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Re: Draft language on MEPs, synchronous, and asynchronous.

From: Assaf Arkin <arkin@intalio.com>
Date: Thu, 01 May 2003 14:02:11 -0700
Message-ID: <3EB18B53.9000509@intalio.com>
To: Geoff Arnold <Geoff.Arnold@Sun.COM>
CC: www-ws-arch@w3.org


In a higher level composition it becomes necessary to distinguish 
whether a MEP is synchronous or asynchronous independent of the protocol 
being used. The protocol may change depending on which service ends up 
being used, but some aspect of the exchange need to be known as it 
affects when activities will be performed with relation to each other, 
how transactions are used, and what type of coordination is required.

For example, a MEP that involves sending one message is inherintly 
asynchronous. The sender may complete it's activity (sending) much 
earlier than the receiver completes its activity (receiving). In 
practice some protocol may force both activities to happen concurrently, 
but at the higher protocol-independent level this cannot be assumed. So 
the assumption must be made that the MEP is asynchronous.

The language proposed here allows us to conclude whether a MEP is 
synchronous or asynchronous by looking at the MEP alone without being 
too dependent on the actual protocols that are being used.


Geoff Arnold wrote:

> Apologies for the delays: clearly the result of an asynchronous
> message pattern!
> Draft language for the WSA Glossary on Message Exchange
> Patterns, Synchronous MEPs, and Asynchronous MEPs.
> In general, my objective is to beef up the MEP concept, to tie
> "synchronous" and "asynchronous" to MEPs, and to note that
> they are really just informally descriptive terms. I've incorporated
> comments from Chris Ferris and others, but any problems are due to me.
> ----------------------------------------------------------------------
> Asynchronous Message Exchange Pattern
> See discussion under Message Exchange Pattern
> -------------------------------------------------
> Synchronous Message Exchange Pattern
> See discussion under Message Exchange Pattern
> -------------------------------------------------
> Message Exchange Pattern (MEP)
> [Derived from 
> http://www.w3.org/TR/2002/WD-soap12-part1-20020626/#soapmep ]
> A MEP is a template that establishes a pattern for the exchange of 
> messages
> between SOAP nodes. A MEP MAY be supported by one or more underlying 
> protocol
> binding instances.
> This section is a logical description of the operation of a MEP. It is 
> not
> intended to describe a real implementation or to imply that a real
> implementation needs to be similarly structured.
> In general the definition of a message exchange pattern:
>   * Is named by a URI.
>   * Describes the life cycle of a message exchange conforming to the 
> pattern.
>   * Describes the temporal/causal relationships of multiple messages 
> exchanged
>     in conformance with the pattern.
>   * Describes the normal and abnormal termination of a message exchange
>     conforming to the pattern.
> Underlying protocol binding specifications can declare their support 
> for one or
> more named MEPs.
> [New language]
> In principle, MEPs may be arbitrarily complex, and may include various
> temporal relationships between messages.  In practice, there is a 
> small number
> of patterns for which the temporal relationships are well (if informally)
> understood. MEPs which describe closely coupled, or lock-step 
> interactions
> are frequently referred to as "synchronous". Examples include RPC-style
> request-response interactions and some kinds of transactional exchanges.
> Other MEPs allow messages to be sent without precise sequencing, and 
> these
> are described as "asynchronous". Examples include a flow of sensor event
> messages which need not be individually acknowledged, and an auction 
> in which
> parties may submit bids at any time during the auction.
> The terms "synchronous" and "asynchronous" are descriptive, and do
> not correspond precisely to properties of MEPs. Occasionally the
> terms may be associated with particular message transport features,
> such as the re-use of a session. While specific implementations may
> support such notions, a dependency on such a feature would violate
> protocol independence, and therefore be problematic.
> Many (most?) web services do not use published MEP's, but instead rely
> on more or less informal patterns and techniques. In such cases, the
> terms "synchronous" and "asynchronous" may be used to indicate the
> type of informal pattern being used. They may also indicate whether
> or not coordination and synchronization techniques such as correlation
> data and particular transport bindings are to be used.
Received on Thursday, 1 May 2003 17:04:30 UTC

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