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

From: Cutler, Roger (RogerCutler) <RogerCutler@ChevronTexaco.com>
Date: Sun, 4 May 2003 13:30:03 -0500
Message-ID: <7FCB5A9F010AAE419A79A54B44F3718E01817E11@bocnte2k3.boc.chevrontexaco.net>
To: "Geoff Arnold" <Geoff.Arnold@sun.com>, "Champion, Mike" <Mike.Champion@SoftwareAG-USA.com>
cc: www-ws-arch@w3.org, Suresh_Damodaran@stercomm.com

I would have thought that email is, by its essence, always asynchronous,
that most MOM systems have well-defined synchronous and asynchronous
modes, and that the last example is a "scheduled asynchronous
communication" and clearly asynchronous.  I do not think that
"synchronous" is in any way synonymous with "scheduled" and I don't
think I can recall any candidate definitions in which that was the case.

I'm sorry, I still think this is just giving up and that in fact the
terms have a domain of validity in which they can be rigorously defined.
It seems to me that doing so, and if possible defining some domain where
they are not meaningful (although I really haven't personally understood
why there would be such a domain or seen it described in a way that I
understand) would be more productive.

I think that Suresh's definition, which I had not seen before, is pretty
good.  The one thing that might be questionable is that it uses the word
"channel".  I do not really know what a channel is.  Possibly
understanding that would help to define the domain in which
synch/asynch has meaning.  However, if "synchronous" depends on having a
channel, I don't exactly see the problem with using the term
"asynchronous" for situations where the concept of channel is somehow
not relevant.

For convenience, here is Suresh's definition again.  Note that it rather
explicitly defines its domain of applicability with the phrasee "over a
communication channel" (whatever that means).  

Synchronous: An entity A  communicates with entity B  synchronously over
a communication channel if and only if A requires a response back from B
and A does not initiate another communication to B using the same
communication channel before it receives that response.

Asynchronous: When A communicates with B  asynchronously, A does not
always require a response back from B. Irrespective of whether A
requires a response or not, A may initiate another communication to B on
the same communication channel.

*"communication" can be a message or a signal.

Here is a version that I have word-smithed a bit.

Synchronous: An entity A communicates with entity B synchronously over a
communication channel if and only if A requires a response back from B
and A does not initiate another communication to B using the same
communication channel before it receives that response or the
communication has failed.  Entity A may consider the communication to
have failed if it does not receive a response back in some length of
time.

Asynchronous: When A communicates with B  asynchronously, A may not
require a response back from B. Whether A  requires a response or not, A
may initiate another communication to B on the same communication
channel at any time.



-----Original Message-----
From: Geoff Arnold [mailto:Geoff.Arnold@sun.com] 
Sent: Saturday, May 03, 2003 6:02 PM
To: Champion, Mike
Cc: www-ws-arch@w3.org
Subject: Re: Draft language on MEPs, synchronous, and asynchronous.



> I for one would be happy to substitute something like:
>
> MEP's involving that require a response from the responder back to the

> initiator before the initator can initiate another communication that 
> responder using the same communication  channel are frequently 
> referred to a "synchronous."
>
> I see that as a friendly amendment that defines "closely coupled in
> time"
> more rigorously.

I actually see this as an excellent argument *against* trying to get
more rigorous. First, the "same communication channel" is hopelessly
transport-specific. (Try that over MOM or SMTP!) Second, a MEP that
requires me to send you a message on the third Tuesday of every month is
perfectly *synchronous*. "Closeness" of coupling is a relative thing.

Used informally, the terms can be usefully descriptive. Let's leave it
at that.
Received on Sunday, 4 May 2003 14:30:38 GMT

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