RabbitMQ is a popular message broker. For some time, i have been working on the same but something didn’t sit in my mind. So, i got back to the basics and started from scratch again. This blog will try to cover how RabbitMQ works and the basics.

Basics of RabbitMQ

There are few terms which are needed to understand how Rabbitmq works.

• Queue
• Exchange
• Producer
• Consumer
• Bind

Queue

Queue is the place where messages are sent. Messages are enqueued/dequeued from the queue.

Exchange

Every message needs to go to the queue via an exchange. There are different purpose exchanges, but the idea is that: a message can be enqueued to the queue only by an exchange.

Producer

Anyone who pushes message to the exchange is a Producer. There can be multiple producers for a queue.

Consumer

Anyone who consumes message from the queue is a Consumer. There can be multiple consumers for a queue.

Bind

Before the exchange can send the message to a queue, it needs to be binded to the queue. So that the exchange can be sure to which queue(s) it has to send the message to.

A very basic producer and consumer

Lets say, I have a producer and a consumer; and I want to produce and consume from a queue.

queue_name = "first_queue"
connection = pika.BlockingConnection(pika.ConnectionParameters("localhost"))
channel = connection.channel()
channel.queue_declare(queue_name)
channel.basic_publish(exchange="", routing_key=queue_name, body="hello, world")

Here, we create a connection and then a channel. We can use this channel to make operations.

queue_declare

Whenever we’re going to use a queue, we need to make sure it exists. Hence, we do a queue_declare.

basic_publish

Used to publish a message to a queue via an exchange. If the exchange_name is empty, RabbitMQ will use the default exchange. routing_key is the queue to which the message is to be delivered.

Let us now consume from the queue first_queue

queue_name = "first_queue"
connection = pika.BlockingConnection(pika.ConnectionParameters("localhost"))
channel = connection.channel()
channel.queue_declare(queue_name)
def callback(ch, method, properties, body):
    print(f" [x] Received {body}")

def consume(queue_name):
    channel.basic_consume(queue=queue_name, on_message_callback=callback, auto_ack=True)
    channel.start_consuming()

consume(queue_name)

We’re mostly doing the same thing, creating a connection, channel, declaring the queue. But instead of basic_publish, we’re using basic_consume and start_consuming

basic_consume

This tells RabbitMq that the client is subscribing to consume from the given queue. And when messages are received send it to the function given in on_message_callback.

start_consuming

This is a blocking function call. It blocks the thread and listens to the messages coming in to the given queue.

This is how a basic producer and a consumer works. If you want multiple consumers listening to different queues, use threads and invoke each start_consuming() in a separate thread.

References

https://www.rabbitmq.com/tutorials/tutorial-one-python

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