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section 2.2.22 Message Exchange Pattern (MEP)

From: Mark Jones <jones@research.att.com>
Date: Mon, 7 Jul 2003 14:17:29 -0400 (EDT)
Message-Id: <200307071817.h67IHTL09985@bual.research.att.com>
To: w3c-wsa-editors@w3.org
Cc: www-ws-arch@w3.org

Editors,

Here is revised material for section 2.2.22, Message Exchange Pattern
(MEP).  I have two caveats.

1. Chris Ferris suggests eliminating the phrase "that describes a
single use of a service", which has been in the text to date and was
also in David Booth's proposed revision which I incorporated.
Personally, I think the definition needs some qualifier such as this
in order to differentiate it MEPs from arbitrary or even
choreographically-related collections of messages that happen to be
exchanged between agents.  Chris's proposal to drop the phrase would
create an overly broad scope for the term MEP.

2. I have retained the layering discussion and example which
illustrates the clarifies the MEP/choreography distinction.  This
material could be moved elsewhere for purposes of overall exposition,
but IMO it should be retained somewhere.

Mark Jones
AT&T

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

2.2.22 Message Exchange Pattern (MEP)
2.2.22.1 Summary
A message exchange pattern is a template for the exchange of messages
between agents that describes a single use of a service.

2.2.22.2 Relationships to other elements
a message exchange pattern is
a template for the exchange of messages between agents
that describes a single use 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 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 actor.  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 actors 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) actor A uses an instance of an MEP (possibly request-response) to communicate
    initially with B.

(2) B then uses a separate, but related instance of an MEP to communicate with C.

(3) Actor A uses another instance of an MEP to communicate with C but gets a reply
    only after C has processed (2).

The example makes it clear that the overall pattern cannot be described
in terms of the inputs and outputs of any single actor.  The pattern involves
constraints and relationships among the messages in the various MEP instances.  It also
illuminates the fact that exchange (1) is in in-out MEP from the
perspective of actor B, and mirrored by an out-in MEP from the
perspective of actor 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 Monday, 7 July 2003 14:17:21 GMT

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