Possible answer to SO question: http://stackoverflow.com/questions/19172707/ruby-event-machine-stop-or-kill-deffered-operation

emws.rb

require 'em-websocket'

EM.run do
  EM::WebSocket.start(:host => '0.0.0.0', :port => 8080) do |ws|


    q = EM::Queue.new
    l = Mutex.new

    ws.onmessage do |msg|

      op = proc do

        l.synchronize do
          q.push Thread.current
          q.size.times do
            q.pop { |t| t.kill }
          end
        end


        puts "Received #{msg}. will now do a looooong process"
        sleep 10 # Thread safe IO here that is safely killed
        true
      end

      callback = proc do |result|
        puts "Done! with #{msg}"
      end

      EM.defer(op, callback)
    end
  end
end