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Re: Correlation Requirements

From: Mark Little <mark.little@arjuna.com>
Date: Mon, 11 Aug 2003 11:15:14 +0100
Message-ID: <002301c35ff1$75341350$850a090a@exhp>
To: "Burdett, David" <david.burdett@commerceone.com>, "'Keith Swenson'" <KSwenson@fsw.fujitsu.com>, "'Monica Martin'" <monica.martin@sun.com>
Cc: "'Martin Chapman'" <martin.chapman@oracle.com>, "'Yves Lafon'" <ylafon@w3.org>, <jdart@tibco.com>, "'Ugo Corda'" <UCorda@SeeBeyond.com>, "'Cummins Fred A'" <fred.cummins@eds.com>, <public-ws-chor@w3.org>
RE: Correlation RequirementsHas anyone looked at the recently released WS-Context specification? It's aims seem to be in line with precisely this kind of correlation.

Mark.
  ----- Original Message ----- 
  From: Burdett, David 
  To: 'Keith Swenson' ; Burdett, David ; 'Monica Martin' 
  Cc: 'Martin Chapman' ; 'Yves Lafon' ; jdart@tibco.com ; 'Ugo Corda' ; 'Cummins Fred A' ; public-ws-chor@w3.org 
  Sent: Monday, August 11, 2003 7:28 AM
  Subject: RE: Correlation Requirements


  I think you have two use cases:
  1. Where there is *no* data inside the "payload" that can be used for corellation purposes, and
  2. Where there *is* data inside the "payload" that can be used for corellation

  Now, since the first case will sometimes exist, when there is a need for corellation, then you really have no option but to put some type of "choreography instance identifier" in data that is carried with the message, or what, for the purposes of this email, I am calling message "metadata" (Note, for SOAP this would be almost be data in a SOAP header).

  However if you always insist that the "choreography instance identifier" is present in the message metadata, then, in the second case, there is a risk that the data inside the payload might be inconsistent with choreography instance identifier in the messsage metadata. This inconsistency is almost certainly incorret and so there is an error which would should be flagged.

  You can avoid this inconsistency, if, message metadata, you reference the data in the payload instead with a "choreography instance reference", but at the expense of more complexity in how the correllation is done since it will be impossible, for example to restrict the type of the correlation which could include a combination of different data of different types. For example you might need to do correllation based on a combination of "supplier identifier, year and order no".

  My *personal* $0.02c, would be to always have a "choreography instance identifier" in the data carried with the message, e.g. the SOAP header, as:
  a) There is always just one way to do correlation at "messaging middleware" level, i.e. in the software layer between the transport protocol software and the applicaiton
  b) The probability of inconsistency between the message
  c) It is *much* simpler!

  Now, before anyone says anything, I know this is talking about a design, but I think that sometimes thinking about design problems actually helps clarify the problems ... with the proviso that you a) record your design decisions (i.e. in emails like this) and b) you are prepared to revisit the problem in the light of a better understanding of the problems/issues. If we try and postpone *all* these things, then we are just creating more problems for later in my opinion!

  David
    -----Original Message-----
    From: Keith Swenson [mailto:KSwenson@fsw.fujitsu.com]
    Sent: Sunday, August 10, 2003 10:46 PM
    To: Burdett, David; 'Monica Martin'
    Cc: 'Martin Chapman'; 'Yves Lafon'; jdart@tibco.com; 'Ugo Corda'; 'Cummins Fred A'; public-ws-chor@w3.org
    Subject: RE: Correlation Requirements


    I would like to understand why it is important to leave so many different ways of carrying correlation information.  Our job is to produce a specification that will ensure interoperability.  If there are an infinite number of ways to communicate correlation information, then we haven't really specified anything, have we?  

    The reason I am probing this is because I want to understand what is the underlying "requirement" that we avoid being prescriptive.  It clearly would be a benefit to the entire industry if we could stick with your requirements 1 & 2, except change 2 to specify exactly which header field MUST contain the choreography instance id.  Why is it that "you don't want to have to be forced to use an identifier in the header."?  Seems to me that the effort and cost to put this in a consistent place would be far less effort and cost that would be incurred by coding all the various point-to-point variations due to each implementation using a different way of coding correlation information.

    -Keith Swenson
      -----Original Message-----
      From: Burdett, David [mailto:david.burdett@commerceone.com]
      Sent: Thursday, August 07, 2003 3:15 PM
      To: 'Monica Martin'; Burdett, David
      Cc: 'Martin Chapman'; 'Yves Lafon'; jdart@tibco.com; 'Ugo Corda'; 'Cummins Fred A'; public-ws-chor@w3.org
      Subject: RE: Correlation Requirements


      Monica 

      The reason I included requirements 2 and 3 is that they reflect two use cases ... 

      If we assume that there has to be some data in the message that can be used for correlation when the message is taking part in a choreography then requirement 2 arises becaus it is possible that there is no data in the payload (or anywhere else) that can be used for correlation purposes.

      Requirement 3 arises because there maybe data that can be used in the payload and therefore you don't want to have to be forced to use an identifier in the header.

      However, I can also see your point that the existing requirement definitions could be a bit too presrcriptive, so how about these as alternatives, I've added a fourth requirement which hopefully makes it clearer. The complete set is as follows ...

      Requirement 1 (not changed) 
      If a message is being sent between roles as part of the "performance" of a choreography, then that message MUST identify the "choreography instance" to which it belongs.

      Requirement 2 (changed) 
      A choreography instance MUST be identified by specifying a separate identifier associated with the payloads in the message where there is no combination of data in the "payload(s)" that can be used to uniquely identify the choreography instance that is being performed.

      Requirement 3 (changed) 
      A choreography instance MAY be identified by referencing a combination of one or more items of data in the "payload(s)" of the message where that combination of data can be used to uniquely identify the choreography instance that is being performed.

      Requirement 4 (new) 
      A choreography  instance MAY be identified by specifying a separate identifier associated with payload(s) in the message even if there is a combination of data in the "payload(s)" that can be used to uniquely identify the choreography instance that is being performed.

      David 
      -----Original Message----- 
      From: Monica Martin [mailto:monica.martin@sun.com] 
      Sent: Thursday, August 07, 2003 3:03 PM 
      To: Burdett, David 
      Cc: 'Martin Chapman'; 'Yves Lafon'; jdart@tibco.com; 'Ugo Corda'; 
      'Cummins Fred A'; public-ws-chor@w3.org 
      Subject: Re: Correlation Requirements 



      Burdett, David wrote: 

      > A very good point Martin - I was presuming "a" solution which is 
      > perhaps premature. 
      > 
      > So let's do this the "right" way and think about it in terms of 
      > requirements so here's my $0.02c on what they might be ... 
      > 
      > Requirement 1 
      > If a message is being sent between roles as part of the "performance" 
      > of a choreography, then that message MUST identify the "choreography 
      > instance" to which it belongs 
      > 
      > Requirement 2 
      > A choreography instance MAY be identified by specifying a unique 
      > identifier in "metadata" (e.g. a SOAP header) associated with the message. 
      > 
      > Requirement 3 
      > A choreography instance MAY be identified by referencing a combination 
      > of one or items of data in the "payload(s)" (e.g. the SOAP body and/or 
      > attachments) of the message. 
      > 
      mm1: I would suggest on Reqt 2 and 3 that we specify the requirement not 
      the solution, of which these requirements appear to do both.  
      Particularly, a choreography instance may be referenced, - do we specify 
      how? 

      > To make these complete, we should also define, roles, performance, 
      > choreography instance, metadata and payload, but that can come later! 
      > 
      > Thoughts? 
      > 
      > David 
      > 
Received on Monday, 11 August 2003 06:16:16 UTC

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