Web services messaging

If we can agree to discuss Web services messaging from the application or "actor" viewpoint rather than the viewpoint of the transport, I think we have a better chance of gaining agreement and adding useful text to the document.   
 
Defining such terms as "synchronous" and "asynchronous" are not likely to be viewed as much of a contribution to industry, whereas defining the behavior of Web services messaging relative to software "agents" probably would be.
 
In the context of application to application communication paradigms, Web services are basically a peer-to-peer system since the agents perform operations to send and receive messages in various patterns.  Web services do not perform RPCs since there is no equivalent of a local procedure call.  Web services also do not define queueing, although persisting messages can be viewed as the same thing.
 
Eric

-----Original Message-----
From: Newcomer, Eric 
Sent: Sunday, March 16, 2003 11:58 AM
To: Champion, Mike; www-ws-arch@w3.org
Subject: RE: Friendly amendment to friendly amendment (was RE: Friendly amendm ent #2c [Re: Straw poll on "synchronous" definitions]


I'd like to say that I'm not sure that defining synchronous and asynchronous are going to help us very much.  I'd like to offer a quote from a book Phil Bernstein and I wrote in 1997 on transaction processing ("Principles of Transaction Processing," page 74):
 
"TP application programs communicate with each other when they run in a distributed setting on different computers.  Even on a single computer, for modularity reasons, it's often convenient to break up the application programs into separate processes.  Application programs therefore have to communicate between processes, which means the programs have to exchange messages.  There are three ways to do this:
 
1.  Use remote procedure calls, which mimic the behavior of local procedure calls within a single process.  When a program in one process calls a program in another process (in another address space), it does so by calling a procedure and getting a value returned just as if the two procedures were running within a single address space.
 
2.  Explicitly send and receive messages, called peer-to-peer messaging.  Rather than use a procedure call with a return, application programs issue operations that send and receive messages.
 
3.  Move messages among queues.  An application sends a message from one program to another by putting the message in a queue.  The queue is a third object, with which both programs communicate.  One program enqueues the message to the queue, and sometime later the receiver program dequeues the message from the queue.
 
These communication paradigms apply to all distributed applications, not just in TP. ..."
 
I think this approach is much clearer and more helpful than trying to define how the technology works under the covers, or describing the messaging systems on which these paradigms are based.  It's clearly possible to use a synchronous protocol to implement a message queuing paradigm, and possible to implement a remote procedure call paradigm using peer-to-peer.  If we look at the capability of Web services based communication technology from this perspective, we can fairly easily decide which of these paradigms it really supports, and how, rather than argue over synchronous vs. asynchronous, which is really a detail rather than an indication of application level behavior.
 
Eric

-----Original Message-----
From: Champion, Mike 
Sent: Sunday, March 16, 2003 11:18 AM
To: www-ws-arch@w3.org
Subject: Friendly amendment to friendly amendment (was RE: Friendly amendm ent #2c [Re: Straw poll on "synchronous" definitions]


 

-----Original Message-----
From: Anne Thomas Manes [mailto:anne@manes.net]
Sent: Saturday, March 15, 2003 6:12 PM
To: Walden Mathews; Christopher B Ferris; www-ws-arch@w3.org
Subject: RE: Friendly amendment #2c [Re: Straw poll on "synchronous" definitions]


The biggest issue I have with Ugo's definition (and all the others) is that they tie synchrony with blocking versus non-blocking. Synchronous means "at the same time". Asynchronous means "not at the same time". Whether or not the sender has to wait idly for a response is a separate issue. 
 
An interaction (one-way, two-way, or multi-way) is synchronous if the sender and receiver must communicate at the same time (the reciever must be available to receive the message when the sender sends it). A one-way message is asynchronous if the sender and receiver do not need to communicate at the same time (the message may be stored and delivered at a later time).  

I must say that I like Anne's wording that synchronous means that "the receiver must be available to receive the message"  better than notions of blocking or simultaneity.  What about changing Ugo's suggestion to:
 
Asynchronous: A request/response interaction is said to be asynchronous
when the request and response are chronologically and procedurally decoupled. In other
words, the client agent can process the response at some indeterminate point in the future when its existence is discovered, for example, by polling, notification by receipt of another message, etc. 
 
Synchronous:  A request/response interaction is said to be synchronous when the client agent must be available to receive and process the response message from the time it issues
it issues the initial request until it is actually received or some failure condition is determined. The exact meaning of "available to receive the message" depends on the characteristics of the client agent (including the transfer protocol it uses); it may, but does not necessarily, imply tight time synchronization, blocking a thread, etc.