Question

Does "run" block executed as a whole in EM (without a context switch)? And in this example, will be there a race condition in the if clause?

EventMachine.run {
  @current_value = 0
  EventMachine::WebSocket.start(:host => "0.0.0.0", :port => 8080, :debug => true) do |ws|
    ws.onopen {
      @current_value += 1
      if @current_value >= 4 # Race condition?
        # Code Block
        @current_value = 0
      end

      ws.onmessage { |msg|
        # puts msg
      }

      ws.onclose {
        # puts "disconnected"
      }
    }  
  end
end

Answer 1

EventMachine, by default, is single threaded, so there really shouldn’t be any race conditions unless you’re introducing threads.

An event-loop model means you perform small, simple actions in rapid sequence instead of long, blocking methods that require their own threads. As such you should never have two pieces of code executing in parallel.

It is your responsibility to yield control to the event-loop frequently at points you define.