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Re: Asynchronous Web Services

From: Edwin Khodabakchian <edwink@collaxa.com>
Date: Fri, 19 Jul 2002 16:54:04 -0700
To: "'Paul Prescod'" <paul@prescod.net>, <www-ws-arch@w3.org>
Message-ID: <002f01c22f7f$90860710$670aa8c0@collaxa.net>


This is a very good explanation. 

How would you address the case where the client initiates the long-lived
transaction through a one-way protocol like SMTP or JMS. It seems to me
that it that case, the client needs to specify the correlationId. 

As to whether the correlationId should be a URI, some clients (like a VB
app) cannot host URIs: those clients interact with the long-lived
transaction through polling (passing in the correlationId as part of
each request).

It seems to me that a correlationId is not a resource but rather a key
used as part of an asynchronous interactions.

Otherwise, I would tend to agree with you that the initiated long-live
transaction should be a resource hosted by the server and whose state
should be available through GET.



> -----Original Message-----
> From: www-ws-arch-request@w3.org 
> [mailto:www-ws-arch-request@w3.org] On Behalf Of Paul Prescod
> Sent: Wednesday, July 17, 2002 4:33 PM
> To: David Orchard; www-ws-arch@w3.org
> Subject: REST, Conversations and Reliability
> David offers the following URI:
> http://dev2dev.bea.com/techtrack/SOAPConversation.jsp
> In my mind, it is a perfect example of a protocol that can be 
> enhanced by applying some REST discipline.
> The BEA proposal introduces a concept of "ConversationID" 
> which represents a conversation. It also introduces a state 
> machine that allows the participants to move through the 
> stages from "no conversation" to "talking" to "finished 
> conversing". It defines ways that headers are used to move 
> through those stages. It also defines how a callback URI can 
> be presented. It has quite a resemblance to the ideas in the 
> HTTPEvents draft.
> Now let me apply a combination of REST discipline and my own 
> thoughts about networking.
> Let's call the recipient of the first message the "server" 
> and the sender of the first message the "client" although at 
> an HTTP level they may switch roles if the exchange is asynchronous.
> The server needs to deal with N incoming conversations and 
> needs to keep them all straight. Also, the server by 
> definition has the capability to host URIs but the client may 
> or may not. For this and other reasons, I feel that the 
> conversation ID should be generated by the recipient, not the 
> sender. Most important: the recipient can trivially generate 
> IDs unique to them. The sender can at best use UUIDs to 
> reduce the chances of collision.
> Second, the conversation ID should be a (surprise!) http URI. 
> It should point to a conversation resource. Obviously if the 
> conversation is necessary to the successful completion of the 
> discussion then it is an important resource and deserves a 
> URI. This isn't just theoretically clean it is extremely 
> important in practice as will become clear in a moment.
> Let's think about reliability.
> What happens if the conversation-constructing message is 
> lost? That's okay. The client can just send it again.
> What happens if the conversation-constructing response is 
> lost? That's okay. The client can just set up a new 
> conversation resource and the server can dispose of the 
> unused one after a timeout.
> Now both partners are in the "conversing" state. But the big 
> difference between the original proposal and the REST 
> proposal is that the REST proposal makes this state explicit 
> in terms of universally addressable resources.
> According to the original proposal, callbacks refer to the 
> conversation ID. In my proposal, callbacks would also refer 
> to the conversation resource. But the conversation resource 
> would be a real data-containing resource. For instance in an 
> instant-messaging application, the conversation resource 
> would list which users are involved with the discussion. In 
> an order negotiation application, the conversation resource 
> could point to the good being bought or sold. Note that the 
> server by definition has access to this information so it is 
> just a case of giving the information a URI so that it may be 
> looked up at runtime by the client or third parties.
> This is important for a variety of reasons. First, it means 
> that clients can be stateless and thus simpler. It means that 
> the client-end of a conversation can migrate from one machine 
> to another merely by passing the conversation ID URI (and 
> authentication information). It means that an (authorized) 
> third-party application like a logger, auditor or security 
> filter can apprise itself of the full state of the 
> conversation just by following the URI in the message. 
> A conversation resource is not in any way tied to any 
> particular nodes/endpoints. Once it is set up, dozens or 
> hundreds of participants can be involved without any major 
> architectural shift. The third, fourth, etc. participants are 
> brought in merely by forwarding them the URI. There are no 
> hard-coded roles of "client" and "server" after the 
> conversation is set up. There is "the server maintaining the 
> conversation" and "everybody else".
> Also, stateless presentation tools like XSLT stylesheets can 
> extract information for rendering the transmitted message. 
> Assertions can be made about conversation resources using 
> RDF. An HTML representation of resources can be used for 
> technical support and debugging.
> Most important: if the client or other participant misses a 
> message, gets state corrupted or otherwise gets confused 
> about the state of the conversation, it can refresh itself 
> with a simple GET. That's a scalable approach to reliability. 
> Under the original protocol, there is no way for a confused 
> client that has missed a message to check whether the 
> conversation is still ongoing and thus it should expect more 
> messages. For instance if the client is momentarily offline, 
> there is no way for it to check whether the server timed-out 
> in the meantime.
> The original proposal says:
> "The ContinueHeader
>        MUST be sent on any messages to operations that are 
> marked in the
>        WSDL as requesting a ContinueHeader."
> I feel that this is too large grained of a constraint. In 
> some cases, conversations will need to nest. For instance 
> there is the conversation that sets up a shopper/seller 
> relationship and then within that there are conversations on 
> the price of individual items. In the REST model these would 
> be just different kinds of conversation resources. Some 
> operations would expect a reference to a "shopping 
> conversation" and some operations would expect a reference to 
> a "product price negotiation" conversation. Of course each 
> resource would have a link to the other so that it is 
> possible to easily go from one to another.
> The original proposal says that the conversation is ended in 
> an unspecified manner. I do not understand why it would 
> specify some things and leave that unspecified. Therefore I 
> would say rather that the conversation is ended when either 
> party DELETEs the conversation resource. There should be some 
> standardized way for the server to indicate that it is doing 
> so to a callback-capable client. Alternately, conversation 
> resources could be immortal (for archival purposes) but could 
> have a flag that says whether they are ongoing or historical.
> I hope this demonstrates that a REST approach is not at all 
> at odds with a "named conversation" approach but a REST 
> approach would say:
>  1. Conversations should be named as everything else on the 
> Web is named, with URIs.
>  2. Conversations should be inspectable and introspectable as 
> everything else on the Web is, through HTTP GET.
>  3. Any authorized party (especially confused clients) should 
> be able to bring itself up to a full understanding of the 
> state of the conversation by looking at the conversation URI 
> (or things linked to the conversation URI). 
>  4. Conversations will almost always have important 
> associated data (the stuff being talked about) and the 
> resource storing that information can easily serve as the 
> conversation resource.
> -- 
> Come discuss XML and REST web services at:
>   Open Source Conference: July 22-26, 2002, conferences.oreillynet.com
>   Extreme Markup: Aug 4-9, 2002,  www.extrememarkup.com/extreme/
Received on Friday, 19 July 2002 19:54:40 UTC

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