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RE: REST, Conversations and Reliability

From: David Orchard <dorchard@bea.com>
Date: Sun, 21 Jul 2002 22:55:56 -0700
To: "'Paul Prescod'" <paul@prescod.net>, <www-ws-arch@w3.org>
Message-ID: <032c01c23144$bfbc96e0$0100007f@beasys.com>


Rest (:-) assured, I haven't forgotten to respond to your note.  In what
little time I've had available since thursday morning, I worked on the
ws-arch document.  That is a higher priority for the working group at this

I have a rather detailed rebuttal that I've only made some progress on -
it's basically that the conversation ID isn't *really* an ID, it's actually
a data structure used by dispatchers to keep the dispatcher stateless.

I'm also strongly against using GET to retrieve a WSDL file for a web
service.  If only the spec that specifies that was done within the W3C, I
would bring it up to the TAG.


> -----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 5:33 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 Monday, 22 July 2002 01:59:16 UTC

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