Usage Examples¶

It requires an accessible RabbitMQ and a Python environment with the amqppy package installed.

Topic Publisher-Subscribers¶

This is one of the most common messaging pattern where the publisher publishes message to an AMQP exchange and the subscriber receives only the messages that are of interest. The subscriber’s interest is modeled by the Topic or in terms of AMQP by the rounting_key.

https://www.rabbitmq.com/img/tutorials/python-five.png

Image from RabbitMQ Topic tutorial.

Topic Subscriber¶

Firstly, we need to start the Topic Subscriber (also known as Consumer). In amqppy the class amqppy.consumer.Worker has this duty.

import amqppy
BROKER = 'amqp://guest:guest@localhost:5672//'

def on_topic_status(exchange, routing_key, headers, body):
    print('Received message from topic \'amqppy.publisher.topic.status\': {}'.format(body))

# subscribe to a topic: 'amqppy.publisher.topic.status'
worker = amqppy.Worker(BROKER)
worker.add_topic(exchange='amqppy.test',
                 routing_key='amqppy.publisher.topic.status',
                 on_topic_callback=on_topic_status)
# it will wait until worker is stopped or an uncaught exception
worker.run()

The subscriber worker will invoke the on_topic_callback every time a message is published with a topic that matches with the specified routing_key: ‘amqppy.publisher.topic.status’. Note that routing_key can contain wildcards therefore, one subscriber might be listening a set of Topics.

Once the topic subscriber is running we able to launch the publisher.

Topic Publisher¶

import amqppy
BROKER = 'amqp://guest:guest@localhost:5672//'

# publish my current status
amqppy.Topic(BROKER).publish(exchange='amqppy.test',
                             routing_key='amqppy.publisher.topic.status',
                             body='RUNNING')

The topic publisher will send a message to the AMQP exchange with the Topic routing_key: ‘amqppy.publisher.topic.status’, therefore, all the subscribed subscribers will receive the message unless they do not share the same queue. In case they share the same queue a round-robin dispatching policy would be applied among subscribers/consumers like happens in `work queues <https://www.rabbitmq.com/tutorials/tutorial-two-python.html>`_*.

RPC Request-Reply¶

This pattern is commonly known as Remote Procedure Call or RPC. And is widely used when we need to run a function request on a remote computer and wait for the result reply.

https://www.rabbitmq.com/img/tutorials/python-six.png

Image from RabbitMQ RPC tutorial

RPC Reply¶

An object of type amqppy.consumer.Worker listens incoming RPC requests and computes the RPC reply in the on_request_callback. In the example below, the RPC consumer listens on Request rounting_key:’amqppy.requester.rpc.division’ and the division would be returned as the RPC reply.

import amqppy
BROKER = 'amqp://guest:guest@localhost:5672//'

def on_rpc_request_division(exchange, routing_key, headers, body):
    args = json.loads(body)
    return args['dividend'] / args['divisor']

# subscribe to a rpc request: 'amqppy.requester.rpc.division'
worker = amqppy.Worker(BROKER)
worker.add_request(exchange='amqppy.test',
                   routing_key='amqppy.requester.rpc.division',
                   on_request_callback=on_rpc_request_division)
# it will wait until worker is stopped or an uncaught exception
worker.run()

RPC Request¶

The code below shows how to do a RPC Request using an instance of class amqppy.publisher.Rpc

import amqppy
BROKER = 'amqp://guest:guest@localhost:5672//'

# do a Rpc request 'amqppy.requester.rpc.division'
result = amqppy.Rpc(BROKER).request(exchange='amqppy.test',
                                    routing_key='amqppy.requester.rpc.division',
                                    body=json.dumps({'dividend': 3.23606797749979, 'divisor': 2.0}))
print('RPC result: {}.'.format(result))