How to create a delayed queue in RabbitMQ?

Question

What is the easiest way to create a delay (or parking) queue with Python, Pika and RabbitMQ? I have seen an similar questions, but none for Python.

I find this an useful idea when designing applications, as it allows us to throttle messages that needs to be re-queued again.

There are always the possibility that you will receive more messages than you can handle, maybe the HTTP server is slow, or the database is under too much stress.

I also found it very useful when something went wrong in scenarios where there is a zero tolerance to losing messages, and while re-queuing messages that could not be handled may solve that. It can also cause problems where the message will be queued over and over again. Potentially causing performance issues, and log spam.

Answer 1

I found this extremely useful when developing my applications. As it gives you an alternative to simply re-queuing your messages. This can easily reduce the complexity of your code, and is one of many powerful hidden features in RabbitMQ.

Steps

First we need to set up two basic channels, one for the main queue, and one for the delay queue. In my example at the end, I include a couple of additional flags that are not required, but makes the code more reliable; such as confirm delivery, delivery_mode and durable. You can find more information on these in the RabbitMQ manual.

After we have set up the channels we add a binding to the main channel that we can use to send messages from the delay channel to our main queue.

channel.queue_bind(exchange='amq.direct',
                   queue='hello')

Next we need to configure our delay channel to forward messages to the main queue once they have expired.

delay_channel.queue_declare(queue='hello_delay', durable=True,  arguments={
  'x-message-ttl' : 5000,
  'x-dead-letter-exchange' : 'amq.direct',
  'x-dead-letter-routing-key' : 'hello'
})

• x-message-ttl (Message - Time To Live)

  This is normally used to automatically remove old messages in the queue after a specific duration, but by adding two optional arguments we can change this behaviour, and instead have this parameter determine in milliseconds how long messages will stay in the delay queue.

• x-dead-letter-routing-key

  This variable allows us to transfer the message to a different queue once they have expired, instead of the default behaviour of removing it completely.

• x-dead-letter-exchange

  This variable determines which Exchange used to transfer the message from hello_delay to hello queue.

Publishing to the delay queue

When we are done setting up all the basic Pika parameters you simply send a message to the delay queue using basic publish.

delay_channel.basic_publish(exchange='',
                      routing_key='hello_delay',
                      body="test",
                      properties=pika.BasicProperties(delivery_mode=2))

Once you have executed the script you should see the following queues created in your RabbitMQ management module.

Example.

import pika

connection = pika.BlockingConnection(pika.ConnectionParameters(
               'localhost'))

# Create normal 'Hello World' type channel.
channel = connection.channel()
channel.confirm_delivery()
channel.queue_declare(queue='hello', durable=True)

# We need to bind this channel to an exchange, that will be used to transfer 
# messages from our delay queue.
channel.queue_bind(exchange='amq.direct',
                   queue='hello')

# Create our delay channel.
delay_channel = connection.channel()
delay_channel.confirm_delivery()

# This is where we declare the delay, and routing for our delay channel.
delay_channel.queue_declare(queue='hello_delay', durable=True,  arguments={
  'x-message-ttl' : 5000, # Delay until the message is transferred in milliseconds.
  'x-dead-letter-exchange' : 'amq.direct', # Exchange used to transfer the message from A to B.
  'x-dead-letter-routing-key' : 'hello' # Name of the queue we want the message transferred to.
})

delay_channel.basic_publish(exchange='',
                      routing_key='hello_delay',
                      body="test",
                      properties=pika.BasicProperties(delivery_mode=2))

print " [x] Sent"