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RE: Snapshot of Web Services Glossary

From: Assaf Arkin <arkin@intalio.com>
Date: Mon, 24 Feb 2003 11:30:24 -0800
To: "Cutler, Roger \(RogerCutler\)" <RogerCutler@chevrontexaco.com>, "Walden Mathews" <waldenm@optonline.net>, "Francis McCabe" <fgm@fla.fujitsu.com>
Cc: <www-ws-arch@w3.org>
Message-ID: <IGEJLEPAJBPHKACOOKHNEEOJDDAA.arkin@intalio.com>



> -----Original Message-----
> From: www-ws-arch-request@w3.org [mailto:www-ws-arch-request@w3.org]On
> Behalf Of Cutler, Roger (RogerCutler)
> Sent: Monday, February 24, 2003 10:30 AM
> To: Walden Mathews; Francis McCabe
> Cc: www-ws-arch@w3.org
> Subject: RE: Snapshot of Web Services Glossary
>
>
> Incidentally, in one of the earlier go-arounds on this subject I believe
> that it was pointed out that one can build a synchronous interaction out
> of asynchronous components.  That is, as I understand it a TCP/IP
> communication can be viewed as synchronous by the application using the
> service, but under the covers it is composed of a conversation of
> asynchronous messages.  This seems to say that whether something is
> synchronous or asynchronous depends on how you are viewing it.  That is,
> the observer is part of the system somehow here.

At the lowest level communication is always asynchronous. The primitives are
send/receive. Ethernet is asynchronous, and so is IP. But you can build a
synchronous protocol using two asynchronous messages flowing in opposite
direction and time-out, which is what TCP does. And on top of that HTTP. You
can also have an asynchronous interaction using synchronous communication,
e.g. send a message using HTTP, wait for it to be received, but don't wait
for it to be processed. If the interaction requires some other response that
will come later on, then the HTTP operation is viewed as asynchronous.

This is not really a matter of observer but of scope. At a minimum a
synchronous operation has two messages flowing in opposite direction. Let's
say I have such a synchronous interaction performed using two WSDL
operations.

At the choreography level I have request/response, so that interaction is
synchronous.
At the operation level I have two separate interactions, send and receive,
so each one is asynchronous
I can use HTTP to send a message and confirm its receipt (but not delivery),
so HTTP usage is synchronous
Messages travel as packets using IP, so the low-level communication is
asynchronous

At every scope you look at what an atomic interaction performs:

- send one side, receive at the other -> always asynchronous
- two asynchronous in opposite directions -> minimum for synchronous

arkin

>
> At the last F2F I got so confused about what "synchronous" really means
> that I claimed the only interaction that I was SURE was synchronous
> would be if I punched you in the nose -- but then somebody told me that
> at the neural level this interaction is not really synchronous.  Again,
> it seems that in order to tell whether something is synchronous or not
> you need to consider not only what is happening with the sender and
> receiver, but also the observer.
>
> -----Original Message-----
> From: Walden Mathews [mailto:waldenm@optonline.net]
> Sent: Monday, February 24, 2003 12:10 PM
> To: Cutler, Roger (RogerCutler); Francis McCabe
> Cc: www-ws-arch@w3.org
> Subject: Re: Snapshot of Web Services Glossary
>
>
> Roger,
>
> I agree with you (this time in public).  While the group complains that
> "blocking" is implementation detail and therefore technically an
> inaccurate description of synchrony, I think the reality is that the
> readership of the architecture document and glossary will tend to be
> programmers and IT people who will best understand in terms of blocking
> threads or processes.  It's going to be a little hard to bridge between
> common imprecise understandings and more esoteric ones based on deeper
> analysis, like the one Frank offered.  See my rough attempt at [1].
>
> In reading through the scenarios, I noticed some strange usage of the
> terms in question, like "send an asynchronous message".  As I understand
> it, synchrony is a property of a communication relationship, and so has
> no meaning when applied to a single message.  Here, it could mean the
> sender doesn't expect a reply at any particular time.  It could also
> mean the equivalent of "unsolicited".  Then there's a reference to a
> "synchronous HTTP POST".  Not sure if "synchronous" adds any meaning
> there.  I can visualize a task to clean up the usage of "synch" and
> "asynch" in the scenarios document.  Part of that could be through
> refinement of their respective definitions, but some of it is in
> scrutinizing their application, too.  Perhaps this is a candidate issue.
>
> Thanks,
>
> Walden
>
> [1] http://lists.w3.org/Archives/Public/www-ws-arch/2003Feb/0283.html
>
> ----- Original Message -----
> From: "Cutler, Roger (RogerCutler)" <RogerCutler@chevrontexaco.com>
> To: "Francis McCabe" <fgm@fla.fujitsu.com>
> Cc: "Assaf Arkin" <arkin@intalio.com>; "David Booth" <dbooth@w3.org>;
> "Martin Chapman" <martin.chapman@oracle.com>; <www-ws-arch@w3.org>;
> "Hugo Haas" <hugo@w3.org>
> Sent: Monday, February 24, 2003 11:29 AM
> Subject: RE: Snapshot of Web Services Glossary
>
>
> >
> > Sort of sounds to me like this definition would shrink the effective
> > scope of "synchronous" essentially to zero, at least as far as matter
> > that concern this WG.  I do not think that this would be useful.
> >
> > -----Original Message-----
> > From: Francis McCabe [mailto:fgm@fla.fujitsu.com]
> > Sent: Sunday, February 23, 2003 5:20 PM
> > To: Cutler, Roger (RogerCutler)
> > Cc: Assaf Arkin; David Booth; Martin Chapman; www-ws-arch@w3.org; Hugo
>
> > Haas
> > Subject: Re: Snapshot of Web Services Glossary
> >
> >
> >
> > here is a very straightforward definition of synchronous:
> >
> > A rendezvous of two activities is synchronous if they complete
> > simultaneously.
> >
> > The language, if not the definition, comes from Communicating
> > Sequential Processes.
> >
> > One might try to sharpen this up by defining simultaneously in terms
> > of clocks etc. But that is not necessary; because an alternative view
> > of this definition is:
> >
> > An activity involved in a synchronous rendezvous may assume that the
> > rendezvous is complete for both sides if it 's side completes.
> >
> > Frank
> >
> >
> >
> > On Saturday, February 22, 2003, at 12:10  PM, Cutler, Roger
> > (RogerCutler) wrote:
> >
> > >
> > > Oh, perhaps I should express an opinion about the alternatives.  My
> > > druthers, for what they are worth, is that the "blocking" definition
>
> > > is the least desirable.  I base this on two factors: 1)I don't
> > > really know what it means in a world where applications can easily
> > > have multiple threads; 2)It does not seem to have any aspect of
> > > timeliness,
> >
> > > or shortness of time, in it -- and my intuitive understanding of
> > > synchronous is that it has something to do with things happening in
> > > a timely manner.  I personally like the ones that are based on how
> > > fast things happen the best.
> > >
> > > That's my opinion, but I am MORE than willing to accept any of the
> > > approaches to the concept, as long as it is just one definition.
> > >
> > > -----Original Message-----
> > > From: Cutler, Roger (RogerCutler)
> > > Sent: Friday, February 21, 2003 9:05 PM
> > > To: 'Assaf Arkin'; David Booth; Martin Chapman; www-ws-arch@w3.org
> > > Cc: 'Hugo Haas'
> > > Subject: RE: Snapshot of Web Services Glossary
> > >
> > >
> > > Yeah, this is the approach to synchronous that I recall impressed me
>
> > > as being MOST different from the others.  I recall that there was a
> > > considerably more formal definition along these lines some months
> > > ago.
> >
> > > Well, if not more formal at least longer, but along the same lines
> > > with the concept of agreeing about the time of day being the key
> > > factor.
> > >
> > > OK, there is the "blocking" thing, as in David's definition, there
> > > is this thing with agreeing about timing of clocks, and there have
> > > also been other definitions that were pretty formal but which ran
> > > along the
> >
> > > lines of "how soon" things happen.
> > >
> > > IMHO there are at least three completely different understandings of
>
> > > what synchronous means floating around.  They all sound really good
> > > to
> >
> > > me, but they are not the same.  I would REALLY like it if we could
> > > agree on one of them and make sure that when we use the word we
> > > agree that we are using the word in that sense.  Or, perhaps we
> > > could subset
> >
> > > them somehow, as in synchronous(1) ... Synchronous(N).
> > >
> > > -----Original Message-----
> > > From: Assaf Arkin [mailto:arkin@intalio.com]
> > > Sent: Friday, February 21, 2003 12:26 PM
> > > To: David Booth; Martin Chapman; www-ws-arch@w3.org
> > > Cc: 'Hugo Haas'
> > > Subject: RE: Snapshot of Web Services Glossary
> > >
> > >
> > >
> > >
> > >
> > >> -----Original Message-----
> > >> From: www-ws-arch-request@w3.org
> > >> [mailto:www-ws-arch-request@w3.org]On
> > >> Behalf Of David Booth
> > >> Sent: Friday, February 21, 2003 10:09 AM
> > >> To: Martin Chapman; www-ws-arch@w3.org
> > >> Cc: 'Hugo Haas'
> > >> Subject: RE: Snapshot of Web Services Glossary
> > >>
> > >>
> > >>
> > >> At 04:35 PM 2/20/2003 -0800, Martin Chapman wrote:
> > >>> hmmm don't like the defn of synchronous:
> > >>
> > >> I struggled with this one, and I'm not sure my proposed wording is
> > >> ideal, but what I was trying to do was more clearly differentiate
> > > between
> > >> synchronous and asynchronous.   The old definition was very vague.
> > >>
> > >> Somehow we need to convey the idea that with "synchronous"
> > >> interactions the parties are synchronized in some way.  (!)  This
> > >> could mean "at the same time", but in the case of two communicating
>
> > >> parties it generally means the sending party waits for the
> > >> receiving party to do something before the sending party continues.
>
> > >> Thus, they
> >
> > >> are "synchronized".  I couldn't figure
> > >> out any better way to precisely capture this.  Any ideas?
> > >
> > > Define that operation involves sending/receiving at initiator site,
> > > and receiving/sending at respondent site. Define "time" to be bound
> > > by
> >
> > > T1
> > > (lower) and T2 (upper). I assume we can all agree to that.
> > >
> > > Given just sending and receiving primitives (e.g. TCP
> > > send()/receive()), initiator and respondent can agree on T1/T2 after
>
> > > concluding
> > operation.
> > > With just these two communication primitives they can synchronize
> > their
> > > clock within some resolution (but don't look for atomic clock type
> > > of synchronization here).
> > >
> > >
> > >> I agree that store-and-forward would NOT be synchronous, but I
> > >> don't see store-and-forward as the opposite of direct
> > >> communication. Communication can certainly be indirect (i.e., go
> > >> through
> > >> intermediaries) but still be synchronous.  So although synchronous
> > >> communication is often direct, I don't see that as a distinguishing
> > >> characteristic.
> > >
> > > An interaction can be synchronous even if it uses some
> > > store-and-forward mechanism, even if both request and response are
> > > stored and forwarded.
> > >
> > > Test for synchronisity of interaction is something like that:
> > >
> > > If initiator sent request at time T1 then it can conclude that
> > > respondent did not start performing interaction before time T1 If
> > > initiator received request at time T2 then it can conclude that
> > > respondent did not continue performing interaction after time T2
> > > (and vice versa)
> > >
> > > You can clearly see this is not true for asynchronous interaction.
> > >
> > > arkin
> > >
> > >>
> > >>> and
> > >>> the fact that the reply (if any) comes back on the same
> > >>> communication channel as the request.
> > >>
> > >> Interesting thought.  Must that always be true?  I could certainly
> > >> imagine an input-output operation in which the input uses one
> > >> communication channel and the output uses another.  So again, I
> > >> don't
> >
> > >> see this as a distinguishing characteristic of synchronous
> > >> communication.
> > >>
> > >> Anyone else have other suggestions for this definition?
> > >>
> > >>
> > >> --
> > >> David Booth
> > >> W3C Fellow / Hewlett-Packard
> > >> Telephone: +1.617.253.1273
> > >
> > >
> > >
> >
> >
> >
>
>
Received on Monday, 24 February 2003 14:34:14 GMT

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