Exchanges, Queues, and Bindings in RabbitMQ

1. Overview

To better understand how RabbitMQ works, we need to dive into its core components.

In this article, we’ll take a look into exchanges, queues, and bindings, and how we can declare them programmatically within a Java application.

2. Setup

As usual, we’ll use the Java client and the official client for the RabbitMQ server.

First, let's add the Maven dependency for the RabbitMQ client:

<dependency>
    <groupId>com.rabbitmq</groupId>
    <artifactId>amqp-client</artifactId>
    <version>5.12.0</version>
</dependency>

Next, let's declare the connection to the RabbitMQ server and open a communication channel:

ConnectionFactory factory = new ConnectionFactory();
factory.setHost("localhost");
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();

Also, a more detailed example of the setup can be found in our Introduction to RabbitMQ.

3. Exchanges

In RabbitMQ, a producer never sends a message directly to a queue. Instead, it uses an exchange as a routing mediator.

Therefore, the exchange decides if the message goes to one queue, to multiple queues, or is simply discarded.

For instance, depending on the routing strategy, we have four exchange types to choose from:

• Direct – the exchange forwards the message to a queue based on a routing key
• Fanout – the exchange ignores the routing key and forwards the message to all bounded queues
• Topic – the exchange routes the message to bounded queues using the match between a pattern defined on the exchange and the routing keys attached to the queues
• Headers – in this case, the message header attributes are used, instead of the routing key, to bind an exchange to one or more queues

Moreover, we also need to declare properties of the exchange:

• Name – the name of the exchange
• Durability – if enabled, the broker will not remove the exchange in case of a restart
• Auto-Delete – when this option is enabled, the broker deletes the exchange if it is not bound to a queue
• Optional arguments

All things considered, let’s declare the optional arguments for the exchange:

Map<String, Object> exchangeArguments = new HashMap<>();
exchangeArguments.put("alternate-exchange", "orders-alternate-exchange");

When passing the alternate-exchange argument, the exchange redirects unrouted messages to an alternative exchange, as we might guess from the argument name.

Next, let's declare a direct exchange with durability enabled and auto-delete disabled:

channel.exchangeDeclare("orders-direct-exchange", BuiltinExchangeType.DIRECT, true, false, exchangeArguments);

4. Queues

Similar to other messaging brokers, the RabbitMQ queues deliver messages to consumers based on a FIFO model.

In addition, when creating a queue, we can define several properties of the queue:

• Name – the name of the queue. If not defined, the broker will generate one
• Durability – if enabled, the broker will not remove the queue in case of a restart
• Exclusive – if enabled, the queue will only be used by one connection and will be removed when the connection is closed
• Auto-delete – if enabled, the broker deletes the queue when the last consumer unsubscribes
• Optional arguments

Further, we'll declare the optional arguments for the queue.

Let's add two arguments, the message TTL and the maximum number of priorities:

Map<String, Object> queueArguments = new HashMap<>();
queueArguments.put("x-message-ttl", 60000);
queueArguments.put("x-max-priority", 10);

Now, let's declare a durable queue with the exclusive and auto-delete properties disabled:

channel.queueDeclare("orders-queue", true, false, false, queueArguments);

5. Bindings

Exchanges use bindings to route messages to specific queues.

Sometimes, they have a routing key attached to them, used by some types of exchanges to filter specific messages and route them to the bounded queue.

Finally, let's bind the queue that we created to the exchange using a routing key:

channel.queueBind("orders-queue", "orders-direct-exchange", "orders-routing-key");

6. Conclusion

In this article, we covered the core components of RabbitMQ – exchanges, topics, and bindings. We also learned about their role in message delivery and how we can manage them from a Java application.

As always, the complete source code for this tutorial is available over on GitHub.