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Re: [rest-discuss] API versioning

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  • Jørn Wildt
    Hi Eric ... wire, so I ve never understood why this solution is so prevalent in REST discussions. Just media types, which are agnostic as to what type of
    Message 1 of 13 , Jan 8, 2013
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      Hi Eric

      > REST has no notion of resource "type" let alone of exposing same on the
      wire, so I've never understood why this solution is so prevalent in REST
      discussions. Just media types, which are agnostic as to what "type" of
      resource is being represented (with rare exceptions as parameters).

      I remember your stance on this from previous discussions. And, yes, a "sales order" or what ever other "type" you need can certainly be represented as (X)HTML. Sure, I am in no way questioning that.

      My question is though - from where does the client get the information it needs to extract, say, the order sum, delivery address, order lines quantity and so on from the generic payload?

      Let me expand with an example:

      - A client of mine sends me a link to a sales order and I want to import that into my local ERP system.

      - If the sales order is only available in (X)HTML, JSON or similar generic media type, how would I then be able to understand the content in enough detail to import it?

      - If the sales order was available in ATOM, I could at least import it as a document having a title and some content. But, okay, you can do that with (X)HTML too - albeit not with JSON and XML.

      - If the sales order was available in an even more domain specific media type, I could do an even better import of data.

      - ... and so on for various levels of domain specific media types ...

      Having a domain specific media type does although not imply any specific "type" of the resource itself - it only says "this particular resource can (also) be represented as XXX" - where XXX is an official application specific media type, for instance vCard or vCal which would allow the client to derive contact and calendar information from a resource - without making *any* kind of assumption about the underlying "type" of the it.

      Actually, I would argue that a generic media type is more "typed" than a domain specific media type. Okay, that sounds like rubbish, I know - but let me explain:

      - as I stated above, a domain specific media type lets you derived useful information from a resource without making any assumptions about its underlying type.

      - whereas a generic media type forces the client to magically assume the resource has a specific type with specific properties you can extract from the generic payload.

      Well, that is my current view on this issue ... and I have been swinging back and forth between your viewpoint and the above viewpoint during the last couple of years :-) Still open for new inputs!

      Thanks, Jørn


      On Tue, Jan 8, 2013 at 6:57 AM, Eric J. Bowman <eric@...> wrote:
       

      Keith Hassen wrote:
      >
      > This seems to be a recommended practice that I've encountered in
      > reading about RESTful implementations.
      >

      Perfectly good HTTP API practice; REST, not so much. The point of
      media types is to provide loose coupling based on shared understanding
      of how the representation is to be decoded. Not tight coupling based
      on interpreting the content of the representation. I've never seen an
      order API that couldn't be represented as HTML; despite any number of
      variations on this theme (or versions within), the media type doesn't
      vary between more order APIs on the Web than I can count.

      REST has no notion of resource "type" let alone of exposing same on the
      wire, so I've never understood why this solution is so prevalent in REST
      discussions. Just media types, which are agnostic as to what "type" of
      resource is being represented (with rare exceptions as parameters).


      >
      > > Anything else is up to the user-agent to work out. This is actually
      > > easier for m2m than h2m; a v1 m2m client should already ignore any
      > > hypertext it doesn't understand, rolling out v2 can then re-use any
      > > non-deprecated bits of the API already present plus the new stuff,
      > > and ignore the deprecated stuff. Can't it?
      >
      > Not always, at least not in my experience. :) I have found that
      > while breaking changes don't happen often, they *do* happen. Add to
      > this the possibility of different types of API consumers that start
      > off implementing your APIs the same way, but slowly (or rapidly!)
      > diverge over time. In some cases new resources (and thus new URIs)
      > will suffice, but in other cases what we are doing is very
      > specifically *evolving* the resource representation to accommodate
      > new requirements.
      >

      Then design for graceful degradation (or progressive enhancement) using
      client-side feature detection. Just saying there's no need to swing
      the conneg hammer to turn this particular screw.

      -Eric


    • Eric J. Bowman
      ... From hypertext. NOT from knowledge of resource type coupled via custom media type (vnd.order*). It seems like you re asking how to inform the client that
      Message 2 of 13 , Jan 8, 2013
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        Jørn Wildt wrote:
        >
        > My question is though - from where does the client get the
        > information it needs to extract, say, the order sum, delivery
        > address, order lines quantity and so on from the generic payload?
        >

        From hypertext. NOT from knowledge of resource type coupled via custom
        media type (vnd.order*). It seems like you're asking how to inform the
        client that this resource type is "order" -- which is simply not
        relevant to REST, where the only client knowledge required is of the
        *interface* to the resource, not the *meaning* of the resource.

        If my order-form <table> has a <th>quantity</th> column, then the
        quantity of any item ID is the corresponding <td>, i.e. you code the
        client with the XPATH expression which yields the answer you're looking
        for, assuming XHTML. If Xforms is used, this XPATH expression is right
        there in the form control binding, i.e. a self-documenting API.

        >
        > Actually, I would argue that a generic media type is more "typed"
        > than a domain specific media type. Okay, that sounds like rubbish, I
        > know - but let me explain:
        >
        > - as I stated above, a domain specific media type lets you derived
        > useful information from a resource without making any assumptions
        > about its underlying type.
        >

        I might agree with that, if generic media types didn't have
        extensibility mechanisms like, oh I dunno, link relations... AFAIC,
        domain-specific media types _are_ assumptions about the underlying
        type, i.e. an "order" is too unique a snowflake to express generically;
        what other conclusion *can* be drawn about how vnd.order* types are
        used in practice?

        >
        > - whereas a generic media type forces the client to magically assume
        > the resource has a specific type with specific properties you can
        > extract from the generic payload.
        >

        OK, this is where these discussions always lose me. A browser needs to
        know *nothing* about resource type, to allow a user to place an order
        from, like, a bazillion different websites. In REST, the client just
        needs to know how to manipulate generic hypertext controls. What is it
        about order processing that makes folks think the client needs to know
        that it's processing an order?

        -Eric

        Now playing, ***** stars for sure:
        http://archive.org/details/gd1970-11-06.aud.evans.3338.sbefail.shnf
      • Jørn Wildt
        ... This is my mistake, using the term client wrongly, sorry. According to the dissertation, a client is the library that connects to the web - and the thing
        Message 3 of 13 , Jan 8, 2013
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          > the client just
          > needs to know how to manipulate generic hypertext controls.
          > What is it
          > about order processing that makes folks think the client needs to know
          > that it's processing an order?

          This is my mistake, using the term "client" wrongly, sorry. According to the dissertation, a client is the library that connects to the web - and the thing driving the client is the user agent.

          And that sort of explains it all ... yes, the client "just needs to know how to manipulate generic hypertext controls.". Let me rephrase my question:

          > > from where does the ***user agent*** get the
          > > information it needs to extract, say, the order sum, delivery
          > > address, order lines quantity and so on from the generic payload?

          But now we are are out of REST country. This is more about application specific details. REST is concerned with the interaction between the client and the server - it does not provide any guidance on how to implement the user agent.

          Thanks for your feedback - I had actually prepared two long e-mails before I got to this point, so you certainly got me thinking (as has happened before when reading your replies :-)

          /Jørn

        • Keith Hassen
          First of all, thanks for the replies and perspective on REST. Backing up to the original question it seems like your particular take on it is that there s no
          Message 4 of 13 , Jan 8, 2013
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            First of all, thanks for the replies and perspective on REST.

            Backing up to the original question it seems like your particular take on it is that there's no need to version URIs or the resource representations provided that a cautious approach is taken to resource evolution.

            To be honest I'm still not sure how this would be handled if two clients diverge in their understanding of the same resource.  Perhaps that divergence leads us to different concepts and thus different resources and I've just been thinking about the problem incorrectly. :)

            Back to your most recent response...

            This seems to be a recommended practice that I've encountered in
            reading about RESTful implementations.


            Perfectly good HTTP API practice; REST, not so much.  The point of
            media types is to provide loose coupling based on shared understanding
            of how the representation is to be decoded.  

            I think I'm actually talking about loose coupling, but this area seems a little fuzzy.  Comprehensive coverage of media type seems to be missing from Fielding's dissertation.  The media type I specified was definitely too specific (vnd.example.com might be a better choice rather than vnd.example.com.order, the latter effectively binding the media type to the representation and starts smelling like RPC modelling rather than the loose coupling promoted by the RESTful style).

            So how tight should this coupling be?  And what about versioning?  I'm tempted to say that if versioning is required, do so in the URi rather than the media type.  The media type seems to be more of an assertion/expectation regarding application context rather than a specification of the resource.

            It's amazing (or maybe not at all amazing) how divided this subject appears to be on the web.  This: http://www.informit.com/articles/article.aspx?p=1566460 is a pretty typical example.

            Thanks again for the feedback.

            K


            On 2013-01-08, at 12:57 AM, Eric J. Bowman wrote:

            Keith Hassen wrote:

            This seems to be a recommended practice that I've encountered in
            reading about RESTful implementations.


            Perfectly good HTTP API practice; REST, not so much.  The point of
            media types is to provide loose coupling based on shared understanding
            of how the representation is to be decoded.  Not tight coupling based
            on interpreting the content of the representation.  I've never seen an
            order API that couldn't be represented as HTML; despite any number of
            variations on this theme (or versions within), the media type doesn't
            vary between more order APIs on the Web than I can count.

            REST has no notion of resource "type" let alone of exposing same on the
            wire, so I've never understood why this solution is so prevalent in REST
            discussions.  Just media types, which are agnostic as to what "type" of
            resource is being represented (with rare exceptions as parameters).


            Anything else is up to the user-agent to work out.  This is actually
            easier for m2m than h2m; a v1 m2m client should already ignore any
            hypertext it doesn't understand, rolling out v2 can then re-use any
            non-deprecated bits of the API already present plus the new stuff,
            and ignore the deprecated stuff.  Can't it?

            Not always, at least not in my experience. :)  I have found that
            while breaking changes don't happen often, they *do* happen.  Add to
            this the possibility of different types of API consumers that start
            off implementing your APIs the same way, but slowly (or rapidly!)
            diverge over time.  In some cases new resources (and thus new URIs)
            will suffice, but in other cases what we are doing is very
            specifically *evolving* the resource representation to accommodate
            new requirements.


            Then design for graceful degradation (or progressive enhancement) using
            client-side feature detection.  Just saying there's no need to swing
            the conneg hammer to turn this particular screw.

            -Eric

          • Alexander Johannesen
            Hiya, ... I think there needs to be made some distinctions between a shared understanding of models and a shared understanding of a resource in that model.
            Message 5 of 13 , Jan 8, 2013
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              Hiya,

              To be honest I'm still not sure how this would be handled if two clients 
              > diverge in their understanding of the same resource.

              I think there needs to be made some distinctions between a shared understanding of models and a shared understanding of a resource in that model. Very often the model stays the same, and so the entities of that models shouldn't change, or at best, provide a framework for dealing with small changes (like backwards compatibility of models).

              What many confuse as an API change in terms of REST is really a model change, not a change to the language itself, which is REST. Even if the internal representation of a resource changes, its place in the model most often does not, and neither does the interface. True REST (TM) is representational through hyperlinks, and applications should be guided through the model rather than having knowledge of it, at least in an ideal world (but people tend to think the overhead is too great for the world of computing to deal with). There's no point in changing the interface nor the model if only representation changes. That is probably better dealt with through content negotiation, or, at worst, inside the representation level itself.

              What's being stressed here is that the separation of interface, model and internal representation most often do not require versioning, at least not on the model and interface level. So;

                 /user/johnson
                 /invoice/1023
                 /process/345/participants

              These are URIs that probably don't change with "versions". So why version them at this level? The model is probably fine, only semantics and representations change, so why not deal with it at that level instead?

              Hope that makes sense. :) If not, gives us some examples of a change from one version to the next, and let's talk about various ways to deal with the changes in question.


              Regards,

              Alex








              On Wed, Jan 9, 2013 at 4:25 PM, Keith Hassen <keith.hassen@...> wrote:
               

              First of all, thanks for the replies and perspective on REST.

              Backing up to the original question it seems like your particular take on it is that there's no need to version URIs or the resource representations provided that a cautious approach is taken to resource evolution.

              To be honest I'm still not sure how this would be handled if two clients diverge in their understanding of the same resource.  Perhaps that divergence leads us to different concepts and thus different resources and I've just been thinking about the problem incorrectly. :)

              Back to your most recent response...

              This seems to be a recommended practice that I've encountered in
              reading about RESTful implementations.


              Perfectly good HTTP API practice; REST, not so much.  The point of
              media types is to provide loose coupling based on shared understanding
              of how the representation is to be decoded.  

              I think I'm actually talking about loose coupling, but this area seems a little fuzzy.  Comprehensive coverage of media type seems to be missing from Fielding's dissertation.  The media type I specified was definitely too specific (vnd.example.com might be a better choice rather than vnd.example.com.order, the latter effectively binding the media type to the representation and starts smelling like RPC modelling rather than the loose coupling promoted by the RESTful style).

              So how tight should this coupling be?  And what about versioning?  I'm tempted to say that if versioning is required, do so in the URi rather than the media type.  The media type seems to be more of an assertion/expectation regarding application context rather than a specification of the resource.

              It's amazing (or maybe not at all amazing) how divided this subject appears to be on the web.  This: http://www.informit.com/articles/article.aspx?p=1566460 is a pretty typical example.

              Thanks again for the feedback.

              K


              On 2013-01-08, at 12:57 AM, Eric J. Bowman wrote:

              Keith Hassen wrote:

              This seems to be a recommended practice that I've encountered in
              reading about RESTful implementations.


              Perfectly good HTTP API practice; REST, not so much.  The point of
              media types is to provide loose coupling based on shared understanding
              of how the representation is to be decoded.  Not tight coupling based
              on interpreting the content of the representation.  I've never seen an
              order API that couldn't be represented as HTML; despite any number of
              variations on this theme (or versions within), the media type doesn't
              vary between more order APIs on the Web than I can count.

              REST has no notion of resource "type" let alone of exposing same on the
              wire, so I've never understood why this solution is so prevalent in REST
              discussions.  Just media types, which are agnostic as to what "type" of
              resource is being represented (with rare exceptions as parameters).


              Anything else is up to the user-agent to work out.  This is actually
              easier for m2m than h2m; a v1 m2m client should already ignore any
              hypertext it doesn't understand, rolling out v2 can then re-use any
              non-deprecated bits of the API already present plus the new stuff,
              and ignore the deprecated stuff.  Can't it?

              Not always, at least not in my experience. :)  I have found that
              while breaking changes don't happen often, they *do* happen.  Add to
              this the possibility of different types of API consumers that start
              off implementing your APIs the same way, but slowly (or rapidly!)
              diverge over time.  In some cases new resources (and thus new URIs)
              will suffice, but in other cases what we are doing is very
              specifically *evolving* the resource representation to accommodate
              new requirements.


              Then design for graceful degradation (or progressive enhancement) using
              client-side feature detection.  Just saying there's no need to swing
              the conneg hammer to turn this particular screw.

              -Eric




              --
               Project Wrangler, SOA, Information Alchemist, UX, RESTafarian, Topic Maps
              --- http://shelter.nu/blog/ ----------------------------------------------
              ------------------ http://www.google.com/profiles/alexander.johannesen ---
            • Jørn Wildt
              If you work with generic media types like plain XML, JSON and(X)HTML (and others) then there is absolutely no reason to version those through the content-type
              Message 6 of 13 , Jan 8, 2013
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                If you work with generic media types like plain XML, JSON and(X)HTML (and others) then there is absolutely no reason to version those through the content-type and content negotiation. I mean, when did the processing rules for XML and JSON last change? Oh, well, the rules for HTML does change - but this is not handled through the media type, but rather through a versioning mechanism inside the payload (the DOCTYPE).

                "Unfortunately" there is a lot of domain specific media types out there - and since domains changes regularly over time, well, you start thinking that versioning must be done through the media type. And this is where things start to break down. If you avoid domain specific media types (like vnd.order+xml and so on), then you can avoid versioning through the media type.

                But, of course, your application still evolves over time. Then what?

                Try these guidelines (and check also Nottingham's blog http://www.mnot.net/blog/2011/10/25/web_api_versioning_smackdown):

                - Do not expose your internal business objects directly on the wire (for instance by JSON serializing them directly). Do wrap your business objects in a (more) stable external format, such that you are free to change the underlying business objects without the public facing format changing too. Keep that transformation in sync with your internal changes.

                - Don't make breaking changes to your public format.

                - Add versioning information inside the payload.

                - If you have to introduce breaking changes then create a new resource for the new version.

                - Put links to never versions of the resource into the original resource (for instance using the Link header). Never clients can look for these links and move on to the new and improved version.

                - Introduce new links, in addition to the old ones, where other resources are linking to your resource (if possible), using new link relation names or similar mechanism. Newer clients will know which of the links to follow - older clients will continue with the link that they have always used.

                - URLs are not identities for your business data - they are identities for various representations of your business data. I mean, api-v1.example.com/person/1234 is obviously not the same resource as api-v2.example.com/person/1234 - but it can certainly represent exactly the same piece of business data.

                (and, yes, this is quite the opposite of what I argued in my first post in this thread)

                /Jørn


              • Keith Hassen
                Yeah we definitely make the distinction between the internal representation and the representation exposed to clients. The changes I m thinking about are
                Message 7 of 13 , Jan 9, 2013
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                  Yeah we definitely make the distinction between the internal representation and the representation exposed to clients.  The changes I'm thinking about are those dictated by consumers, so in my case I'm thinking of a scenario where two consumers start with the same requirements for a representation of a resource (the thing that is returned to the client when issuing a GET on a certain resource), but then diverge over time.

                  I'm actually having a hard time thinking of a *breaking* change for consumers that can't be accommodated by some kind of additive transformation to the original.  In the worst case, you may have data duplication, for example a user might start off being:

                  { name:"John Smith", address:"123 nowhere ave." }

                  but due to changing requirements needs to change to:

                  { name:"John Smith", address:{street_address:"123 nowhere ave.", postal:"12345", phone:"555-5555"}}

                  In which case the new "address" representation could be added to the original representation using a different name:

                  { name:"John Smith", address:"123 nowhere ave.", full_address:{...}}

                  This allows for compatibility between old and new client requirements.


                  On 2013-01-09, at 12:48 AM, Alexander Johannesen wrote:

                  Hiya,

                  To be honest I'm still not sure how this would be handled if two clients 
                  > diverge in their understanding of the same resource.

                  I think there needs to be made some distinctions between a shared understanding of models and a shared understanding of a resource in that model. Very often the model stays the same, and so the entities of that models shouldn't change, or at best, provide a framework for dealing with small changes (like backwards compatibility of models).

                  What many confuse as an API change in terms of REST is really a model change, not a change to the language itself, which is REST. Even if the internal representation of a resource changes, its place in the model most often does not, and neither does the interface. True REST (TM) is representational through hyperlinks, and applications should be guided through the model rather than having knowledge of it, at least in an ideal world (but people tend to think the overhead is too great for the world of computing to deal with). There's no point in changing the interface nor the model if only representation changes. That is probably better dealt with through content negotiation, or, at worst, inside the representation level itself.

                  What's being stressed here is that the separation of interface, model and internal representation most often do not require versioning, at least not on the model and interface level. So;

                     /user/johnson
                     /invoice/1023
                     /process/345/participants

                  These are URIs that probably don't change with "versions". So why version them at this level? The model is probably fine, only semantics and representations change, so why not deal with it at that level instead?

                  Hope that makes sense. :) If not, gives us some examples of a change from one version to the next, and let's talk about various ways to deal with the changes in question.


                  Regards,

                  Alex








                  On Wed, Jan 9, 2013 at 4:25 PM, Keith Hassen <keith.hassen@...> wrote:
                   

                  First of all, thanks for the replies and perspective on REST.

                  Backing up to the original question it seems like your particular take on it is that there's no need to version URIs or the resource representations provided that a cautious approach is taken to resource evolution.

                  To be honest I'm still not sure how this would be handled if two clients diverge in their understanding of the same resource.  Perhaps that divergence leads us to different concepts and thus different resources and I've just been thinking about the problem incorrectly. :)

                  Back to your most recent response...

                  This seems to be a recommended practice that I've encountered in
                  reading about RESTful implementations.


                  Perfectly good HTTP API practice; REST, not so much.  The point of
                  media types is to provide loose coupling based on shared understanding
                  of how the representation is to be decoded.  

                  I think I'm actually talking about loose coupling, but this area seems a little fuzzy.  Comprehensive coverage of media type seems to be missing from Fielding's dissertation.  The media type I specified was definitely too specific (vnd.example.com might be a better choice rather than vnd.example.com.order, the latter effectively binding the media type to the representation and starts smelling like RPC modelling rather than the loose coupling promoted by the RESTful style).

                  So how tight should this coupling be?  And what about versioning?  I'm tempted to say that if versioning is required, do so in the URi rather than the media type.  The media type seems to be more of an assertion/expectation regarding application context rather than a specification of the resource.

                  It's amazing (or maybe not at all amazing) how divided this subject appears to be on the web.  This: http://www.informit.com/articles/article.aspx?p=1566460 is a pretty typical example.

                  Thanks again for the feedback.

                  K


                  On 2013-01-08, at 12:57 AM, Eric J. Bowman wrote:

                  Keith Hassen wrote:

                  This seems to be a recommended practice that I've encountered in
                  reading about RESTful implementations.


                  Perfectly good HTTP API practice; REST, not so much.  The point of
                  media types is to provide loose coupling based on shared understanding
                  of how the representation is to be decoded.  Not tight coupling based
                  on interpreting the content of the representation.  I've never seen an
                  order API that couldn't be represented as HTML; despite any number of
                  variations on this theme (or versions within), the media type doesn't
                  vary between more order APIs on the Web than I can count.

                  REST has no notion of resource "type" let alone of exposing same on the
                  wire, so I've never understood why this solution is so prevalent in REST
                  discussions.  Just media types, which are agnostic as to what "type" of
                  resource is being represented (with rare exceptions as parameters).


                  Anything else is up to the user-agent to work out.  This is actually
                  easier for m2m than h2m; a v1 m2m client should already ignore any
                  hypertext it doesn't understand, rolling out v2 can then re-use any
                  non-deprecated bits of the API already present plus the new stuff,
                  and ignore the deprecated stuff.  Can't it?

                  Not always, at least not in my experience. :)  I have found that
                  while breaking changes don't happen often, they *do* happen.  Add to
                  this the possibility of different types of API consumers that start
                  off implementing your APIs the same way, but slowly (or rapidly!)
                  diverge over time.  In some cases new resources (and thus new URIs)
                  will suffice, but in other cases what we are doing is very
                  specifically *evolving* the resource representation to accommodate
                  new requirements.


                  Then design for graceful degradation (or progressive enhancement) using
                  client-side feature detection.  Just saying there's no need to swing
                  the conneg hammer to turn this particular screw.

                  -Eric





                  --
                   Project Wrangler, SOA, Information Alchemist, UX, RESTafarian, Topic Maps
                  --- http://shelter.nu/blog/ ----------------------------------------------
                  ------------------ http://www.google.com/profiles/alexander.johannesen ---

                • Keith Hassen
                  Thanks, the link below is interesting. I think the rules you listed below are good ones and generally make sense. Maybe it is *almost* always true that we
                  Message 8 of 13 , Jan 9, 2013
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                    Thanks, the link below is interesting.  I think the rules you listed below are good ones and generally make sense.  Maybe it is *almost* always true that we can find a way to either a) make the change non-breaking or b) create a new resource.  In the rare cases where it must be done, then specifying a version in the URI might be necessary, but should be limited to very exceptional cases that I'm having a hard time devising. :)

                    K


                    On 2013-01-09, at 1:45 AM, Jørn Wildt wrote:

                    If you work with generic media types like plain XML, JSON and(X)HTML (and others) then there is absolutely no reason to version those through the content-type and content negotiation. I mean, when did the processing rules for XML and JSON last change? Oh, well, the rules for HTML does change - but this is not handled through the media type, but rather through a versioning mechanism inside the payload (the DOCTYPE).

                    "Unfortunately" there is a lot of domain specific media types out there - and since domains changes regularly over time, well, you start thinking that versioning must be done through the media type. And this is where things start to break down. If you avoid domain specific media types (like vnd.order+xml and so on), then you can avoid versioning through the media type.

                    But, of course, your application still evolves over time. Then what?

                    Try these guidelines (and check also Nottingham's blog http://www.mnot.net/blog/2011/10/25/web_api_versioning_smackdown):

                    - Do not expose your internal business objects directly on the wire (for instance by JSON serializing them directly). Do wrap your business objects in a (more) stable external format, such that you are free to change the underlying business objects without the public facing format changing too. Keep that transformation in sync with your internal changes.

                    - Don't make breaking changes to your public format.

                    - Add versioning information inside the payload.

                    - If you have to introduce breaking changes then create a new resource for the new version.

                    - Put links to never versions of the resource into the original resource (for instance using the Link header). Never clients can look for these links and move on to the new and improved version.

                    - Introduce new links, in addition to the old ones, where other resources are linking to your resource (if possible), using new link relation names or similar mechanism. Newer clients will know which of the links to follow - older clients will continue with the link that they have always used.

                    - URLs are not identities for your business data - they are identities for various representations of your business data. I mean, api-v1.example.com/person/1234 is obviously not the same resource as api-v2.example.com/person/1234 - but it can certainly represent exactly the same piece of business data.

                    (and, yes, this is quite the opposite of what I argued in my first post in this thread)

                    /Jørn



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