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RE: MEP text

From: <jones@research.att.com>
Date: Fri, 27 Jun 2003 12:46:47 -0400 (EDT)
Message-Id: <200306271646.h5RGklH08335@bual.research.att.com>
To: dmh@contivo.com, www-ws-arch@w3.org

	From: Dave Hollander <dmh@contivo.com>
	To: www-ws-arch@w3.org
	Date: Fri, 27 Jun 2003 09:04:47 -0700
	Subject: RE: MEP text

	Thanks Mark. 

	I have one big concern.

	> a message exchange pattern may be expressed
	> in a choreography description language.

	I fear that this may end up in circular difinition.
	I expect that Choreography will need to ground upon the definition 
	MEP's. 

It would only be circular if we said that choreography was subsumed by
MEPs, which it is not.  The way I see it, MEPs talk in terms of the
messages exchanged at a a rather atomic binding/operation level.
Choreography builds on the foundation that the MEPs provide.  Since
one-way messages can also be simple MEPs in their own right, you can
potentially express MEPs such as request-response in a choreography
language.  You would not typically want to do so for things like the
SOAP HTTP request-response binding, but you might use choreography to
describe an asynchronous request-response rendezvous that uses a SOAP
module for callbacks.

	I am assuming that MEPs describe the arrows in the diagram, not
	the entire diagram. If not, wouldn't it be clearer to use the word
	MEP in (2) and (3)? eg: B then uses an MEP to send a separate ...

Yeah, since an MEP is a message and its response(s), I was using
message and MEP rather interchangeable in (2) and (3).  That could
be tightened up.

	------------------------------
	    A------------>B
	     \            |
	      \ (3)       | (2)
	       \          V
	        --------->C

	In this pattern:

	(1) node A uses an MEP (possibly request-response) to communicate
	    initially with B.

	(2) B then sends a separate, but related message to C.

	(3) Node A sends another message to C but gets a reply only after C
	    has processed (2).

	-----------------------------------------------------




	I also have a few minor concerns:

I inherited both of the following clauses from the original text
and left them in.

	> a message exchange pattern has
	> a unique identifier

There may some merit in keeping this, particularly if the WSDL
group uniquely identifies its various patterns.

	Shouldn't that be "may have"?

	> a message exchange pattern is
	> the life cycle of a message exchange

I would just as soon lose this one altogether.

	Is "life cycle" defined somewhere? I doubt there is any widespread
	understanding of a precise difinition for life cycle.

	I know I would have difficulty defining when the life cycle ends
	for a "message exchange". In one sense, the message exchange 
	within this wg will span the entire existance of the WG. 
	I would make the same assumption in thinking about two agents
	and their message exchanges.


	daveh

Mark Jones
AT&T


	-----Original Message-----
	From: Mark Jones [mailto:jones@research.att.com]
	Sent: Friday, June 27, 2003 8:49 AM
	To: www-ws-arch@w3.org
	Subject: MEP text



	Per my action item to flesh out section 2.2.22 on MEPs, here is a new
	synthesis of my January f2f MEP text with the current doc structure.
	It includes a discussion of MEPs at both the binding/operation level
	and the choreography level.  It also includes both the SOAP and WSDL
	perspectives on MEPs.  Finally, there is an example to motivate the
	choreography issues and a stab at a layering discussion.  Some of the
	material can be moved elsewhere for editorial purposes.

	Mark Jones
	AT&T

	================================================================

	2.2.22 Message Exchange Pattern (MEP)
	2.2.22.1 Summary
	A message exchange pattern is a minimal set of messages, together with
	their sender and receivers, that constitutes a single use of a
	service.

	2.2.22.2 Relationships to other elements
	a message exchange pattern is set of messages between agents that
	corresponds to a single instantiation of a service

	a message exchange pattern is
	a feature of the architecture

	a message exchange pattern has
	a unique identifier

	a message exchange pattern is
	the life cycle of a message exchange

	a message exchange pattern describes
	the temporal and causal relationships, if any, of multiple
	messages exchanged in conformance with the pattern.

	a message exchange pattern describes
	the normal and abnormal termination of any message exchange conforming
	to the pattern.

	a message exchange pattern may be expressed
	in a choreography description language.

	a message exchange pattern may realize
	message correlation

	a message exchange pattern may describe
	a service invocation.

	2.2.22.3 Description

	Distributed applications in a Web services architecture communicate
	via message exchanges.  A Message Exchange Pattern (MEP) is a template
	that establishes a pattern for the exchange of (one-way) messages
	between agents.  These message exchanges are logically factored into
	patterns that may compose at different levels.  These patterns can be
	described by state machines that indicate the flow of the message, the
	handling of faults that may arise, and the correlation of messsages.

	At the SOAP messaging level, an MEP refers to an exchange of messages
	in various invoking-response patterns.  Each message at this level may
	travel across multiple transports en route to its destination.  A
	message and its response(s) are correlated, either implicitly in the
	underlying protocol (e.g., request-response in HTTP) or by other
	correlation techniques implemented at the binding level.  The
	exchanges may be synchronous or asynchronous.  An asynchronous
	exchange involves some form of rendezvous to associate the message and
	its responses, typically due to separate invocations of the underlying
	transport or to long response time intervals.

	Web service description languages at the level of WSDL view MEPs from
	the perspective of a particular service node.  A simple
	request-reponse MEP, for example, appears as an incoming message which
	invokes an operation and an associated outgoing message with a reply.
	Extremely simple applications based on single message exchanges may be
	adequately characterized at the operation level.  More complex
	applications require multiple, related message exchanges; choreography
	describes patterns where the units of communication are themselves
	instances of MEPs.  Especially at this higher level of abstraction,
	the communicating nodes are seen as peers which play various roles in
	more complex applications.  These choreographic patterns form the
	communication structure of the application.

	Consider the following simple structure:

	          (1)
	    A------------>B
	     \            |
	      \ (3)       | (2)
	       \          V
	        --------->C

	In this pattern:

	(1) node A uses an MEP (possibly request-response) to communicate
	    initially with B.

	(2) B then sends a separate, but related message to C.

	(3) Node A sends another message to C but gets a reply only after C
	    has processed (2).

	The example makes it clear that the overall pattern can't be described
	from the perspective of any single node.  The pattern involves
	constraints and relationships among the various messages.  It also
	illuminates the fact that exchange (1) is in in-out MEP from the
	perspective of node B, and mirrored by an out-in MEP from the
	perspective of node A.  Finally, an actual application instantiates
	this communication pattern and completes the picture by adding
	computation at A, B and C to carry out application-specific
	operations.

	The following stack roughly captures the typical layering described
	above:

	     application
	        |
	        | (application instantiates some choreographic structure
	        |  and provides message content)
	        V
	     choreography
	        |
	        | (application + choreography yields an XML Infoset,
	        |   attachments, and messaging features including the
	        |   MEP)
	        V
	     message transport binding
	        |
	        | (the binding produces a serialization, implements
	        |   required features, manages MEP-level coordination
	        |   for associating request/responses, etc.)
	        V
	     transfer/transport protocol

	It is instructive to consider to consider the kinds of fault reporting
	that occur in such a layering.  Consider a fault at the transport
	protocol level.  This transport level may itself be able to manage
	certain faults (e.g., re-tries), but it may also simply report the
	fault to the binding level.  Similarly the binding level may manage
	the fault (e.g., by re-initiating the underlying protocol) or may
	report a SOAP fault.  The choreography and application layers may be
	intertwined or separated depending on how they are architected.
	There is also no rigid distinction between the choreography and
	binding layers; binding-level MEPs are essentially simple choreographies.
	Conceptually, the choreographic level can enforce constraints on
	message order, maintain state consistency, communicate choreographic
	faults to the application, etc. in ways that transcend particular
	bindings and transports.
Received on Friday, 27 June 2003 12:46:39 GMT

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