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June 19, 2010

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Simple RabbitMQ Subscriber Using Bunny

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This is a very simple example of an RabbitMQ subscriber in Ruby using the bunny gem. It uses the dead_letter pattern to resend failures to another queue if processing fails. The original message is ack'd so it does not get reprocessed. Something else probably needs to be monitoring the messages on the dead_letter queue.

Signal.trap('INT') { @subscriber.shutdown }
Signal.trap('TERM') { @subscriber.shutdown }

class Subscriber
  def initialize
    @bunny = Bunny.new(...)
    @queue = ...
    @dead_letter = ...
  end
  
  def run
    @queue.subscribe(:ack => true) do |msg|
      payload = JSON.parse(msg[:payload])
      begin
        do_something_with_the_payload(payload)
      rescue StandardError, Timeout::Error => error
        routing_key = msg[:delivery_details][:routing_key]
        @dead_letter.publish(payload, :key => routing_key)
      end
    end
  end

  def shutdown
    @queue.unsubscribe rescue nil
    @bunny.stop rescue nil
    exit
  end
end

@subscriber = Subscriber.new
@subscriber.run

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June 16, 2010

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The Self-Pipe Trick

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Unicorn is Unix mentions the use of the self-pipe trick, mixing select with Unix signals. D. J. Bernstein, the creator of daemontools states he came up with it here. The problem is that a signal handler might be called while select is starting up. The solution is to select for read on a pipe and write to the pipe from within the signal handler.

  # self_pipe example in Ruby
  SELF_PIPE = IO.pipe
  ['INT', 'TERM'].each { |sig| trap(sig) {shutdown}}

  def shutdown
    SELF_PIPE[1].puts '.'
  end

  selected = select(SELF_PIPE, nil, nil, nil)
  puts "SELF_PIPE success" if selected[0][0] == SELF_PIPE[0]

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June 13, 2010

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Lorem Ipsum

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