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

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  • 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 1 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 2 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 3 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 4 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 5 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 6 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 7 of 28 , Dec 4, 2012
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                  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 8 of 28 , Dec 5, 2012
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                    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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