async_sinatra_example_2.rb

require 'async_sinatra'

class App < Sinatra::Base
  register Sinatra::Async

  @next_id = 0
  def self.next_id
    @next_id += 1
  end

  @connected_clients = {}
  def self.connected_clients
    @connected_clients
  end

  aget '/' do
    timed_out = false

    # If you cluster, this needs to come from a global id provider or be a
    # uuid of some kind to avoid collisions.
    my_id = self.class.next_id

    # Add ourselves to the connection list
    self.class.connected_clients[my_id] = self

    cleanup = lambda do
      # Remove ourselves from the connection list
      self.class.connected_clients.delete my_id
    end

    timeout = EM::Timer.new(7) do
      timed_out = true

      # Tell the database we're no longer waiting
      db.remove_client my_id

      body { '{"state": "not_found"}' }

      cleanup.call
    end

    # Add this client to the list of waiting clients, n.b. if can be hard to
    # "match" clients accross a cluster, you'll need some kind of inter-server
    # protocol to pipe data between them if that's the plan.
    db.add_client my_id

    # Try to find a matching client:
    db.find_matching_clients my_id do |matching_client|

      # presumably, after we've matched up with someone, we don't want to be
      # matched ourselves. N.B. I have casually skirted around the concurrency
      # issue of finding a matching client /THEN/ removing ourselves, which
      # has bad concurrency implications, in that we could end up getting
      # paired with another client between finding one, and removing this
      # client. This should be avoided by clients doing one or the other, or
      # some kind of shared lock between trying to find a client or not. An
      # example would be to use a function that instead does a "find or add
      # this client" method that is atomic (e.g. in a single transaction).
      db.remove_client my_id

      other = self.class.connected_clients[matching_client]

      # here we do something with other...

      body { ... whatver ... }

      cleanup.call
    end

  end
end