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

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  • 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 1 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 2 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 3 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 4 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 5 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 6 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 7 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 8 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 9 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 10 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 11 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 12 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 13 of 28 , Dec 3, 2012
                            • 0 Attachment
                              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 14 of 28 , Dec 4, 2012
                              • 0 Attachment
                                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 15 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 16 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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