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