API Versioning within web applications

Question

I am currently in the process of designing a versioned API for a new website. I understand how to namespace the routes but I am stuck on the best way to implement versioned methods within a model.

The code samples below are using the rails framework but the principle of the matter should be consistent between most web frameworks.

The routes currently look something like:

MyApp::Application.routes.draw do
  namespace :api do
    namespace :v1 do
      resources :products, :only => [:index, :show]
    end
  end
end

And the controller:

class Api::V1::ProductsController < V1Controller
  respond_to :json, :xml

  def index
    respond_with @products = Product.scoped
  end

  def show
    respond_with @product = Product.find(params[:id])
  end
end

So obviously we're just exposing the attributes available on Product here, this solution works great if you're only going to have one version of the API. What happens when you want to release V2 and V2 needs to reimplement the way that Product's name is displayed (while maintaining backwards compatibility with V1 - at least in the short term)?

As far as I see it you have a couple of options...

1. Drop support for V1 immediately and deal with the fallout (worst possible solution)
2. You start overriding the to_[format] methods (I am pretty sure you do this with as_[format] but that's beside the point) to include a new attribute... name_2 - this seems equally dumb
3. Implement some kind of proxy class that is responsible for exposing only the methods that we're after
4. Let views handle creating some kind of hash that the versioned controllers and call to[format] on...

Three and Four are the only ones that I can actually think makes any kind of sense... Three would look something like:

# model
class Api::V1::Product < Struct.new(:product)
  def to_json
    attributes.to_json
  end

  def to_xml
    attributes.to_xml
  end

private
  def attributes
    {:name => product.name} # add all the attributes you want to expose
  end
end

# Controller
class Api::V1::ProductsController < V1Controller
  respond_to :json, :xml

  def show
    respond_with @product = Api::V1::Product.new(Product.find(params[:id]))
  end
end

What have other people done in the past?

Answer 1

Instead of one app serving V1 and V2 and V... you deploy one app for each version. One app is going to answer api.domain.com/v1, then another app is going to answer api.domain.com/v2 and so on.

That is how service oriented applications are best organised, each service should be isolated, an independent deployment.

Serving all versions from a single app defeats the purpose of service oriented design, since each time you make a change in one service you will need to test and deploy for all.