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RabbitMQ tutorial - "Hello world!"

Introduction​

info

Prerequisites​

This tutorial assumes RabbitMQ is installed and running on localhost on the standard port (5672). In case you use a different host, port or credentials, connections settings would require adjusting.

Where to get help​

If you're having trouble going through this tutorial you can contact us through GitHub Discussions or RabbitMQ community Discord.

RabbitMQ is a message broker: it accepts and forwards messages. You can think about it as a post office: when you put the mail that you want posting in a post box, you can be sure that the letter carrier will eventually deliver the mail to your recipient. In this analogy, RabbitMQ is a post box, a post office, and a letter carrier.

The major difference between RabbitMQ and the post office is that it doesn't deal with paper, instead it accepts, stores, and forwards binary blobs of data ‒ messages.

RabbitMQ, and messaging in general, uses some jargon.

  • Producing means nothing more than sending. A program that sends messages is a producer :

  • A queue is the name for the post box in RabbitMQ. Although messages flow through RabbitMQ and your applications, they can only be stored inside a queue. A queue is only bound by the host's memory & disk limits, it's essentially a large message buffer.

    Many producers can send messages that go to one queue, and many consumers can try to receive data from one queue.

    This is how we represent a queue:

  • Consuming has a similar meaning to receiving. A consumer is a program that mostly waits to receive messages:

Note that the producer, consumer, and broker do not have to reside on the same host; indeed in most applications they don't. An application can be both a producer and consumer, too.

Hello World!​

(using the Pika Python client)​

In this part of the tutorial we'll write two small programs in Python; a producer (sender) that sends a single message, and a consumer (receiver) that receives messages and prints them out. It's the "Hello World" of messaging.

In the diagram below, "P" is our producer and "C" is our consumer. The box in the middle is a queue - a message buffer that RabbitMQ keeps on behalf of the consumer.

Our overall design will look like:

Producer sends messages to the "hello" queue. The consumer receives messages from that queue.

RabbitMQ libraries​

RabbitMQ speaks multiple protocols. This tutorial uses AMQP 0-9-1, which is an open, general-purpose protocol for messaging. There are a number of clients for RabbitMQ in many different languages. In this tutorial series we're going to use Pika 1.0.0, which is the Python client recommended by the RabbitMQ team. To install it you can use the pip package management tool:

python -m pip install pika --upgrade

Now we have Pika installed, we can write some code.

Sending​

Our first program send.py will send a single message to the queue. The first thing we need to do is to establish a connection with RabbitMQ server.

#!/usr/bin/env python
import pika

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

We're connected now, to a broker on the local machine - hence the localhost. If we wanted to connect to a broker on a different machine we'd simply specify its name or IP address here.

Next, before sending we need to make sure the recipient queue exists. If we send a message to non-existing location, RabbitMQ will just drop the message. Let's create a hello queue to which the message will be delivered:

channel.queue_declare(queue='hello')

At this point we're ready to send a message. Our first message will just contain a string Hello World! and we want to send it to our hello queue.

In RabbitMQ a message can never be sent directly to the queue, it always needs to go through an exchange. But let's not get dragged down by the details ‒ you can read more about exchanges in the third part of this tutorial. All we need to know now is how to use a default exchange identified by an empty string. This exchange is special ‒ it allows us to specify exactly to which queue the message should go. The queue name needs to be specified in the routing_key parameter:

channel.basic_publish(exchange='',
                      routing_key='hello',
                      body='Hello World!')
print(" [x] Sent 'Hello World!'")

Before exiting the program we need to make sure the network buffers were flushed and our message was actually delivered to RabbitMQ. We can do it by gently closing the connection.

connection.close()

Sending doesn't work!​

If this is your first time using RabbitMQ and you don't see the "Sent" message then you may be left scratching your head wondering what could be wrong. Maybe the broker was started without enough free disk space (by default it needs at least 50 MB free) and is therefore refusing to accept messages. Check the broker log file to see if there is a resource alarm logged and reduce the free disk space threshold if necessary. The Configuration guide will show you how to set disk_free_limit.

Receiving​

Our second program receive.py will receive messages from the queue and print them on the screen.

Again, first we need to connect to RabbitMQ server. The code responsible for connecting to Rabbit is the same as previously.

The next step, just like before, is to make sure that the queue exists. Creating a queue using queue_declare is idempotent ‒ we can run the command as many times as we like, and only one will be created.

channel.queue_declare(queue='hello')

You may ask why we declare the queue again ‒ we have already declared it in our previous code. We could avoid that if we were sure that the queue already exists. For example if send.py program was run before. But we're not yet sure which program to run first. In such cases it's a good practice to repeat declaring the queue in both programs.

Listing queues​

You may wish to see what queues RabbitMQ has and how many messages are in them. You can do it (as a privileged user) using the rabbitmqctl tool:

sudo rabbitmqctl list_queues

On Windows, omit the sudo:

rabbitmqctl.bat list_queues

Receiving messages from the queue is more complex. It works by subscribing a callback function to a queue. Whenever we receive a message, this callback function is called by the Pika library. In our case this function will print on the screen the contents of the message.

def callback(ch, method, properties, body):
    print(f" [x] Received {body}")

Next, we need to tell RabbitMQ that this particular callback function should receive messages from our hello queue:

channel.basic_consume(queue='hello',
                      auto_ack=True,
                      on_message_callback=callback)

For that command to succeed we must be sure that a queue which we want to subscribe to exists. Fortunately we're confident about that ‒ we've created a queue above ‒ using queue_declare.

The auto_ack parameter will be described later on.

And finally, we enter a never-ending loop that waits for data and runs callbacks whenever necessary, and catch KeyboardInterrupt during program shutdown.

print(' [*] Waiting for messages. To exit press CTRL+C')
channel.start_consuming()
if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        print('Interrupted')
        try:
            sys.exit(0)
        except SystemExit:
            os._exit(0)

Putting it all together​

send.py (source)

#!/usr/bin/env python
import pika

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

channel.queue_declare(queue='hello')

channel.basic_publish(exchange='', routing_key='hello', body='Hello World!')
print(" [x] Sent 'Hello World!'")
connection.close()

receive.py (source)

#!/usr/bin/env python
import pika, sys, os

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

    channel.queue_declare(queue='hello')

    def callback(ch, method, properties, body):
        print(f" [x] Received {body}")

    channel.basic_consume(queue='hello', on_message_callback=callback, auto_ack=True)

    print(' [*] Waiting for messages. To exit press CTRL+C')
    channel.start_consuming()

if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        print('Interrupted')
        try:
            sys.exit(0)
        except SystemExit:
            os._exit(0)

Now we can try out our programs in a terminal. First, let's start a consumer, which will run continuously waiting for deliveries:

python receive.py
# => [*] Waiting for messages. To exit press CTRL+C

Now start the producer in a new terminal. The producer program will stop after every run:

python send.py
# => [x] Sent 'Hello World!'

The consumer will print the message:

# => [*] Waiting for messages. To exit press CTRL+C
# => [x] Received 'Hello World!'

Hurray! We were able to send our first message through RabbitMQ. As you might have noticed, the receive.py program doesn't exit. It will stay ready to receive further messages, and may be interrupted with Ctrl-C.

Try to run send.py again in a new terminal.

We've learned how to send and receive a message from a named queue. It's time to move on to part 2 and build a simple work queue.