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Re: [rest-discuss] client keeps its state

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  • Eric J. Bowman
    ... No, saying this has nothing to do with REST or declaring it off-topic to rest-discuss would be extreme; I did neither. ;-) ... Agreed. To more explicitly
    Message 1 of 33 , Apr 5, 2010
      Stuart Charlton wrote:
      >> The problem is, how does a machine user deduce which of the presented
      >> state-transition options will advance it towards its goal? This is a
      >> problem orthogonal to REST, which is not to say off-topic to rest-
      >> discuss. Once the client component arrives at the proper steady-
      >> state REST doesn't enter the equation again, until the user requests
      >> some transition to the next steady-state in their specific
      >> application.
      > That's rather extreme.

      No, saying this has nothing to do with REST or declaring it off-topic
      to rest-discuss would be extreme; I did neither. ;-)

      > Implementers clearly are curious how to retain the constraints of the
      > architecture and build m2m agents. While the techniques for building
      > goal-directed agents aren't particular to REST, they're certainly of
      > interest to this audience, and it's been a sorely lacking area of
      > exploration, IMO.

      Agreed. To more explicitly state my position: Discussions of m2m REST
      consistently violate the layered-system and self-descriptive-messaging
      constraints. We need to change the discussion so these m2m agents are
      manipulating the API, not the other way 'round...

      First, user and user-agent are combined into client-component. This
      leads to (amongst other horrors) APIs where a separate media type is
      used to represent each resource state, enforcing a 1:1 relationship
      between resource state and application state -- itself a violation of
      the layered-system constraint -- by solving a vocabulary problem over
      the wire, i.e. at the protocol layer.

      Solving a vocabulary problem over the wire with custom media types
      results from a violation of the layered-system constraint, and carries
      that error forward. The resulting API violates the self-descriptive-
      messaging constraint and HTTP by failing to use well-known, registered
      media types to derive application steady-states.

      All led to from the notion that an m2m client is a user-agent not a
      user, and that a REST application instructs these m2m user-agents what
      to do. Which is entirely backwards from what a REST application *is*.
      The user informs the user-agent of the next step, the series of steps
      from initial URI to completion of some task is defined as a "REST
      application". Not the other way around! Not even for m2m! No!

      This mess may be avoided from the get-go by applying some REST
      discipline and recognizing that a user and a user-agent are indeed
      separate layers in a system, regardless of the nature of the user.
      So, in order to have a discussion about m2m REST, we must distinguish
      between user and user-agent, avoiding the paper tiger of machine vs.
      human user-agents -- such a distinction being a violation of the
      layered-system constraint.

      The distinction between human and machine belongs in the user component
      of a REST system. The problem is, how do we inform the user of the
      meaning of the possible state transitions? When the user is human, the
      solution is simple -- natural language. When the user is a machine,
      the solution is no less simple -- machine language -- just harder to
      implement. Either way, these domain-specific (even if standardized)
      vocabularies must be embedded within the standard methods, media types
      and link relations making up the REST API.

      The first thing you need in a REST API are standard link relations,
      methods and media types to instruct user-agents how to arrive at an
      application steady-state when a URI is dereferenced. Domain-specific
      vocabularies are used which allow the user-agent to inform the user
      what options there are and what information is required to proceed,
      i.e. natural-language descriptions of form fields and submission
      buttons in a shopping-cart system.

      It's the human user instructing the user-agent how to proceed.

      Domain-specific vocabularies which allow the user-agent to inform a
      machine user what options there are and what information is required
      to proceed, i.e. machine-language descriptions of form fields and
      submission buttons in a shopping-cart system, are embedded within the
      steady-state just like natural-language vocabularies, except as
      metadata instead of as content.

      It's the machine user instructing the user-agent how to proceed.

      This is RESTful m2m development and must be emphasized. It must also be
      emphasized that "user decides what to do" isn't part of a REST
      application -- it *defines* any given REST application (what the user
      is trying to do). So please, folks, stop writing m2m HTTP APIs which
      instruct the *user* how to proceed and calling the result a REST

      REST ends at "user-agent informs the user what it can do", while "user
      decides what to do" is out-of-scope. This isn't extremist, it's central
      to having the entire m2m discussion; the point is, the discussion must
      be framed properly as "how does the user-agent inform the user of its
      options" not "how does the API instruct the user of the next step"
      (which leaps right across the user-agent layer, while standing the
      definition of "REST application" on its head, you see).

    • Andrew Wahbe
      ... Yes I see how that s confusing. By client I mean the thing running the application -- perhaps distributed user-agent is the right terminology here.
      Message 33 of 33 , Apr 6, 2010
        On Tue, Apr 6, 2010 at 6:07 PM, Eric J. Bowman <eric@...> wrote:
        Andrew Wahbe wrote:
        > But from a REST perspective, you could think of them being part of a
        > single distributed client...

        Not sure what you mean.  In REST, "client" specifically means "client
        connector", so do you mean a single distributed client connector, or a
        single distributed user agent?  Or is it a single distributed user,
        driving numerous user agents (like Google driving googlebot)?

        Yes I see how that's confusing. By "client" I mean the "thing running the application" -- perhaps "distributed user-agent" is the right terminology here. Consider an application that consists of multiple hypermedia formats, could be VoiceXML + CCXML or Atom + HTML. It could be the case that the markup is processed by a single process or it could be that different processes are handling the individual markup languages and coordinating somehow. The server is just seeing the HTTP requests and shouldn't really care how the user agent is internally constructed. Of course as I mentioned cookies break this -- it's another way that they are not ideal. VoiceXML/CCXML systems can sometimes be broken into as many as 3 separate components all making requests related to a single application session: the CCXML processor, the VoiceXML processor and a speech processor (performing speech recognition and fetching grammar files). Some of the related protocols have mechanisms to try and coordinate cookies: e.g. http://tools.ietf.org/html/draft-ietf-speechsc-mrcpv2-20#section-6.2.15

        Anyways, it's just food for thought.

        Actually, at second glance, CCXML seems more akin to Xforms -- is it an
        MVC application the server transfers to the user agent?  MVC on the
        user agent is a powerful REST design pattern that can be adapted to


        That's maybe one way to think about it. It is a finite state machine that communicates via messages/events to resources in an underlying client platform. Events cause state transitions, transitions handlers can send messages back to the platform or place HTTP requests to transition to a new page (or do various other things). I see parallels between this model and an Ajax application -- which can be thought of as a state machine: each "view" is a different state often labelled with a URI fragment (e.g. #inbox in Gmail)

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