API version checking using Sinatra's before filter; supports nested resources

gistfile1.rb

require 'sinatra'

# Set the version of the API being run here
#
MAJOR_VERSION = 1
MINOR_VERSION = 0
VERSION_REGEX = %r{/api/v(\d)\.(\d)}

helpers do
  def version_compatible?(nums)
    return MAJOR_VERSION == nums[0].to_i && MINOR_VERSION >= nums[1].to_i
  end
end

# Enforce compatibility before the call. Rewrite the
# URL in the request to remove the API versioning stuff
#
before VERSION_REGEX do
  if version_compatible?(params[:captures])
    request.path_info = request.path_info.match(VERSION_REGEX).post_match
  else
    halt 400, "Version not compatible with this server"
  end
end

before !VERSION_REGEX do
    halt 404, "Resource not found"
end

# Reach this route using
#   http://localhost:4567/api/vX.Y/hello
#
get '/hello' do
  "Hello there, compatible client."
end

Oh, it makes sense. That's nice, thanks.

Question, how do you deal with code / responses that change between versions - it's not quite clear to me how to handle that scenario?

If you are running multiple versions of your API, the important thing is to ensure that you segregate your code -- avoid having a situation where say a method does different things depending on which version of the API it is attending to. In a production environment I would run a couple of different servers and route to one or the other depending on the version of the API that is being requested.

Short summary - this Sinatra filtering will only work to verify what it's getting is version compatible, it won't solve the is issue of maintaining multiple API versions.