RE: Reliable Messaging - Summary of Threads

MessageMartin,

As you pointed out these are all three different and orthogonal uses that
may relate to the same operation you are doing. I'm not sure if these are
different definitions, since they all use the word asynchronous
consistently, they just describe different aspects of what you are doing.

The word synchronous means happening or occuring at the same time, so
another way to say that something is asynchronous is to say that it is time
independent. What is asynchronous depends on what you are doing.

If you are talking about the interaction between two services, if the
request/response binds the clinet and server to perform that operation at
the same time, then the operation is synchronous. But you could use an
asynchronous transport protocol to send the message and receive the
response.

If the transport protocol uses the same communication channel for sending a
reponse and then receiving a reply, then it's synchronous. If it uses two
different connections, pipelines out-of-order request/responses over the
same connection, or is connectionless, then it's asynchronous. So you could
have a UDP protocol or even a TCP/IP protocol that is asynchronous and do a
synchronous operation using it.

You could do a synchronous operation through an asynchronous API. In this
case even though a response will come when the operation completes you will
not be waiting for the response but either poll for it sometime after it
arrived (and the operation completed) or be notified sometimes after it
arrived. The latter is often known as an asynchronous callback since it
notifies you when an operation has completed sometime after it has
completed.

So you make a valid point that we should all indicate precisely what we mean
when we use the term asynchronous. Are we talking about the operation
between the services, the transport protocol, or the API we use to conduct
the operation.

arkin

  -----Original Message-----
  From: www-ws-arch-request@w3.org [mailto:www-ws-arch-request@w3.org]On
Behalf Of Martin Chapman
  Sent: Friday, December 13, 2002 4:59 PM
  To: 'Cutler, Roger (RogerCutler)'; 'Assaf Arkin'; www-ws-arch@w3.org
  Subject: RE: Reliable Messaging - Summary of Threads


  I personally have three different definitions of asynchronous which are
more or less orthogonal to each other and can be combined.
      1. async programming model - this is where your thread blocks  at the
application level until a reply or fault is received.
      2. async transport - where the reply comes back on a different
transport connection from the request.
      3. time independent - the sender and the receiver do not have to be up
and running at the same time for the communication to happen
         (i.e. some for intermediary or queue is involved)

  Whether these meet other peoples definitions I am not sure, but it would
be good to get some agreed definitions for the architecture.

  Martin.
    -----Original Message-----
    From: www-ws-arch-request@w3.org [mailto:www-ws-arch-request@w3.org] On
Behalf Of Cutler, Roger (RogerCutler)
    Sent: Friday, December 13, 2002 1:28 PM
    To: Assaf Arkin; www-ws-arch@w3.org
    Subject: RE: Reliable Messaging - Summary of Threads


    Thanks for the support.

    One thing this note reminded me of -- I have seen a number of different
definitions of "synchronous" floating around this group.  In fact, if my
memory serves, there are three major ones.  One concentrates on the idea
that a call "blocks" if it is synchronous, another has a complicated logic
that I cannot recall and the third (contained in one of the references on
the two army problem) concentrates on the length of time it takes for a
message to arrive.  The formality of all of these definitions indicates to
me that all have had considerable thought put into them and that all are, in
their context, "correct".  They are, however, also different.

    -----Original Message-----
    From: Assaf Arkin [mailto:arkin@intalio.com]
    Sent: Friday, December 13, 2002 2:27 PM
    To: Cutler, Roger (RogerCutler); www-ws-arch@w3.org
    Subject: RE: Reliable Messaging - Summary of Threads



      3 - There is concern about the "two army" problem, which essentially
says that it is not possible, given certain assumptions about the types of
interactions, for all parties in the communication to reliably reach
consensus about what has happened.  I have been trying to encourage the
objective of documenting the scenarios that can come up in and their
relative importance and possibly mitigating factors or strategies.  I
haven't seen people violently disagreeing but I wouldn't call this a
consensus point of view.  I consider the ebXML spec as weak in discussing
the two-army problem.

      The two army problem assumes you are using a non-reliable medium for
all your communication and proves that it is impossible for the sender to
reach confidence that the message has arrived and is processed in 100% of
cases.

      You can increase your level of confidence by using message + ack and
being able to resend a message and receive a duplicate ack. That get's you
close to a 100% but not quite there, but it means that in most cases the
efficient solution (using asynchronous messaging) would work, and so
presents a viable option.

      In my opinion it is sufficient for a low level protocol to give you
that level of reliability. And that capability is generic enough that we
would want to address it at the protocol level in a consistent manner, so we
reduce at least one level of complexity for the service developer. It is
also supported by a variety of transport protocols and mediums.

      This still doesn't mean you can get two distributed services to
propertly communicate with each other in all cases. A problem arises if
either the message was not received (and is not processed), a message was
received but no ack recevied (and is processed) or a message was received
and an ack was received but the message is still not processed.

      That problem is not unique to asynchronous messaging, in fact it also
presents itself when synchronous messaging is used. With synchronous
messaging you have 100% confidence that a message was received, but no
confidence that it will be processed. Furthermore, you may fail before you
are able to persist that information, in which case your confidence is lost.

      If you do not depend on the result of the message being processed than
you would simply regard each message that is sent as being potentially
processed. You use the ack/resend mechanism as a way to increase the
probability that the message indeed reaches its destination, so a majority
of your messages will be received and.

      I argue that using ack/resend you could reach the same level of
confidence that the message will be processed as if you were using a
synchronous protocol, but could do so more efficiently.

      If you do depend on the message being processes, then you are in a
different class of problem, and simply having a reliable protocol is not
sufficient since it does not address the possibility that the message was
received, acked but not processed. It in fact presents the same problem that
would arise when synchronous protocols are used.

      This is best solved at a higher layer. There are two possible
solutions, both of which are based on the need to reach a concensus between
two systems. One solution is based on a two-phase commit protocol, which
could be extended to use asynchronous patterns. A more efficient solution in
terms of message passing would be to use state transitions that coordinate
through the exchange of well defined messages. This could be modeled using a
choreography language.

      Since this is outside the scope of this discussion I will not go into
details, but if anyone is interested I would recommend looking at protocols
for handling failures in distributed systems (in particular Paxos). In my
understanding these protocols are applicable for modeling at the
choreography language and are more efficient than using transactional
protocols and two-phase commit.

      My only point here was to highlight that a solution involving
ack/resend is sufficient to give you the same level of confidence that a
message would be processed as if you were using a synchronous operation, and
that solutions for achieving 100% confidence are required whether you are
using asynchronous or synchronous messaging.

      This is in support of Roger's recommendation for adding ack support to
XMLP.

       regards,
       arkin