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Re: [rest-discuss] URI design, part 2

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  • Eric J. Bowman
    ... Well, what are you expecting to GET from /cancel, or are you just using that URL to invoke a procedure? If so, then there are a few places Roy s thesis
    Message 1 of 28 , Nov 30, 2012
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      Max Toro wrote:
      >
      > What I'd love to get is an answer like: POST /orders/1/cancel is not
      > REST compliant because chapter x of Fielding's dissertation explicitly
      > or implicitly says it's not allowed or it's discouraged. After knowing
      > what is or isn't REST then I'd love to learn more about the pros and
      > cons of different architectural and implementation styles.
      >

      Well, what are you expecting to GET from /cancel, or are you just using
      that URL to invoke a procedure? If so, then there are a few places Roy's
      thesis admonishes against it in Chapter 6 -- the rest of REST is about
      positive, rather than negative, reinforcement of the identification of
      resources constraint. Suggested reading: 6.5.2; 6.2.1, in particular:
      "REST [defines] a resource to be the semantics of what the author
      intends to identify."

      Not the semantics of a method invocation. What does /cancel identify?
      Sounds to me like a method of tunneling DELETE through POST which
      identifies nothing, iow a procedure endpoint, which is characteristic
      of various styles but not of the REST style. The hypertext constraint
      only makes sense if your resources make sense, in that their URLs are
      identifiers rather than endpoints.

      Roy's thesis really must be considered in its entirety, to understand
      the uniform interface constraint (of which identification of resources
      is a sub-constraint). Chapter 1 introduces the notion of applying
      design-by-constraint to networked software architecture, as inspired
      by Eames IIRC. "A style is a named set of constraints on architectural
      elements that induces the set of properties desired of the
      architecture." (4.3)

      Chapter 2 defines terminology associated with networked software
      architecture, which is required for understanding Chapter 3, which lays
      out a methodology for evaluating various styles and applies this to
      several examples. Most importantly, Chapter 3 identifies the
      constraints associated with various styles, and describes the properties
      they induce in a system (which may or may not be beneficial or
      detrimental to the system you're designing; remember there is no best
      architecture, only that which is best for your system). Which is of
      course required for understanding Chapter 4.

      Chapter 4 considers the problems raised by the WWW, and suggests that
      the architecture may be improved by applying design-by-constraint to it,
      in order to address those problems. First, by identifying the desirable
      properties of the early Web, and the constraints responsible for them;
      next, by extending that architecture by adding additional constraints,
      resulting in a new hybrid style consisting of aspects of existing
      styles. Of course, this is required for understanding Chapter 5.

      "REST provides a set of architectural constraints that, WHEN APPLIED
      AS A WHOLE, emphasizes scalability of component interactions, generality
      of interfaces, independent deployment of components, and intermediary
      components to reduce interaction latency, enforce security, and
      encapsulate legacy systems." (4.4)

      Which brings us to Chapter 5, and the short answer to your question:
      "POSTing to /cancel violates the Identification of Resources constraint,
      and is therefore unRESTful." But I've found that just giving that
      answer tends to upset folks who've only read Chapter 5, then they get
      defensive about why can't they call their API RESTful, accusations of
      pedantry follow, and threads devolve into general ugliness, heheh...

      My point is, you'll have a much harder time trying to understand REST
      by being told bluntly what is and isn't RESTful, than you will by
      reading Roy's thesis in its entirety. As computer science dissertations
      go, Roy produced a functional work of art, much as an Eames chair isn't
      just a piece of furniture. Understanding what the constraints are, and
      where they come from, is vital to understanding how they're applied to
      the Web to derive REST, and why they must be implemented as a whole to
      achieve REST.

      Only then will it become apparent when they're being violated, as in
      the example given of POSTing to an unGETtable /cancel URL. That level
      of understanding comes from the bottom up, not from the top down, IMO.
      Knowing what is or isn't REST depends on understanding the pros and
      cons of various architectural styles, because that's what constraints
      are, and constraints must be understood before their application to the
      Web as REST can be understood.

      More importantly, understanding REST as a style makes one a better
      architect, because sometimes it's advantageous to deviate from REST's
      constraints. Which is why I'm always on about how saying something
      isn't REST is not a value judgment, just a fact. Understanding Roy's
      thesis allows you to identify the constraints you have applied, and
      understand that as its own architectural style derived from REST, to
      use as a guide to developing that system -- but also to understand
      which desirable properties of REST you're forfeiting in the bargain.

      Making informed decisions about which constraints to apply, is making
      use of REST as a tool for long-term development. It may not be
      feasible to apply all the constraints initially, if REST is truly what
      your system needs. In which case your system design can account for
      this, becoming more RESTful over time, instead of painting yourself
      into a corner where the system needs re-architecting rather than
      implementing another constraint as an extension.

      -Eric
    • Eric J. Bowman
      ... This is why Roy s thesis is an invaluable tool, at least for me anyway as I ve been at this since 93 and had a front-row seat at the transition from the
      Message 2 of 28 , Dec 1, 2012
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        "Eric J. Bowman" wrote:
        >
        > Making informed decisions about which constraints to apply, is making
        > use of REST as a tool for long-term development. It may not be
        > feasible to apply all the constraints initially, if REST is truly what
        > your system needs. In which case your system design can account for
        > this, becoming more RESTful over time, instead of painting yourself
        > into a corner where the system needs re-architecting rather than
        > implementing another constraint as an extension.
        >

        This is why Roy's thesis is an invaluable tool, at least for me anyway
        as I've been at this since '93 and had a front-row seat at the
        transition from the old Web to HTTP 1.1 -- Roy's experience applying
        his concepts to a system I was familiar with is included in his thesis,
        which helped me to incrementally apply REST to my own work as I gained
        understanding of it. Now, I can confidently design systems today which
        may be easily extended to be fully RESTful tomorrow.

        So I really don't stress about the unRESTful aspects of my systems,
        they're "by design" and will continue to work themselves out over time.
        Which is exactly what you want from an architectural style, particularly
        for the Web. Successful websites tend to grow users over time, and it
        ain't easy to chuck one system for another. What's needed is a system
        which uses progressively less resources per user, as usage increases.

        (Just check out the shoddy uptime records of certain big boys on the
        block to see how badly this desirable property is needed, and note that
        they don't begin to implement REST.)

        Using REST as a tool enables the development of such systems by allowing
        the architect to chart a steady course, deciding which constraints (like
        identification of resources) are crucial to get right from the get-go,
        and which may be put off until they satisfy a cost-benefit analysis.

        For example, most popular CMS software defaults to no-cache. But this
        doesn't really matter until you're starting to bump into the transfer
        limits of your hosting plan, at which time you can implement caching,
        after having some time to test the setup first. You can then fine-tune
        your cache settings over time, and later integrate with a CDN. This is
        usually done ad-hoc, what I'm saying is that knowledge of REST can make
        this go smoothly rather than being a comedy of errors (quite common).

        If your system doesn't grow to need it, then you can probably do without
        all the added development expense (plus complexity driving up long-term
        maintenance costs) of implementing non-native caching on that CMS. REST
        is your friend when it comes to planning for and managing these expenses
        and complexities vis-a-vis your cashflow. Budgeting for ho-hum websites
        is easy; successful websites are another story entirely, where knowledge
        of REST as a tool can be your competitive edge in funding growth from
        cashflow instead of debt or outside capital while turning a profit --
        instead of piling up red ink on wasteful infrastructure.

        Yeah, I know. That just isn't how things are done on the Web! :-D

        -Eric
      • Greg Young
        /CRMapp?order=1&query=status until intermediaries decide to ignore you because the ? ... They shouldn t but they do :) ... -- Le doute n est pas une condition
        Message 3 of 28 , Dec 1, 2012
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           /CRMapp?order=1&query=status

          until intermediaries decide to ignore you because the ?

          :(

          They shouldn't but they do :)

          On Fri, Nov 30, 2012 at 10:56 PM, Eric J. Bowman <eric@...> wrote:
          /CRMapp?order=1&query=status



          --
          Le doute n'est pas une condition agréable, mais la certitude est absurde.
        • Eric J. Bowman
          ... They re configurable, and while this may have been a problem in the past, I don t believe it to be anything but an edge case, now. URIs are opaque and
          Message 4 of 28 , Dec 1, 2012
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            Greg Young wrote:
            >
            > /CRMapp?order=1&query=status
            >
            > until intermediaries decide to ignore you because the ?
            >
            > :(
            >
            > They shouldn't but they do :)
            >

            They're configurable, and while this may have been a problem in the
            past, I don't believe it to be anything but an edge case, now. URIs
            are opaque and should be considered in their entirety as cache keys.
            The best a developer can do, is design to REST -- my example certainly
            doesn't violate any constraints -- and let the deployed architecture
            catch up.

            There's much FUD out there about query strings, particularly in REST
            discussions, which amounts to unfounded myth. I see no reason to avoid
            query strings because they *might* be ignored by a small percentage of
            caches, economy of scale still kicks in vs. no-cache. So go ahead and
            treat URIs as opaque when doing REST development, the downside is
            more than compensated for by the upside, more so as time goes by.

            -Eric
          • Erik Mogensen
            This post is mostly aimed at Max Toro, but Eric provided a nice entry point for me :-) ... Exactly. And we don t know what the author intends to identify just
            Message 5 of 28 , Dec 1, 2012
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              This post is mostly aimed at Max Toro, but Eric provided a nice entry point for me :-)

              On Sat, Dec 1, 2012 at 8:08 AM, Eric J. Bowman <eric@...> wrote:
               

              Max Toro wrote:
              >
              > What I'd love to get is an answer like: POST /orders/1/cancel is not
              > REST compliant because chapter x of Fielding's dissertation explicitly
              > or implicitly says it's not allowed or it's discouraged. 
              >

              Well, what are you expecting to GET from /cancel, or are you just using
              that URL to invoke a procedure? [...] 
               Suggested reading: 6.5.2; 6.2.1, in particular:
              "REST [defines] a resource to be the semantics of what the author
              intends to identify."

              Exactly.  And we don't know what the author intends to identify just by looking at the path /orders/1/cancel.

              Just looking at the syntax of a single request (e.g. the method POST and/or the path /orders/1/cancel) is not enough to determine if the system adheres to any particular architectural style.

              The author may well intend that /cancel identifies a resource, and allow lots of interaction in it.  It might even have an HTML representation, with a nice <form method="post"><input type="submit" value="CANCEL"></form> in it.

              But this is still not enough to determine if a system adheres to the constraints: For even though the server provides such hypermedia controls, media types and does everything "by the book", it is still up to the *client* to actually *use* these hypermedia controls.  In fact it is up to the author of the agent itself.

              If you write code in the client that hard wires a button called "cancel" to the "POST /orders/1/cancel" HTTP request then the client isn't really honouring the hypermedia controls laid out above.  If, however, it _soft wires_ the same button _because_ of the hypermedia control above, then you're doing it "more right" I would say.  The point is that the client should be written in such a way that it uses _only_ the hypermedia controls that it receives at run-time.

              Roy's thesis really must be considered in its entirety, [...]

              +1 to such a degree that 1 > 1.  The rest of the stuff you wrote was great.  I just want to highlight the incorrectness of the question, as others said earlier that REST doesn't say anything about if POST /foo is or isn't...  For all we know /orders/1/cancel is a picture of a dog, or it could be a SOAP endpoint for a nice game of Global Thermonuclear War.

              The constraints of REST are too often thought / taught to deal with the server side, but in my experience, it has much more to do with the client side, how much knowledge the client has on things like:

              - what URLs can it use? (it shouldn't know any; but bind to one at run-time, preferably via a configuration parameter)
              - when it knows one URL (e.g. "/orders/1"), can it add "/cancel" to it to cancel it?  (no, unless it was told to add "/cancel" by a hypermedia control)
              - does it know the "type" of a resource, e.g. that /orders/1 is an Order (this is a point of contention on the list; browsers don't know about books or a bank account, but work fine nonetheless.)
              - when it does know the type of the URL is an order, does it pull up a pre-coded "order" UI (bad), or does it create a UI based on what it finds in the response (good)


              -- 
              -mogsie-
            • Max Toro
              ... To clarify, /orders/1/cancel is used to modify a resource, using POST. A GET request would result in a Method Not Allowed response. This is also explained
              Message 6 of 28 , Dec 1, 2012
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                > Well, what are you expecting to GET from /cancel, or are you just using
                > that URL to invoke a procedure? If so, then there are a few places Roy's
                > thesis admonishes against it in Chapter 6 -- the rest of REST is about
                > positive, rather than negative, reinforcement of the identification of
                > resources constraint. Suggested reading: 6.5.2; 6.2.1, in particular:
                > "REST [defines] a resource to be the semantics of what the author
                > intends to identify."

                To clarify, /orders/1/cancel is used to modify a resource, using POST.
                A GET request would result in a Method Not Allowed response.

                This is also explained on Subbu's book, chapter 2.6 "When and How to
                Use Controllers to Operate on Resources":

                "Problem: You want to know how to tackle write operations that involve
                modifying more than one resource atomically, or whose mapping to PUT
                or DELETE is not obvious. Solution: Designate a controller resource
                for each distinct operation. Let clients use the HTTP method POST to
                submit a request to trigger the operation... If the outcome is the
                modification of one or more existing resources, return response code
                303 (See Other) with a Location with a URI that clients can use to
                fetch a representation of those modifications."

                > Not the semantics of a method invocation. What does /cancel identify?
                > Sounds to me like a method of tunneling DELETE through POST which
                > identifies nothing, iow a procedure endpoint, which is characteristic
                > of various styles but not of the REST style. The hypertext constraint
                > only makes sense if your resources make sense, in that their URLs are
                > identifiers rather than endpoints.

                If I understand correctly, you are saying that if I need to affect a
                resource then I should use the uniform interface on that resource URI,
                and not another URI.

                > Which brings us to Chapter 5, and the short answer to your question:
                > "POSTing to /cancel violates the Identification of Resources constraint,
                > and is therefore unRESTful." But I've found that just giving that
                > answer tends to upset folks who've only read Chapter 5, then they get
                > defensive about why can't they call their API RESTful, accusations of
                > pedantry follow, and threads devolve into general ugliness, heheh...

                After reading that chapter again I'm not sure my example violates
                anything, but I'd love to get more clarification from you. Is it the
                use of a verb in the URI? or not using the URI of the resource I'm
                trying to modify directly?
                --
                Max Toro


                On Sat, Dec 1, 2012 at 4:08 AM, Eric J. Bowman <eric@...> wrote:
                > Max Toro wrote:
                >>
                >> What I'd love to get is an answer like: POST /orders/1/cancel is not
                >> REST compliant because chapter x of Fielding's dissertation explicitly
                >> or implicitly says it's not allowed or it's discouraged. After knowing
                >> what is or isn't REST then I'd love to learn more about the pros and
                >> cons of different architectural and implementation styles.
                >>
                >
                > Well, what are you expecting to GET from /cancel, or are you just using
                > that URL to invoke a procedure? If so, then there are a few places Roy's
                > thesis admonishes against it in Chapter 6 -- the rest of REST is about
                > positive, rather than negative, reinforcement of the identification of
                > resources constraint. Suggested reading: 6.5.2; 6.2.1, in particular:
                > "REST [defines] a resource to be the semantics of what the author
                > intends to identify."
                >
                > Not the semantics of a method invocation. What does /cancel identify?
                > Sounds to me like a method of tunneling DELETE through POST which
                > identifies nothing, iow a procedure endpoint, which is characteristic
                > of various styles but not of the REST style. The hypertext constraint
                > only makes sense if your resources make sense, in that their URLs are
                > identifiers rather than endpoints.
                >
                > Roy's thesis really must be considered in its entirety, to understand
                > the uniform interface constraint (of which identification of resources
                > is a sub-constraint). Chapter 1 introduces the notion of applying
                > design-by-constraint to networked software architecture, as inspired
                > by Eames IIRC. "A style is a named set of constraints on architectural
                > elements that induces the set of properties desired of the
                > architecture." (4.3)
                >
                > Chapter 2 defines terminology associated with networked software
                > architecture, which is required for understanding Chapter 3, which lays
                > out a methodology for evaluating various styles and applies this to
                > several examples. Most importantly, Chapter 3 identifies the
                > constraints associated with various styles, and describes the properties
                > they induce in a system (which may or may not be beneficial or
                > detrimental to the system you're designing; remember there is no best
                > architecture, only that which is best for your system). Which is of
                > course required for understanding Chapter 4.
                >
                > Chapter 4 considers the problems raised by the WWW, and suggests that
                > the architecture may be improved by applying design-by-constraint to it,
                > in order to address those problems. First, by identifying the desirable
                > properties of the early Web, and the constraints responsible for them;
                > next, by extending that architecture by adding additional constraints,
                > resulting in a new hybrid style consisting of aspects of existing
                > styles. Of course, this is required for understanding Chapter 5.
                >
                > "REST provides a set of architectural constraints that, WHEN APPLIED
                > AS A WHOLE, emphasizes scalability of component interactions, generality
                > of interfaces, independent deployment of components, and intermediary
                > components to reduce interaction latency, enforce security, and
                > encapsulate legacy systems." (4.4)
                >
                > Which brings us to Chapter 5, and the short answer to your question:
                > "POSTing to /cancel violates the Identification of Resources constraint,
                > and is therefore unRESTful." But I've found that just giving that
                > answer tends to upset folks who've only read Chapter 5, then they get
                > defensive about why can't they call their API RESTful, accusations of
                > pedantry follow, and threads devolve into general ugliness, heheh...
                >
                > My point is, you'll have a much harder time trying to understand REST
                > by being told bluntly what is and isn't RESTful, than you will by
                > reading Roy's thesis in its entirety. As computer science dissertations
                > go, Roy produced a functional work of art, much as an Eames chair isn't
                > just a piece of furniture. Understanding what the constraints are, and
                > where they come from, is vital to understanding how they're applied to
                > the Web to derive REST, and why they must be implemented as a whole to
                > achieve REST.
                >
                > Only then will it become apparent when they're being violated, as in
                > the example given of POSTing to an unGETtable /cancel URL. That level
                > of understanding comes from the bottom up, not from the top down, IMO.
                > Knowing what is or isn't REST depends on understanding the pros and
                > cons of various architectural styles, because that's what constraints
                > are, and constraints must be understood before their application to the
                > Web as REST can be understood.
                >
                > More importantly, understanding REST as a style makes one a better
                > architect, because sometimes it's advantageous to deviate from REST's
                > constraints. Which is why I'm always on about how saying something
                > isn't REST is not a value judgment, just a fact. Understanding Roy's
                > thesis allows you to identify the constraints you have applied, and
                > understand that as its own architectural style derived from REST, to
                > use as a guide to developing that system -- but also to understand
                > which desirable properties of REST you're forfeiting in the bargain.
                >
                > Making informed decisions about which constraints to apply, is making
                > use of REST as a tool for long-term development. It may not be
                > feasible to apply all the constraints initially, if REST is truly what
                > your system needs. In which case your system design can account for
                > this, becoming more RESTful over time, instead of painting yourself
                > into a corner where the system needs re-architecting rather than
                > implementing another constraint as an extension.
                >
                > -Eric
              • Max Toro
                Thanks for you answer Erik. I don t want to repeat myself, so please see my answer to Eric, I d love to get your comments as well. Rather than good or bad I d
                Message 7 of 28 , Dec 1, 2012
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                  Thanks for you answer Erik. I don't want to repeat myself, so please
                  see my answer to Eric, I'd love to get your comments as well. Rather
                  than good or bad I'd like to determine if my example is REST or not.
                  --
                  Max Toro


                  On Sat, Dec 1, 2012 at 10:30 AM, Erik Mogensen <erik@...> wrote:
                  > This post is mostly aimed at Max Toro, but Eric provided a nice entry point
                  > for me :-)
                  >
                  > On Sat, Dec 1, 2012 at 8:08 AM, Eric J. Bowman <eric@...>
                  > wrote:
                  >>
                  >>
                  >>
                  >> Max Toro wrote:
                  >> >
                  >> > What I'd love to get is an answer like: POST /orders/1/cancel is not
                  >> > REST compliant because chapter x of Fielding's dissertation explicitly
                  >> > or implicitly says it's not allowed or it's discouraged.
                  >> >
                  >>
                  >> Well, what are you expecting to GET from /cancel, or are you just using
                  >> that URL to invoke a procedure? [...]
                  >>
                  >> Suggested reading: 6.5.2; 6.2.1, in particular:
                  >> "REST [defines] a resource to be the semantics of what the author
                  >> intends to identify."
                  >
                  >
                  > Exactly. And we don't know what the author intends to identify just by
                  > looking at the path /orders/1/cancel.
                  >
                  > Just looking at the syntax of a single request (e.g. the method POST and/or
                  > the path /orders/1/cancel) is not enough to determine if the system adheres
                  > to any particular architectural style.
                  >
                  > The author may well intend that /cancel identifies a resource, and allow
                  > lots of interaction in it. It might even have an HTML representation, with
                  > a nice <form method="post"><input type="submit" value="CANCEL"></form> in
                  > it.
                  >
                  > But this is still not enough to determine if a system adheres to the
                  > constraints: For even though the server provides such hypermedia controls,
                  > media types and does everything "by the book", it is still up to the
                  > *client* to actually *use* these hypermedia controls. In fact it is up to
                  > the author of the agent itself.
                  >
                  > If you write code in the client that hard wires a button called "cancel" to
                  > the "POST /orders/1/cancel" HTTP request then the client isn't really
                  > honouring the hypermedia controls laid out above. If, however, it _soft
                  > wires_ the same button _because_ of the hypermedia control above, then
                  > you're doing it "more right" I would say. The point is that the client
                  > should be written in such a way that it uses _only_ the hypermedia controls
                  > that it receives at run-time.
                  >
                  >> Roy's thesis really must be considered in its entirety, [...]
                  >
                  >
                  > +1 to such a degree that 1 > 1. The rest of the stuff you wrote was great.
                  > I just want to highlight the incorrectness of the question, as others said
                  > earlier that REST doesn't say anything about if POST /foo is or isn't...
                  > For all we know /orders/1/cancel is a picture of a dog, or it could be a
                  > SOAP endpoint for a nice game of Global Thermonuclear War.
                  >
                  > The constraints of REST are too often thought / taught to deal with the
                  > server side, but in my experience, it has much more to do with the client
                  > side, how much knowledge the client has on things like:
                  >
                  > - what URLs can it use? (it shouldn't know any; but bind to one at run-time,
                  > preferably via a configuration parameter)
                  > - when it knows one URL (e.g. "/orders/1"), can it add "/cancel" to it to
                  > cancel it? (no, unless it was told to add "/cancel" by a hypermedia
                  > control)
                  > - does it know the "type" of a resource, e.g. that /orders/1 is an Order
                  > (this is a point of contention on the list; browsers don't know about books
                  > or a bank account, but work fine nonetheless.)
                  > - when it does know the type of the URL is an order, does it pull up a
                  > pre-coded "order" UI (bad), or does it create a UI based on what it finds in
                  > the response (good)
                  >
                  >
                  > --
                  > -mogsie-
                • Erik Wilde
                  hello max. ... i guess most people agree that the only reasonable answer to a question such as is URI X RESTful is: tell me more about your design. i d
                  Message 8 of 28 , Dec 1, 2012
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                    hello max.

                    On 2012-12-01 08:17 , Max Toro wrote:
                    > Thanks for you answer Erik. I don't want to repeat myself, so please
                    > see my answer to Eric, I'd love to get your comments as well. Rather
                    > than good or bad I'd like to determine if my example is REST or not.

                    i guess most people agree that the only reasonable answer to a question
                    such as "is URI X RESTful" is: "tell me more about your design."

                    i'd like to get one tiny bit of clarification from you: when saying you
                    have a resource X/operation, did you choose "cancel" on purpose as
                    something that could be mapped to a DELETE X fairly easily, or do you
                    also consider something such as X/additem, where the hypothetical
                    operation would not delete the resource, but change its status outside
                    of the CRUD realm?

                    thanks,

                    dret.
                  • Max Toro
                    Did not choose DELETE because cancel does not delete the resource, it executes some logic which in the end sets it s status field to Canceled. The
                    Message 9 of 28 , Dec 1, 2012
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                      Did not choose DELETE because cancel does not delete the resource, it
                      executes some logic which in the end sets it's status field to
                      Canceled.
                      The implementation of PATCH with a body 'status=Canceled' can be
                      tricky if you also accept changes to other fields, which may or may
                      not have some logic associated to them. I think that using a
                      subresource as Nicholas Shanks suggested is a good solution, e.g.

                      PUT /orders/1/status

                      Canceled

                      The implementation can do something like this:

                      switch (value) {
                      case "Canceled":
                      cancelOrder(id);
                      ...
                      }
                      --
                      Max Toro


                      On Sat, Dec 1, 2012 at 2:58 PM, Erik Wilde <dret@...> wrote:
                      > hello max.
                      >
                      >
                      > On 2012-12-01 08:17 , Max Toro wrote:
                      >>
                      >> Thanks for you answer Erik. I don't want to repeat myself, so please
                      >> see my answer to Eric, I'd love to get your comments as well. Rather
                      >> than good or bad I'd like to determine if my example is REST or not.
                      >
                      >
                      > i guess most people agree that the only reasonable answer to a question such
                      > as "is URI X RESTful" is: "tell me more about your design."
                      >
                      > i'd like to get one tiny bit of clarification from you: when saying you have
                      > a resource X/operation, did you choose "cancel" on purpose as something that
                      > could be mapped to a DELETE X fairly easily, or do you also consider
                      > something such as X/additem, where the hypothetical operation would not
                      > delete the resource, but change its status outside of the CRUD realm?
                      >
                      > thanks,
                      >
                      > dret.
                    • Mike Schinkel
                      ... +1 As an aside you ll find maintenance and validation easier if your status values are all lowercase (all upper would be okay too, but uglier.) Has nothing
                      Message 10 of 28 , Dec 1, 2012
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                        On Dec 1, 2012, at 9:21 PM, Max Toro <maxtoroq@...> wrote:
                        > PUT /orders/1/status
                        > Canceled

                        +1

                        As an aside you'll find maintenance and validation easier if your status values are all lowercase (all upper would be okay too, but uglier.)

                        Has nothing to do with REST, it's just easier to deal with when you don't have to worry what is the proper casing of status values. FWIW.

                        -Mike
                      • Matt McClure
                        ... This seems like a really important point. As I started reading about hypermedia APIs, the authors seemed to be evangelizing the benefit that server
                        Message 11 of 28 , Dec 2, 2012
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                          On Sat, Dec 1, 2012 at 8:30 AM, Erik Mogensen <erik@...> wrote:
                          But this is still not enough to determine if a system adheres to the constraints: For even though the server provides such hypermedia controls, media types and does everything "by the book", it is still up to the *client* to actually *use* these hypermedia controls.  In fact it is up to the author of the agent itself.

                          If you write code in the client that hard wires a button called "cancel" to the "POST /orders/1/cancel" HTTP request then the client isn't really honouring the hypermedia controls laid out above.  If, however, it _soft wires_ the same button _because_ of the hypermedia control above, then you're doing it "more right" I would say.  The point is that the client should be written in such a way that it uses _only_ the hypermedia controls that it receives at run-time.

                          This seems like a really important point. As I started reading about hypermedia APIs, the authors seemed to be evangelizing the benefit that server programmers could change URIs as requirements evolved. But it seems you can only realize that benefit easily if you can guarantee that all your clients use the hypermedia controls, or that you force the issue by breaking clients that aren't honoring that agreement. Hypermedia APIs seem to raise the expectations on client programmers more than for server programmers.

                          As a server programmer, I'm interested to find great examples of hypermedia APIs (and ecosystems, documentation, etc.) that help their client programmers do the right thing.

                          Even then, if you can't prevent client developers from "deep linking" into your API, how do you handle breaking changes for those clients?
                        • Erik Wilde
                          hello matt. ... this is a very good question. one important aspect is that it s always bad if clients are tested against just one implementation. server
                          Message 12 of 28 , Dec 2, 2012
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                            hello matt.

                            On 2012-12-02 08:44 , Matt McClure wrote:
                            > Even then, if you can't prevent client developers from "deep linking"
                            > into your API, how do you handle breaking changes for those clients?

                            this is a very good question. one important aspect is that it's always
                            bad if clients are tested against just one implementation. server
                            programmers often put quite a bit of effort into designing pretty URIs,
                            which is good. but this also lures client programmers into relying on
                            those patterns, and as long as you just test against one implementation,
                            things often work fine.

                            however, often there only is one implementation to test against, so what
                            then? something i've been thinking about (and nothing more, and also i
                            haven't seen any frameworks supporting this) would be that server
                            frameworks could have a "test" switch, that would start serving "ugly"
                            and pretty much random URIs, instead of the pretty designed ones. a test
                            installation could then be easily provided to client programmers, and
                            their code would break very quickly in all places where they took
                            shortcuts, instead of following links.

                            i haven't seen support for this so far, but it seems to me that in many
                            REST-oriented server-side frameworks, this might not be all that hard to
                            add. if somebody has seen such a thing in practice, please let me know!

                            cheers,

                            dret.

                            --
                            erik wilde | mailto:dret@... - tel:+1-510-2061079 |
                            | UC Berkeley - School of Information (ISchool) |
                            | http://dret.net/netdret http://twitter.com/dret |
                          • Eric J. Bowman
                            ... DELETE /orders/1 doesn t have to delete the resource, it can move it to, say, /canceled/1. In which case you re only changing one property of the order
                            Message 13 of 28 , Dec 3, 2012
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                              Max Toro wrote:
                              >
                              > Did not choose DELETE because cancel does not delete the resource, it
                              > executes some logic which in the end sets it's status field to
                              > Canceled.
                              >

                              DELETE /orders/1 doesn't have to delete the resource, it can move it to,
                              say, /canceled/1. In which case you're only changing one property of
                              the order "object" on your server -- its mapping. REST isn't CRUD. In
                              a hypertext API, it's often best to have different URI paths for the
                              same set of server objects, if their policies vary based on a property
                              like status...

                              Web servers configure policies based on path, so add another URI mapping
                              instead of making server logic more complex by trying to apply different
                              policies based on parsing content for 'canceled=true'. Try to think in
                              terms of hypertext applications instead of serializing objects to
                              hypertext -- status isn't a field in a hypertext document in REST when
                              its semantics overlap that of HTTP status responses, especially if
                              status varies by user role:

                              The customer checking /orders/1 knows they've successfully canceled it,
                              because their DELETE request responded 200 OK with the order and new
                              status in the message body, and that URL now responds to GET with 410
                              Gone. The 410 response may include the order as its entity, or not,
                              for the customer role.

                              Other user roles, i.e. admin, may get redirected to /canceled/1, where
                              they are subject to different policies than exist for an active order.
                              Admins may DELETE from /canceled to actually delete a canceled order,
                              while customers are 403 Forbidden from entering /canceled to begin with.
                              Also, just because you're using the DELETE method, doesn't mean the UI
                              has to say "delete" on the cancel control.

                              >
                              > The implementation of PATCH with a body 'status=Canceled' can be
                              > tricky if you also accept changes to other fields, which may or may
                              > not have some logic associated to them.
                              >

                              What matters is user intent. Clear intention to cancel an order should
                              unambiguously be its own operation from the UI-design perspective; from
                              the protocol perspective, this idempotent user action should be made
                              explicitly visible on the wire by selecting the most appropriate HTTP
                              method.

                              Of all manipulations we may allow for an order, this is the only one
                              that's self-explanatory without transferring a representation, another
                              indication that DELETE has the semantics we're after, provided we don't
                              get hung up on having to delete something -- which we don't!

                              If, to the requesting user, canceling an order makes it go away, then
                              using DELETE meets the self-descriptive messaging and uniform interface
                              constraints of REST. What happens to the order is an implementation
                              detail, hidden behind the uniform interface. To the world-at-large,
                              the traffic pattern of an order cancellation looks like exactly what it
                              is, as the principle of generality has been followed.

                              The visibility of DELETE allows intermediaries to mark cached copies
                              of /orders/1 as stale. This optimization is built-in to the deployed
                              infrastructure of the Web, all it does here is ensure the requesting
                              user doesn't re-load a stale copy of /orders/1 which fails to reflect
                              the results of the action just taken. This can't be done with PATCH,
                              even if you can get close by marking the request as idempotent.

                              -Eric
                            • Eric J. Bowman
                              ... Yes, such examples are out there, but in them, /cancel is NOT a resource in the REST sense so they must be some style of RPC... REST, not so much.
                              Message 14 of 28 , Dec 3, 2012
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                                Max Toro wrote:
                                >
                                > > Well, what are you expecting to GET from /cancel, or are you just
                                > > using that URL to invoke a procedure? If so, then there are a few
                                > > places Roy's thesis admonishes against it in Chapter 6 -- the rest
                                > > of REST is about positive, rather than negative, reinforcement of
                                > > the identification of resources constraint. Suggested reading:
                                > > 6.5.2; 6.2.1, in particular: "REST [defines] a resource to be the
                                > > semantics of what the author intends to identify."
                                >
                                > To clarify, /orders/1/cancel is used to modify a resource, using POST.
                                > A GET request would result in a Method Not Allowed response.
                                >

                                Yes, such examples are out there, but in them, /cancel is NOT a resource
                                in the REST sense so they must be some style of RPC... REST, not so
                                much. Representational State Transfer means just that -- resources are
                                manipulated by transferring representations of their current, intended,
                                desired etc. state. Chapter 5.4:

                                "Requests and responses have the appearance of a remote invocation
                                style, but REST messages are targeted at a conceptual resource rather
                                than an implementation identifier."

                                This example is a REST anti-pattern, as I cannot deduce the current
                                (sub)state of the order (active or canceled) by dereferencing the URL
                                I'm given for manipulating that (sub)state. Just making a toggle POST
                                also fails to transfer any representation of anything, let alone
                                application state, and isn't proper HTTP (which can never be proper
                                REST).

                                REST isn't about optimizing upstream traffic, it's about optimizing GET.
                                What advantage does a subresource give when it contains no content from
                                the parent resource? My goal with subresources is to increase the cache
                                stickiness of their parent resources. Replace the subresource content
                                in the parent resource, with a link or a hypertext control linked to the
                                subresource. The link or hypertext control remains static, and cached,
                                as the content of the subresource varies.

                                That's a RESTful pattern, as the subresource now has a representation
                                (other than that of the 406 error) we can transfer and manipulate to
                                effect change of the parent resource.

                                >
                                > > Not the semantics of a method invocation. What does /cancel
                                > > identify? Sounds to me like a method of tunneling DELETE through
                                > > POST which identifies nothing, iow a procedure endpoint, which is
                                > > characteristic of various styles but not of the REST style. The
                                > > hypertext constraint only makes sense if your resources make sense,
                                > > in that their URLs are identifiers rather than endpoints.
                                >
                                > If I understand correctly, you are saying that if I need to affect a
                                > resource then I should use the uniform interface on that resource URI,
                                > and not another URI.
                                >

                                Absolutely not! My example changed /order/1 by manipulating /order/1
                                /status. The /order/1 resource includes its status, but its 200 OK
                                representations only include links to the /status subresource, derived
                                from the /order/1 resource such that manipulating a representation
                                of /order/1/status updates /order/1 (on the server, you can do anything
                                you want; on the client, the cached link/control in /order/1 now returns
                                a different value).

                                There's no cost to adding a URI like this, nor does it preclude changing
                                order status via PUT/PATCH to /order/1. The difference is that /status
                                uses REST's uniform interface, unlike /cancel. The /order/1/status URL
                                is only presented within a hypertext control which explains how to
                                manipulate it -- picture a drop-down list with the current status
                                highlighted, meeting the hypertext constraint. You can always GET the
                                status of an order even if you don't have a copy of that order, a
                                useful separation of concerns beyond just optimizing GET, promoting
                                serendipitous re-use.

                                >
                                > > Which brings us to Chapter 5, and the short answer to your question:
                                > > "POSTing to /cancel violates the Identification of Resources
                                > > constraint, and is therefore unRESTful." But I've found that just
                                > > giving that answer tends to upset folks who've only read Chapter 5,
                                > > then they get defensive about why can't they call their API
                                > > RESTful, accusations of pedantry follow, and threads devolve into
                                > > general ugliness, heheh...
                                >
                                > After reading that chapter again I'm not sure my example violates
                                > anything, but I'd love to get more clarification from you. Is it the
                                > use of a verb in the URI? or not using the URI of the resource I'm
                                > trying to modify directly?
                                >

                                The biggest problem, is that an RPC endpoint which has no GET function
                                while improperly listening for a method itself as a trigger rather than
                                taking action based on the content of the entity required by that method
                                and in obeyance of the semantics of that method, is so far away from
                                REST that I don't know where to start except by urging that Roy's
                                thesis be read in its entirety. Because it's obvious to me that this
                                violates the first three uniform interface constraints, making adherence
                                to the fourth irrelevant:

                                "REST is defined by four interface constraints: identification of
                                resources; manipulation of resources through representations; self-
                                descriptive messages; and, hypermedia as the engine of application
                                state."

                                Note that "noun/verb" terminology is not present in the dissertation.
                                But, yeah, if your URI is a "verb" you're probably getting REST wrong.
                                There is no "cancel" method in the uniform interface. There are two
                                basic means of solving this -- one, is refactor your cancel operation
                                to use DELETE; two, tunnel your proprietary cancel method through POST.

                                http://tech.groups.yahoo.com/group/rest-discuss/message/19210

                                Making this operation its own URI doesn't make it more RESTful, as the
                                resulting URI is only a resource in the HTTP/URI sense, but not the
                                REST sense, of the term. Utilizing subresources to break out more
                                dynamic aspects of content and cache them separately is RESTful; if the
                                contents of the GET are also allowable content of a PUT then we're
                                letting hypertext drive application state instead of listening for POST
                                events to trigger server-object methods we've failed to refactor to the
                                uniform REST interface.

                                -Eric
                              • Max Toro
                                Thank you very much for your responses Eric. The short answer is (if I understood correctly): POST /cancel is not REST because it lacks visibility, since it s
                                Message 15 of 28 , Dec 4, 2012
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                                  Thank you very much for your responses Eric.

                                  The short answer is (if I understood correctly): POST /cancel is not
                                  REST because it lacks visibility, since it's not possible to
                                  understand the client's intent by examining the request.

                                  Never thought about HTTP/URI resource vs. REST resource, you say that
                                  a URI that doesn't implement GET is probably (or always) not REST.
                                  --
                                  Max Toro


                                  On Mon, Dec 3, 2012 at 1:17 PM, Eric J. Bowman <eric@...> wrote:
                                  > Max Toro wrote:
                                  >>
                                  >> > Well, what are you expecting to GET from /cancel, or are you just
                                  >> > using that URL to invoke a procedure? If so, then there are a few
                                  >> > places Roy's thesis admonishes against it in Chapter 6 -- the rest
                                  >> > of REST is about positive, rather than negative, reinforcement of
                                  >> > the identification of resources constraint. Suggested reading:
                                  >> > 6.5.2; 6.2.1, in particular: "REST [defines] a resource to be the
                                  >> > semantics of what the author intends to identify."
                                  >>
                                  >> To clarify, /orders/1/cancel is used to modify a resource, using POST.
                                  >> A GET request would result in a Method Not Allowed response.
                                  >>
                                  >
                                  > Yes, such examples are out there, but in them, /cancel is NOT a resource
                                  > in the REST sense so they must be some style of RPC... REST, not so
                                  > much. Representational State Transfer means just that -- resources are
                                  > manipulated by transferring representations of their current, intended,
                                  > desired etc. state. Chapter 5.4:
                                  >
                                  > "Requests and responses have the appearance of a remote invocation
                                  > style, but REST messages are targeted at a conceptual resource rather
                                  > than an implementation identifier."
                                  >
                                  > This example is a REST anti-pattern, as I cannot deduce the current
                                  > (sub)state of the order (active or canceled) by dereferencing the URL
                                  > I'm given for manipulating that (sub)state. Just making a toggle POST
                                  > also fails to transfer any representation of anything, let alone
                                  > application state, and isn't proper HTTP (which can never be proper
                                  > REST).
                                  >
                                  > REST isn't about optimizing upstream traffic, it's about optimizing GET.
                                  > What advantage does a subresource give when it contains no content from
                                  > the parent resource? My goal with subresources is to increase the cache
                                  > stickiness of their parent resources. Replace the subresource content
                                  > in the parent resource, with a link or a hypertext control linked to the
                                  > subresource. The link or hypertext control remains static, and cached,
                                  > as the content of the subresource varies.
                                  >
                                  > That's a RESTful pattern, as the subresource now has a representation
                                  > (other than that of the 406 error) we can transfer and manipulate to
                                  > effect change of the parent resource.
                                  >
                                  >>
                                  >> > Not the semantics of a method invocation. What does /cancel
                                  >> > identify? Sounds to me like a method of tunneling DELETE through
                                  >> > POST which identifies nothing, iow a procedure endpoint, which is
                                  >> > characteristic of various styles but not of the REST style. The
                                  >> > hypertext constraint only makes sense if your resources make sense,
                                  >> > in that their URLs are identifiers rather than endpoints.
                                  >>
                                  >> If I understand correctly, you are saying that if I need to affect a
                                  >> resource then I should use the uniform interface on that resource URI,
                                  >> and not another URI.
                                  >>
                                  >
                                  > Absolutely not! My example changed /order/1 by manipulating /order/1
                                  > /status. The /order/1 resource includes its status, but its 200 OK
                                  > representations only include links to the /status subresource, derived
                                  > from the /order/1 resource such that manipulating a representation
                                  > of /order/1/status updates /order/1 (on the server, you can do anything
                                  > you want; on the client, the cached link/control in /order/1 now returns
                                  > a different value).
                                  >
                                  > There's no cost to adding a URI like this, nor does it preclude changing
                                  > order status via PUT/PATCH to /order/1. The difference is that /status
                                  > uses REST's uniform interface, unlike /cancel. The /order/1/status URL
                                  > is only presented within a hypertext control which explains how to
                                  > manipulate it -- picture a drop-down list with the current status
                                  > highlighted, meeting the hypertext constraint. You can always GET the
                                  > status of an order even if you don't have a copy of that order, a
                                  > useful separation of concerns beyond just optimizing GET, promoting
                                  > serendipitous re-use.
                                  >
                                  >>
                                  >> > Which brings us to Chapter 5, and the short answer to your question:
                                  >> > "POSTing to /cancel violates the Identification of Resources
                                  >> > constraint, and is therefore unRESTful." But I've found that just
                                  >> > giving that answer tends to upset folks who've only read Chapter 5,
                                  >> > then they get defensive about why can't they call their API
                                  >> > RESTful, accusations of pedantry follow, and threads devolve into
                                  >> > general ugliness, heheh...
                                  >>
                                  >> After reading that chapter again I'm not sure my example violates
                                  >> anything, but I'd love to get more clarification from you. Is it the
                                  >> use of a verb in the URI? or not using the URI of the resource I'm
                                  >> trying to modify directly?
                                  >>
                                  >
                                  > The biggest problem, is that an RPC endpoint which has no GET function
                                  > while improperly listening for a method itself as a trigger rather than
                                  > taking action based on the content of the entity required by that method
                                  > and in obeyance of the semantics of that method, is so far away from
                                  > REST that I don't know where to start except by urging that Roy's
                                  > thesis be read in its entirety. Because it's obvious to me that this
                                  > violates the first three uniform interface constraints, making adherence
                                  > to the fourth irrelevant:
                                  >
                                  > "REST is defined by four interface constraints: identification of
                                  > resources; manipulation of resources through representations; self-
                                  > descriptive messages; and, hypermedia as the engine of application
                                  > state."
                                  >
                                  > Note that "noun/verb" terminology is not present in the dissertation.
                                  > But, yeah, if your URI is a "verb" you're probably getting REST wrong.
                                  > There is no "cancel" method in the uniform interface. There are two
                                  > basic means of solving this -- one, is refactor your cancel operation
                                  > to use DELETE; two, tunnel your proprietary cancel method through POST.
                                  >
                                  > http://tech.groups.yahoo.com/group/rest-discuss/message/19210
                                  >
                                  > Making this operation its own URI doesn't make it more RESTful, as the
                                  > resulting URI is only a resource in the HTTP/URI sense, but not the
                                  > REST sense, of the term. Utilizing subresources to break out more
                                  > dynamic aspects of content and cache them separately is RESTful; if the
                                  > contents of the GET are also allowable content of a PUT then we're
                                  > letting hypertext drive application state instead of listening for POST
                                  > events to trigger server-object methods we've failed to refactor to the
                                  > uniform REST interface.
                                  >
                                  > -Eric
                                • Jan Algermissen
                                  ... Maybe of interest: http://tech.groups.yahoo.com/group/rest-discuss/message/18998 jan
                                  Message 16 of 28 , Dec 4, 2012
                                  • 0 Attachment
                                    On Dec 4, 2012, at 11:40 PM, Max Toro <maxtoroq@...> wrote:

                                    > Thank you very much for your responses Eric.
                                    >
                                    > The short answer is (if I understood correctly): POST /cancel is not
                                    > REST because it lacks visibility, since it's not possible to
                                    > understand the client's intent by examining the request.

                                    Maybe of interest: http://tech.groups.yahoo.com/group/rest-discuss/message/18998

                                    jan


                                    >
                                    > Never thought about HTTP/URI resource vs. REST resource, you say that
                                    > a URI that doesn't implement GET is probably (or always) not REST.
                                    > --
                                    > Max Toro
                                    >
                                    > On Mon, Dec 3, 2012 at 1:17 PM, Eric J. Bowman <eric@...> wrote:
                                    > > Max Toro wrote:
                                    > >>
                                    > >> > Well, what are you expecting to GET from /cancel, or are you just
                                    > >> > using that URL to invoke a procedure? If so, then there are a few
                                    > >> > places Roy's thesis admonishes against it in Chapter 6 -- the rest
                                    > >> > of REST is about positive, rather than negative, reinforcement of
                                    > >> > the identification of resources constraint. Suggested reading:
                                    > >> > 6.5.2; 6.2.1, in particular: "REST [defines] a resource to be the
                                    > >> > semantics of what the author intends to identify."
                                    > >>
                                    > >> To clarify, /orders/1/cancel is used to modify a resource, using POST.
                                    > >> A GET request would result in a Method Not Allowed response.
                                    > >>
                                    > >
                                    > > Yes, such examples are out there, but in them, /cancel is NOT a resource
                                    > > in the REST sense so they must be some style of RPC... REST, not so
                                    > > much. Representational State Transfer means just that -- resources are
                                    > > manipulated by transferring representations of their current, intended,
                                    > > desired etc. state. Chapter 5.4:
                                    > >
                                    > > "Requests and responses have the appearance of a remote invocation
                                    > > style, but REST messages are targeted at a conceptual resource rather
                                    > > than an implementation identifier."
                                    > >
                                    > > This example is a REST anti-pattern, as I cannot deduce the current
                                    > > (sub)state of the order (active or canceled) by dereferencing the URL
                                    > > I'm given for manipulating that (sub)state. Just making a toggle POST
                                    > > also fails to transfer any representation of anything, let alone
                                    > > application state, and isn't proper HTTP (which can never be proper
                                    > > REST).
                                    > >
                                    > > REST isn't about optimizing upstream traffic, it's about optimizing GET.
                                    > > What advantage does a subresource give when it contains no content from
                                    > > the parent resource? My goal with subresources is to increase the cache
                                    > > stickiness of their parent resources. Replace the subresource content
                                    > > in the parent resource, with a link or a hypertext control linked to the
                                    > > subresource. The link or hypertext control remains static, and cached,
                                    > > as the content of the subresource varies.
                                    > >
                                    > > That's a RESTful pattern, as the subresource now has a representation
                                    > > (other than that of the 406 error) we can transfer and manipulate to
                                    > > effect change of the parent resource.
                                    > >
                                    > >>
                                    > >> > Not the semantics of a method invocation. What does /cancel
                                    > >> > identify? Sounds to me like a method of tunneling DELETE through
                                    > >> > POST which identifies nothing, iow a procedure endpoint, which is
                                    > >> > characteristic of various styles but not of the REST style. The
                                    > >> > hypertext constraint only makes sense if your resources make sense,
                                    > >> > in that their URLs are identifiers rather than endpoints.
                                    > >>
                                    > >> If I understand correctly, you are saying that if I need to affect a
                                    > >> resource then I should use the uniform interface on that resource URI,
                                    > >> and not another URI.
                                    > >>
                                    > >
                                    > > Absolutely not! My example changed /order/1 by manipulating /order/1
                                    > > /status. The /order/1 resource includes its status, but its 200 OK
                                    > > representations only include links to the /status subresource, derived
                                    > > from the /order/1 resource such that manipulating a representation
                                    > > of /order/1/status updates /order/1 (on the server, you can do anything
                                    > > you want; on the client, the cached link/control in /order/1 now returns
                                    > > a different value).
                                    > >
                                    > > There's no cost to adding a URI like this, nor does it preclude changing
                                    > > order status via PUT/PATCH to /order/1. The difference is that /status
                                    > > uses REST's uniform interface, unlike /cancel. The /order/1/status URL
                                    > > is only presented within a hypertext control which explains how to
                                    > > manipulate it -- picture a drop-down list with the current status
                                    > > highlighted, meeting the hypertext constraint. You can always GET the
                                    > > status of an order even if you don't have a copy of that order, a
                                    > > useful separation of concerns beyond just optimizing GET, promoting
                                    > > serendipitous re-use.
                                    > >
                                    > >>
                                    > >> > Which brings us to Chapter 5, and the short answer to your question:
                                    > >> > "POSTing to /cancel violates the Identification of Resources
                                    > >> > constraint, and is therefore unRESTful." But I've found that just
                                    > >> > giving that answer tends to upset folks who've only read Chapter 5,
                                    > >> > then they get defensive about why can't they call their API
                                    > >> > RESTful, accusations of pedantry follow, and threads devolve into
                                    > >> > general ugliness, heheh...
                                    > >>
                                    > >> After reading that chapter again I'm not sure my example violates
                                    > >> anything, but I'd love to get more clarification from you. Is it the
                                    > >> use of a verb in the URI? or not using the URI of the resource I'm
                                    > >> trying to modify directly?
                                    > >>
                                    > >
                                    > > The biggest problem, is that an RPC endpoint which has no GET function
                                    > > while improperly listening for a method itself as a trigger rather than
                                    > > taking action based on the content of the entity required by that method
                                    > > and in obeyance of the semantics of that method, is so far away from
                                    > > REST that I don't know where to start except by urging that Roy's
                                    > > thesis be read in its entirety. Because it's obvious to me that this
                                    > > violates the first three uniform interface constraints, making adherence
                                    > > to the fourth irrelevant:
                                    > >
                                    > > "REST is defined by four interface constraints: identification of
                                    > > resources; manipulation of resources through representations; self-
                                    > > descriptive messages; and, hypermedia as the engine of application
                                    > > state."
                                    > >
                                    > > Note that "noun/verb" terminology is not present in the dissertation.
                                    > > But, yeah, if your URI is a "verb" you're probably getting REST wrong.
                                    > > There is no "cancel" method in the uniform interface. There are two
                                    > > basic means of solving this -- one, is refactor your cancel operation
                                    > > to use DELETE; two, tunnel your proprietary cancel method through POST.
                                    > >
                                    > > http://tech.groups.yahoo.com/group/rest-discuss/message/19210
                                    > >
                                    > > Making this operation its own URI doesn't make it more RESTful, as the
                                    > > resulting URI is only a resource in the HTTP/URI sense, but not the
                                    > > REST sense, of the term. Utilizing subresources to break out more
                                    > > dynamic aspects of content and cache them separately is RESTful; if the
                                    > > contents of the GET are also allowable content of a PUT then we're
                                    > > letting hypertext drive application state instead of listening for POST
                                    > > events to trigger server-object methods we've failed to refactor to the
                                    > > uniform REST interface.
                                    > >
                                    > > -Eric
                                    >
                                  • Eric J. Bowman
                                    ... I d phrase it differently: POST /cancel violates self-descriptiveness as user intent cannot be discerned by examining the request. This would hold true
                                    Message 17 of 28 , Dec 5, 2012
                                    • 0 Attachment
                                      Max Toro wrote:
                                      >
                                      > The short answer is (if I understood correctly): POST /cancel is not
                                      > REST because it lacks visibility, since it's not possible to
                                      > understand the client's intent by examining the request.
                                      >

                                      I'd phrase it differently: "POST /cancel violates self-descriptiveness
                                      as user intent cannot be discerned by examining the request." This
                                      would hold true even if GET /cancel had a 200 OK response. Even that's
                                      oversimplifying in that part of the reason POST is wrong here, is
                                      because DELETE is right. But, no, I do not believe setting up an URL to
                                      listen for POST and taking action not based on anything in the POST
                                      request, is ever an example of the REST style, regardless of the nature
                                      of the resource.

                                      My real short answer is, "POST /cancel doesn't use REST's uniform
                                      interface."

                                      Neither is it an example of the REST style for POST to have a definition
                                      which varies by resource, iow sometimes it works without an entity
                                      (method-tunnel listener), sometimes it doesn't (create resource); even
                                      if hypertext-driven, such nonstandard method usage inevitably results
                                      in coupling between client and server, instead of relying on shared
                                      understanding of standardized method semantics. Fixing the POST to have
                                      an entity that's ignored might make it valid HTTP and solve this, but it
                                      still wouldn't be an example of the REST style, where the requested
                                      state change is transferred in the entity (with a few exceptions) -- not
                                      defined per resource by the method being tunneled through / triggered by
                                      POST.

                                      Conceptually, it still violates the identification of resources
                                      constraint, which is why it isn't visible regardless of how we implement
                                      it in terms of hypertext or method selection. Using the uniform
                                      interface means refactoring internal methods like 'cancel' to fit the
                                      resource/representation model of REST such that the request methods and
                                      response codes used map generically to those of your chosen protocol.
                                      Assigning them URIs to toggle with semantically-void messages is simply
                                      some other style not encompassed by REST's uniform interface constraint,
                                      not something that's superficially fixable by improving visibility or
                                      adding a 200 OK response.

                                      >
                                      > Never thought about HTTP/URI resource vs. REST resource, you say that
                                      > a URI that doesn't implement GET is probably (or always) not REST.
                                      >

                                      More like, "REST resources are concepts, not actions." Just using URIs
                                      doesn't bestow compliance with the identification of resources
                                      constraint upon a system, it's more nuanced than that. I can't imagine
                                      why a bona-fide REST resource would ever deliberately fail to have a
                                      retrieval mapping, no.

                                      I think the more important takeaway is method selection. We have DELETE
                                      in order to avoid such convoluted POST-based cancel mechanisms. It
                                      isn't RESTful to use POST when another method's semantics happen to
                                      closely describe user intent. From Roy's blog:

                                      "POST only becomes an issue when it is used in a situation for which
                                      some other method is ideally suited: e.g., retrieval of information
                                      that should be a representation of some resource (GET), complete
                                      replacement of a representation (PUT), or any of the other standardized
                                      methods that tell intermediaries something more valuable than 'this may
                                      change something.' The other methods are more valuable to
                                      intermediaries because they say something about how failures can be
                                      automatically handled and how intermediate caches can optimize their
                                      behavior. POST does not have those characteristics, but that doesn't
                                      mean we can live without it. POST serves many useful purposes in HTTP,
                                      including the general purpose of 'this action isn't worth
                                      standardizing.'"

                                      http://roy.gbiv.com/untangled/2009/it-is-okay-to-use-post

                                      Since DELETE is inherently self-descriptive of user intent to cancel an
                                      order, and is already "listened for" rather than requiring an entity be
                                      transferred, and has inherent idempotency matching the inherently
                                      idempotent user intent captured by a cancel request, DELETE must be the
                                      proper method for implementation using HTTP.

                                      Now, maybe your media type doesn't support DELETE, in which case
                                      contriving a /cancel URI may very well be called for until such time
                                      as it does. I won't fault anyone for doing this out of pragmatism, so
                                      long as they understand it as a mismatch, inconsistent with the
                                      architectural style they're following.

                                      REST is a tool which allows you, over time, to make improvements to the
                                      architecture you have in accordance with an idealized model. To me,
                                      accepting mismatches for what they are, is far more useful than coming
                                      up with convoluted explanations for why they aren't, for the sake of
                                      buzzword compliance. Change is easier when it's based on having been
                                      right all along, rather than based on having to admit error. ;-)

                                      -Eric
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