RabbitMQ Messaging & Async Systems · Lesson

Topic Exchange for Flexible Routing

Master the Topic exchange for complex routing patterns using wildcard matching. Design flexible and scalable message routing topologies.

Topic Exchange for Flexible Routing is a free RabbitMQ Messaging & Async Systems lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the RabbitMQ Messaging & Async Systems learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Topic Exchange: Flexible Routing

Welcome to the Topic Exchange! This exchange type offers the most flexible message routing, letting you send messages to queues based on complex patterns.

Unlike the simple Fanout or direct-match Direct exchange, Topic exchanges use special wildcards in routing keys to match messages dynamically.

Understanding Topic Routing Keys

With a Topic exchange, routing keys aren't exact matches. They are strings made of words separated by dots (.), much like parts of a filename or URL.

Example: animal.rabbit.fast
Example: log.error.database

Each word provides a level of detail, allowing for hierarchical routing.

The Single-Word Wildcard: `*`

The asterisk (*) wildcard matches exactly one word in a routing key segment.

Binding key: animal.*.fast matches animal.rabbit.fast
Binding key: animal.*.fast does not match animal.dog.lazy.fast (too many words)
Binding key: *.error.* matches log.error.database

It's great for matching a specific position in the key.

The Zero-or-More Wildcard: `#`

The hash (#) wildcard matches zero or more words in a routing key. It's much more powerful than *.

Binding key: log.# matches log.error, log.info.web, log.debug.cache.entry
Binding key: animal.# matches animal.rabbit, animal.cat.sleepy

Use # to capture all messages that start with a certain pattern.

Producer: Declaring Topic Exchange

First, our producer needs to declare the Topic exchange. The type is simply "topic".

Try running this snippet to set up the exchange:

import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;

public class TopicProducerSetup {
    private static final String EXCHANGE_NAME = "topic_logs";

    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost"); // Connect to local RabbitMQ
        try (Connection connection = factory.newConnection();
             Channel channel = connection.createChannel()) {

            channel.exchangeDeclare(EXCHANGE_NAME, "topic");
            System.out.println("Topic exchange '" + EXCHANGE_NAME + "' declared.");
        }
    }
}

Producer: Sending Messages

Now, let's send some messages using different routing keys. The exchange will use these keys to decide which queues receive the messages.

Notice how the keys are dot-separated, allowing for detailed categories.

import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import java.nio.charset.StandardCharsets;

public class TopicMessageSender {
    private static final String EXCHANGE_NAME = "topic_logs";

    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        try (Connection connection = factory.newConnection();
             Channel channel = connection.createChannel()) {

            channel.exchangeDeclare(EXCHANGE_NAME, "topic");

            sendMessage(channel, "animal.rabbit.fast", "A fast rabbit runs.");
            sendMessage(channel, "animal.cat.sleepy", "A sleepy cat naps.");
            sendMessage(channel, "log.error.db", "Database connection failed!");
            sendMessage(channel, "log.info.web", "User logged in.");

            System.out.println("Sent various topic messages.");
        }
    }

    private static void sendMessage(Channel channel, String routingKey, String message) throws Exception {
        channel.basicPublish(EXCHANGE_NAME, routingKey, null, message.getBytes(StandardCharsets.UTF_8));
        System.out.println(" [x] Sent '" + routingKey + ":'" + message + "'");
    }
}

Consumer: Binding with `*`

Consumers bind their queues to the Topic exchange using binding keys that can contain wildcards. This consumer uses *.orange.*.

It will receive messages like animal.orange.fast but not fruit.apple.sweet or animal.orange.

import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.DeliverCallback;

public class TopicConsumerOrange {
    private static final String EXCHANGE_NAME = "topic_logs";

    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        Connection connection = factory.newConnection();
        Channel channel = connection.createChannel();

        channel.exchangeDeclare(EXCHANGE_NAME, "topic");
        String queueName = channel.queueDeclare().getQueue();

        String bindingKey = "*.orange.*"; // Matches e.g., 'animal.orange.fast'
        channel.queueBind(queueName, EXCHANGE_NAME, bindingKey);
        System.out.println(" [x] Waiting for messages matching: '" + bindingKey + "'");

        DeliverCallback deliverCallback = (consumerTag, delivery) -> {
            String message = new String(delivery.getBody(), "UTF-8");
            System.out.println(" [x] Received '" + delivery.getEnvelope().getRoutingKey() + ":'" + message + "'");
        };
        channel.basicConsume(queueName, true, deliverCallback, consumerTag -> {});
    }
}

Consumer: Binding with `#`

This consumer uses the log.# binding key. This means it will receive all messages whose routing key starts with log., regardless of how many words follow.

This is extremely useful for systems like logging, where you might want a 'catch-all' listener for a category.

import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.DeliverCallback;

public class TopicConsumerLogs {
    private static final String EXCHANGE_NAME = "topic_logs";

    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        Connection connection = factory.newConnection();
        Channel channel = connection.createChannel();

        channel.exchangeDeclare(EXCHANGE_NAME, "topic");
        String queueName = channel.queueDeclare().getQueue();

        String bindingKey = "log.#"; // Matches e.g., 'log.error.db', 'log.info.web'
        channel.queueBind(queueName, EXCHANGE_NAME, bindingKey);
        System.out.println(" [x] Waiting for messages matching: '" + bindingKey + "'");

        DeliverCallback deliverCallback = (consumerTag, delivery) -> {
            String message = new String(delivery.getBody(), "UTF-8");
            System.out.println(" [x] Received '" + delivery.getEnvelope().getRoutingKey() + ":'" + message + "'");
        };
        channel.basicConsume(queueName, true, deliverCallback, consumerTag -> {});
    }
}

Topic Exchange Use Cases

Topic exchanges shine in scenarios requiring flexible and hierarchical routing:

Logging Systems: Route logs based on severity, source, and component (e.g., app.error.auth, app.info.payment).
Real-time Data Streams: Filter sensor data (e.g., sensor.room1.temp, sensor.room2.humidity).
Content Distribution: Deliver news articles to subscribers interested in specific categories or regions.

They provide fine-grained control over message flow.

Topic Routing Challenge

Consider the following messages and binding keys. Which messages will be delivered to a queue bound with the key animal.*.#.fast?

Recap: Topic Exchange Power

You've mastered the Topic exchange! It's an incredibly powerful tool for building flexible and scalable messaging architectures.

Uses dot-separated routing keys.
* matches exactly one word.
# matches zero or more words.
Ideal for logging, real-time analytics, and content filtering.

By using wildcards, you can create sophisticated routing rules to ensure messages go exactly where they're needed.

Frequently asked questions

Is the “Topic Exchange for Flexible Routing” lesson free?

Yes — the full text of “Topic Exchange for Flexible Routing” is free to read here on the web, and the RabbitMQ Messaging & Async Systems course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the RabbitMQ Messaging & Async Systems course, upgrade to CoddyKit PRO.

What will I learn in “Topic Exchange for Flexible Routing”?

Master the Topic exchange for complex routing patterns using wildcard matching. Design flexible and scalable message routing topologies. You practise RabbitMQ Messaging & Async Systems with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start RabbitMQ Messaging & Async Systems?

No prior experience is required. RabbitMQ Messaging & Async Systems on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Topic Exchange for Flexible Routing” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this RabbitMQ Messaging & Async Systems lesson?

Yes. Every RabbitMQ Messaging & Async Systems lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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