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19251Re: [rest-discuss] API versioning

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  • Keith Hassen
    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





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