RabbitMQ tutorial - Publish/Subscribe SUPPRESS-RHS
(using the Pika Python client)
What This Tutorial Focuses On
In the previous tutorial we created a work queue. The assumption behind a work queue is that each task is delivered to exactly one worker. In this part we’ll do something completely different – we’ll deliver a message to multiple consumers. This pattern is known as “publish/subscribe”.
To illustrate the pattern, we’re going to build a simple logging system. It will consist of two programs – the first will emit log messages and the second will receive and print them.
In our logging system every running copy of the receiver program will get the messages. That way we’ll be able to run one receiver and direct the logs to disk; and at the same time we’ll be able to run another receiver and see the logs on the screen.
Essentially, published log messages are going to be broadcast to all the receivers.
In previous parts of the tutorial we sent and received messages to and from a queue. Now it’s time to introduce the full messaging model in Rabbit.
Let’s quickly go over what we covered in the previous tutorials:
- A producer is a user application that sends messages.
- A queue is a buffer that stores messages.
- A consumer is a user application that receives messages.
The core idea in the messaging model in RabbitMQ is that the producer never sends any messages directly to a queue. Actually, quite often the producer doesn’t even know if a message will be delivered to any queue at all.
Instead, the producer can only send messages to an exchange. An exchange is a very simple thing. On one side it receives messages from producers and the other side it pushes them to queues. The exchange must know exactly what to do with a message it receives. Should it be appended to a particular queue? Should it be appended to many queues? Or should it get discarded. The rules for that are defined by the exchange type.
There are a few exchange types available:
fanout. We’ll focus on the last one – the fanout. Let’s create
an exchange of that type, and call it
The fanout exchange is very simple. As you can probably guess from the name, it just broadcasts all the messages it receives to all the queues it knows. And that’s exactly what we need for our logger.
To list the exchanges on the server you can run the ever useful
rabbitmqctl:sudo rabbitmqctl list_exchanges
In this list there will be some
amq.*exchanges and the default (unnamed) exchange. These are created by default, but it is unlikely you’ll need to use them at the moment.
The default exchange
In previous parts of the tutorial we knew nothing about exchanges, but still were able to send messages to queues. That was possible because we were using a default exchange, which we identify by the empty string (
Recall how we published a message before:channel.basic_publish(exchange='', routing_key='hello', body=message)
exchangeparameter is the name of the exchange. The empty string denotes the default or nameless exchange: messages are routed to the queue with the name specified by
routing_key, if it exists.
Now, we can publish to our named exchange instead:
channel.basic_publish(exchange='logs', routing_key='', body=message)
As you may remember previously we were using queues that had
specific names (remember
task_queue?). Being able to name
a queue was crucial for us – we needed to point the workers to the
same queue. Giving a queue a name is important when you
want to share the queue between producers and consumers.
But that’s not the case for our logger. We want to hear about all log messages, not just a subset of them. We’re also interested only in currently flowing messages not in the old ones. To solve that we need two things.
Firstly, whenever we connect to Rabbit we need a fresh, empty queue. To
do it we could create a queue with a random name, or, even better -
let the server choose a random queue name for us. We can do this by
queue parameter to
result = channel.queue_declare(queue='')
At this point
result.method.queue contains a random queue name. For example
it may look like
Secondly, once the consumer connection is closed, the queue should be
deleted. There’s an
exclusive flag for that:
result = channel.queue_declare(queue='', exclusive=True)
You can learn more about the
exclusive flag and other queue
properties in the guide on queues.
We’ve already created a fanout exchange and a queue. Now we need to tell the exchange to send messages to our queue. That relationship between exchange and a queue is called a binding.
From now on the
logs exchange will append messages to our queue.
You can list existing bindings using, you guessed it,rabbitmqctl list_bindings
Putting it all together
The producer program, which emits log messages, doesn’t look much
different from the previous tutorial. The most important change is that
we now want to publish messages to our
logs exchange instead of the
nameless one. We need to supply a
routing_key when sending, but its
value is ignored for
#!/usr/bin/env python import pika import sys connection = pika.BlockingConnection( pika.ConnectionParameters(host='localhost')) channel = connection.channel() channel.exchange_declare(exchange='logs', exchange_type='fanout') message = ' '.join(sys.argv[1:]) or "info: Hello World!" channel.basic_publish(exchange='logs', routing_key='', body=message) print(" [x] Sent %r" % message) connection.close()
As you see, after establishing the connection we declared the exchange. This step is necessary as publishing to a non-existing exchange is forbidden.
The messages will be lost if no queue is bound to the exchange yet, but that’s okay for us; if no consumer is listening yet we can safely discard the message.
#!/usr/bin/env python import pika connection = pika.BlockingConnection( pika.ConnectionParameters(host='localhost')) channel = connection.channel() channel.exchange_declare(exchange='logs', exchange_type='fanout') result = channel.queue_declare(queue='', exclusive=True) queue_name = result.method.queue channel.queue_bind(exchange='logs', queue=queue_name) print(' [*] Waiting for logs. To exit press CTRL+C') def callback(ch, method, properties, body): print(" [x] %r" % body) channel.basic_consume( queue=queue_name, on_message_callback=callback, auto_ack=True) channel.start_consuming()
We’re done. If you want to save logs to a file, just open a console and type:
python receive_logs.py > logs_from_rabbit.log
If you wish to see the logs on your screen, spawn a new terminal and run:
And of course, to emit logs type:
rabbitmqctl list_bindings you can verify that the code actually
creates bindings and queues as we want. With two
programs running you should see something like:
sudo rabbitmqctl list_bindings # => Listing bindings ... # => logs exchange amq.gen-JzTY20BRgKO-HjmUJj0wLg queue  # => logs exchange amq.gen-vso0PVvyiRIL2WoV3i48Yg queue  # => ...done.
The interpretation of the result is straightforward: data from
logs goes to two queues with server-assigned names. And
that’s exactly what we intended.
To find out how to listen for a subset of messages, let’s move on to tutorial 4