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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 AMQP 1.0 Go client)​

In this part of the tutorial we'll write two small programs in Go; a producer that sends a single message, and a consumer that receives messages and prints them out. We'll gloss over some of the detail in the Go AMQP 1.0 client API, concentrating on this very simple thing just to get started. 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.

The AMQP 1.0 Go client library​

RabbitMQ speaks multiple protocols. This tutorial uses AMQP 1.0 over the same port as AMQP 0-9-1 (5672 by default). It requires RabbitMQ 4.0 or later.

Use the RabbitMQ AMQP 1.0 Go client (rabbitmq-amqp-go-client), not the classic AMQP 0-9-1 client (amqp091-go). See AMQP 1.0 client libraries and the Go client package.

Install with:

go get github.com/rabbitmq/rabbitmq-amqp-go-client

Now we have the client installed, we can write some code.

Sending​

We'll call our message publisher (sender) send.go and our message consumer receive.go. The publisher will connect to RabbitMQ, send a single message, then exit.

In send.go, we need to import the library first:

package main

import (
	"context"
	"log"

	rmq "github.com/rabbitmq/rabbitmq-amqp-go-client/pkg/rabbitmqamqp"
)

Set up the connection to the broker using an environment and connection:

const brokerURI = "amqp://guest:guest@localhost:5672/"

func main() {
	ctx := context.Background()
	env := rmq.NewEnvironment(brokerURI, nil)
	conn, err := env.NewConnection(ctx)
	if err != nil {
		log.Panicf("Failed to connect to RabbitMQ: %v", err)
	}
	defer func() {
		_ = env.CloseConnections(context.Background())
	}()

The environment holds shared settings, and the connection represents a socket to the broker. Deferring CloseConnections ensures resources are released when the program exits.

Declare a quorum queue named "hello":

	_, err = conn.Management().DeclareQueue(ctx, &rmq.QuorumQueueSpecification{Name: "hello"})
	if err != nil {
		log.Panicf("Failed to declare a queue: %v", err)
	}

Declaring a queue is idempotent: it will only be created if it does not already exist. Quorum queues are durable and replicated across RabbitMQ nodes.

Create a publisher and send a message:

	publisher, err := conn.NewPublisher(ctx, &rmq.QueueAddress{Queue: "hello"}, nil)
	if err != nil {
		log.Panicf("Failed to create publisher: %v", err)
	}
	defer func() { _ = publisher.Close(context.Background()) }()

	body := "Hello World!"
	res, err := publisher.Publish(ctx, rmq.NewMessage([]byte(body)))
	if err != nil {
		log.Panicf("Failed to publish a message: %v", err)
	}
	switch res.Outcome.(type) {
	case *rmq.StateAccepted:
	default:
		log.Panicf("Unexpected publish outcome: %v", res.Outcome)
	}
	log.Printf(" [x] Sent %s\n", body)
}

With AMQP 1.0, Publish returns an outcome. Check for StateAccepted to confirm the broker accepted the message.

Here's the whole send.go file.

Sending doesn't work!​

If this is your first time using RabbitMQ and you don't see the " [x] 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.

Receiving​

That's it for our publisher. Our consumer listens for messages from RabbitMQ, so unlike the publisher which publishes a single message, we'll keep the consumer running to listen for messages and print them out.

The code in receive.go is similar. Set up the environment and connection, declare the same queue, then create a consumer:

package main

import (
	"context"
	"errors"
	"log"

	rmq "github.com/rabbitmq/rabbitmq-amqp-go-client/pkg/rabbitmqamqp"
)

const brokerURI = "amqp://guest:guest@localhost:5672/"

func main() {
	ctx := context.Background()
	env := rmq.NewEnvironment(brokerURI, nil)
	conn, err := env.NewConnection(ctx)
	if err != nil {
		log.Panicf("Failed to connect to RabbitMQ: %v", err)
	}
	defer func() {
		_ = env.CloseConnections(context.Background())
	}()

	_, err = conn.Management().DeclareQueue(ctx, &rmq.QuorumQueueSpecification{Name: "hello"})
	if err != nil {
		log.Panicf("Failed to declare a queue: %v", err)
	}

	consumer, err := conn.NewConsumer(ctx, "hello", nil)
	if err != nil {
		log.Panicf("Failed to create consumer: %v", err)
	}
	defer func() { _ = consumer.Close(context.Background()) }()

	log.Printf(" [*] Waiting for messages. To exit press CTRL+C")
	for {
		delivery, err := consumer.Receive(ctx)
		if err != nil {
			if errors.Is(err, context.Canceled) {
				return
			}
			log.Panicf("Failed to receive a message: %v", err)
		}
		msg := delivery.Message()
		var body string
		if len(msg.Data) > 0 {
			body = string(msg.Data[0])
		}
		log.Printf("Received a message: %s", body)
		err = delivery.Accept(ctx)
		if err != nil {
			log.Panicf("Failed to accept message: %v", err)
		}
	}
}

Note that we declare the queue here as well. Because we might start the consumer before the publisher, we want to make sure the queue exists before we try to consume messages from it.

With AMQP 1.0, the consumer must settle each message by calling Accept (or Discard / Requeue). Here we call delivery.Accept(ctx) after printing the message.

Here's the whole receive.go file.

Putting it all together​

Create a go.mod file for your project:

module rabbitmq-tutorials

go 1.21

require github.com/rabbitmq/rabbitmq-amqp-go-client v0.7.0

Then compile and run the examples. In one terminal, run the consumer:

go run receive.go

Then run the publisher:

go run send.go

The consumer will print the message it gets from the publisher via RabbitMQ. The consumer will keep running, waiting for messages (use Ctrl+C to stop it), so try running the publisher from another terminal.

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

Time to move on to part 2 and build a simple work queue.