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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 php-amqplib Client)​

In this part of the tutorial we'll write two programs in PHP that communicate using RabbitMQ. This tutorial uses a client library that requires PHP 7.x or 8.x.

First program will be a producer that sends a single message, and the second one will be a consumer that receives messages and prints them out. We'll gloss over some of the detail in the php-amqplib 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 php-amqplib client library​

RabbitMQ speaks multiple protocols. This tutorial covers 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. We'll use the php-amqplib in this tutorial, and Composer for dependency management.

Add a composer.json file to your project:

{
    "require": {
        "php-amqplib/php-amqplib": "^3.2"
    }
}

Provided you have Composer installed and functional, you can run the following:

php composer.phar install

There's also a Composer installer for Windows.

Now we have the php-amqplib library installed, we can write some code.

Sending​

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

In send.php, we need to include the library and use the necessary classes:

require_once __DIR__ . '/vendor/autoload.php';
use PhpAmqpLib\Connection\AMQPStreamConnection;
use PhpAmqpLib\Message\AMQPMessage;

then we can create a connection to the server:

$connection = new AMQPStreamConnection('localhost', 5672, 'guest', 'guest');
$channel = $connection->channel();

The connection abstracts the socket connection, and takes care of protocol version negotiation and authentication and so on for us. Here we connect to a RabbitMQ node on the local machine - hence the localhost. If we wanted to connect to a node on a different machine or to a host hosting a proxy recommended for PHP clients, we'd simply specify its hostname or IP address here.

Next we create a channel, which is where most of the API for getting things done resides.

To send, we must declare a queue for us to send to; then we can publish a message to the queue:

$channel->queue_declare('hello', false, false, false, false);

$msg = new AMQPMessage('Hello World!');
$channel->basic_publish($msg, '', 'hello');

echo " [x] Sent 'Hello World!'\n";

Declaring a queue is idempotent - it will only be created if it doesn't exist already. The message content is a byte array, so you can encode whatever you like there.

Lastly, we close the channel and the connection:

$channel->close();
$connection->close();

Here's the whole send.php class.

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​

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

The code (in receive.php) has almost the same include and uses as send:

require_once __DIR__ . '/vendor/autoload.php';
use PhpAmqpLib\Connection\AMQPStreamConnection;

Setting up is the same as the publisher; we open a connection and a channel, and declare the queue from which we're going to consume. Note this matches up with the queue that send publishes to.

$connection = new AMQPStreamConnection('localhost', 5672, 'guest', 'guest');
$channel = $connection->channel();

$channel->queue_declare('hello', false, false, false, false);

echo " [*] Waiting for messages. To exit press CTRL+C\n";

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.

We're about to tell the server to deliver us the messages from the queue. We will define a PHP callable that will receive the messages sent by the server. Keep in mind that messages are sent asynchronously from the server to the clients.

$callback = function ($msg) {
  echo ' [x] Received ', $msg->body, "\n";
};

$channel->basic_consume('hello', '', false, true, false, false, $callback);

try {
    $channel->consume();
} catch (\Throwable $exception) {
    echo $exception->getMessage();
}

Our code will block while our $channel has callbacks. Whenever we receive a message our $callback function will be passed the received message.

Here's the whole receive.php class

Putting it all together​

Now we can run both scripts. In a terminal, run the consumer (receiver):

php receive.php

then, run the publisher (sender):

php send.php

The consumer will print the message it gets from the sender via RabbitMQ. The receiver will keep running, waiting for messages (Use Ctrl-C to stop it), so try running the sender 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

PHP Connection Proxy​

While this tutorial strives to keep things simple and focus on explaining RabbitMQ concepts, it is important to call out something that is specific to PHP applications. In many cases PHP application will not be able to use long-lived connections that RabbitMQ assumes, creating a condition known as high connection churn.

To avoid this, PHP users are recommended to use a special proxy in production when possible. The proxy avoids connection churn or at least significantly reduces it.

Now it is time to move on to part 2 and build a simple work queue.