Earlier this month I gave a presentation at ComoRichWeb on RabbitMQ and one question from an attendee was “Is it possible to publish a message to be consumed at a later date?” I answered that it wasn’t possible to the best of my knowledge, but that there might be some hack to accomplish it. Well, this evening while trying to figure out how to use a push vs. polling model for timed notifications I discovered a clever hack using temporary queues, x-message-ttl and dead letter exchanges.

The main idea behind this is utilizing a new feature available in 2.8.0, dead-letter exchanges. This AMQP extension allows you to specify an exchange on a queue that messages should be published to when a message either expires or is rejected with requeue set to false.

With this in mind, we can simply create a queue for messages we want to be delivered later with an x-message-ttl set to the duration we want to wait before it is delivered. And to ensure the message is transferred to another queue we simply define the x-dead-letter-exchange to an exchange we created (in this case I’ll call it immediate) and bind a queue to it (the “right.now.queue”).

In coffeescript with node-amqp this looks like this:

amqp = require 'amqp'
conn = amqp.createConnection()
  
key = "send.later.#{new Date().getTime()}"
conn.on 'ready', ->
  conn.queue key, {
    arguments:{
      "x-dead-letter-exchange":"immediate"
    , "x-message-ttl": 5000
    }
  }

Next I define the immediate exchange, bind a queue to it and subscribe.

conn.exchange 'immediate'

  conn.queue 'right.now.queue', {autoDelete: false, durable: true}, (q) ->
    q.bind('immediate', 'right.now.queue')
    q.subscribe (msg, headers, deliveryInfo) ->
      console.log msg
      console.log headers

Finally, after defining the queue I created earlier we want publish a message on it. So to revisit the earlier queue definition we add a publish call to publish directly to the queue (using the default exchange).

conn.on 'ready', ->
  conn.queue key, {
    arguments:{
      "x-dead-letter-exchange":"immediate"
    , "x-message-ttl": 5000
    }
  }, ->
    conn.publish key, {v:1}, {contentType:'application/json'}

The result of running this is we’ll see a 5 second wait and then the message content and headers get dumped to the console. Since the queue is only used temporarily in this scenario I also set the x-expires attribute of the queue to expire in a reasonable amount of time after the message expires. This makes sure we don’t wind up with a ton of unused queues just sitting around.

Here’s the result of this exercise in its entirety.

amqp = require 'amqp'
events = require 'events'
em = new events.EventEmitter()
conn = amqp.createConnection()
  
key = "send.later.#{new Date().getTime()}"
conn.on 'ready', ->
  conn.queue key, {
    arguments:{
      "x-dead-letter-exchange":"immediate"
    , "x-message-ttl": 5000
    , "x-expires": 6000
    }
  }, ->
    conn.publish key, {v:1}, {contentType:'application/json'}
  
  conn.exchange 'immediate'

  conn.queue 'right.now.queue', {
      autoDelete: false
    , durable: true
  }, (q) ->
    q.bind('immediate', 'right.now.queue')
    q.subscribe (msg, headers, deliveryInfo) ->
      console.log msg
      console.log headers

You can get this exercise in full on github.

This is pretty interesting and I plan to experiment further with utilizing this in one of my production node.js applications that use interval based polling to trigger scheduled events.