model
>>	that is the same as mine? (can I steer it into my comfort zone)
>>	I would like to participate in a choreography to sell 
>>paint such that 
>>the choreography but I need to be sure that it understands
>>	the concept of "deferred payment"? (I understand the overall 
>>conversation but I never heard of that word before)
>>
>>The bracketed part at the end of each query tries to 
>>re-phrase what is 
>>being asked in party terms.
>>
>>To be able to support such queries requires that a choreography makes 
>>visible the semantics of liveness, roles and allows information about 
>>roles and behaviour to be inferred.
>>
>>The query about transaction models might require the use of clear 
>>separation into system and conceptual artifacts of a choreography. It 
>>might also require some form of behavioural equivalence such a 
>>bi-simulation over all of part of the choreography description.
>>
>>The last query, about deferred payment, might be a combination of 
>>behavioural equivalence and some form of ontology matching. The 
>>ontology matching might be used to find an equivalence relationship 
>>between "deferred payment" and "late payment".
>>
>>I am not claiming that we can achieve all of this. What I am 
>>trying to 
>>do is put some meat on what semantics might mean to a choreography 
>>description language. I do like the idea of Web Services locating a 
>>choroegraphies with which they can be shown to be compatible and then
>>joining those choreographies in the right way.
>>
>>To do the inferencing example we would be well served if we utilised 
>>RDF or something similar. The use of concept matching, "deferred 
>>payment" vs "late payment", would probably necessitate the use of 
>>ontologies (and so OWL or OWL-light) to express the 
>>ontologies and the 
>>choreography would make some reference to the ontology that it uses 
>>which could then enrich the choreography description with the 
>>appropriate concepts.
>>
>>Summary
>>So to summarise I think that there are two areas in which semantics 
>>play a role in choreography. Firstly to locate a choreography 
>>and check 
>>it matches the requirements of a requesting Web Service. Secondly to 
>>reuse ontologies and the concepts that they describe as terms in a 
>>choreography description linking the description to the choreography.
>>
>>Phew! That is as much as I can write on this topic without making it 
>>totally unreadable, which it probably is anyway.
>>
>>Cheers
>>
>>Steve T
>>
>>p.s. As a small anecdote on the importance of semantics and 
>>how easy it 
>>is to get it wrong:
>>
>>My father worked for the GEC in the UK for many years. He was 
>>responsible for QA (both hardware and software). He told me at the 
>>weekend
>>that he has asked his PA to draft a letter to be sent to some 
>>sub-contracting company that GEC were using at that time. In 
>>dictation 
>>he said
>>something like:
>>
>>	"I would like to see your QA procedures so that I can 
>>assess the 
>>effectiveness of your QA system."
>>
>>Alas in production the letter sent turned out to be:
>>
>>	"I would like to see your QA procedures so that I can 
>>assess the 
>>effectivemess of your QA system."
>>
>>As you can see even a single letter can change the meaning of things.
>>
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>>    
>>
>
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Received on Monday, 1 December 2003 09:56:23 GMT

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