rabbitmq : dead letter exchange example with python/pika

new_task.py

#!/usr/bin/env python
# http://www.rabbitmq.com/tutorials/tutorial-two-python.html
import pika
import sys

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

message = ' '.join(sys.argv[1:]) or "Hello World!"
channel.basic_publish(exchange='',
                      routing_key='task_queue',
                      body=message,
)
print " [x] Sent %r" % (message,)
connection.close()

receive_dl.py

#!/usr/bin/env python
# http://www.rabbitmq.com/tutorials/tutorial-two-python.html
import pika

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

channel.exchange_declare(exchange='dlx',
                         type='direct')

result = channel.queue_declare(queue='dl')
queue_name = result.method.queue
channel.queue_bind(exchange='dlx',
                   routing_key='task_queue', # x-dead-letter-routing-key
                   queue=queue_name)

print ' [*] Waiting for dead-letters. To exit press CTRL+C'

def callback(ch, method, properties, body):
    print " [x] %r" % (properties,)
    print " [reason] : %s : %r" % (properties.headers['x-death'][0]['reason'], body)
    ch.basic_ack(delivery_tag = method.delivery_tag)

channel.basic_consume(callback,
                      queue='dl')

channel.start_consuming()

worker.py

#!/usr/bin/env python
# http://www.rabbitmq.com/tutorials/tutorial-two-python.html
import pika
import time
import random

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

channel.queue_declare(queue='task_queue',
  arguments={
  'x-message-ttl' : 1000,
  "x-dead-letter-exchange" : "dlx",
#  "x-dead-letter-routing-key" : "dl", # if not specified, queue's routing-key is used
}
)
print ' [*] Waiting for messages. To exit press CTRL+C'

def callback(ch, method, properties, body):
    print " [x] Received %r" % (body,)
    if random.random() < 0.5:
        ch.basic_ack(delivery_tag = method.delivery_tag)
        time.sleep(5)
        print " [x] Done"
    else:
        ch.basic_reject(delivery_tag = method.delivery_tag, requeue=False)
        print " [x] Rejected"

channel.basic_qos(prefetch_count=1)
channel.basic_consume(callback,
                      queue='task_queue')

channel.start_consuming()

I've run the above code and I faced a problem setting up the dlx exchange as DIRECT without uncommenting #file-worker-py-L15 and setting my routing-key on task-queue declaration. It seems that with DIRECT dead letter exchanges you must specify a dead letter routing key because the broker is not using the routing key of the message when published and dropping it silently. I don't know if it is a version issue. Mine is v3.1.3

As another approach, you can also specify the x-dead-letter-routing-key without the x-dead-letter-exchange when setting dead letter arguments for a queue to dead letter directly to a queue with name as set by x-dead-letter-routing-key.

@anabelengp faced the same problem. thank you to pointing, spent some time to figure it out.