Mirrored Queues for HA
Implement mirrored queues to replicate messages across multiple cluster nodes. Achieve high availability for your queues, ensuring messages survive node failures.
Mirrored Queues for HA 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.
Intro to Mirrored Queues
In a RabbitMQ cluster, queues are by default located on a single node. If that node fails, any messages in its queues (and the queues themselves) become unavailable until the node recovers.
Mirrored queues solve this by replicating queue contents across multiple nodes. This ensures high availability (HA) and fault tolerance for your messages.
Why Use Mirrored Queues?
Imagine a critical application where losing messages or experiencing downtime is unacceptable. Mirrored queues provide:
- High Availability: If the node hosting the primary queue fails, a replica can take over seamlessly.
- Data Durability: Messages are stored on multiple nodes, protecting against single-node failures.
- Fault Tolerance: The system can continue operating even if some nodes go offline.
They are essential for robust, production-grade RabbitMQ deployments.
Master & Replica Architecture
When a queue is mirrored, one node hosts the master (or primary) queue. All other nodes hosting mirrors run replicas.
All operations for a mirrored queue (publishing, consuming, adding messages) are first handled by the master. The master then replicates these operations to all its replicas.
Configuring Mirroring with Policies
You don't configure mirroring directly on individual queues. Instead, you use policies. A policy is a set of rules that apply to queues whose names match a specific pattern.
This allows you to define mirroring behavior for many queues at once, or for queues created in the future, without modifying client code.
Policy Example: Mirror All Queues
Here's how to create a policy named ha-all that mirrors all queues (matching ".*") to all nodes in the cluster ("ha-mode":"all").
You'd typically run this command on one of your RabbitMQ cluster nodes via the command line.
rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"all"}' --apply-to queuesUnderstanding ha-mode Options
The ha-mode argument in a policy defines how mirroring should behave:
all: The queue will be mirrored to all nodes in the cluster.exactly: The queue will be mirrored to a specific number of nodes (e.g.,{"ha-mode":"exactly", "ha-params":2}).nodes: The queue will be mirrored to a specific list of named nodes (e.g.,{"ha-mode":"nodes", "ha-params":["rabbit@node1", "rabbit@node2"]}).
Producers & Mirrored Queues
From a producer's perspective, interacting with a mirrored queue is no different than a non-mirrored one. The client connects to any node in the cluster, and RabbitMQ handles routing the message to the master queue for mirroring.
Try running this simple Java producer:
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
public class Producer {
private final static String QUEUE_NAME = "my_mirrored_queue";
public static void main(String[] argv) throws Exception {
ConnectionFactory factory = new ConnectionFactory();
factory.setHost("localhost"); // Connect to any cluster node
try (Connection connection = factory.newConnection();
Channel channel = connection.createChannel()) {
// Declare a durable queue (important for mirrored queues)
channel.queueDeclare(QUEUE_NAME, true, false, false, null);
String message = "Hello, Mirrored Queue!";
channel.basicPublish("", QUEUE_NAME, null, message.getBytes("UTF-8"));
System.out.println(" [x] Sent '" + message + "'");
}
}
}Consumers & Mirrored Queues
Similarly, consumers don't need special logic to consume from a mirrored queue. They simply connect to a node and subscribe to the queue.
If the master queue fails, RabbitMQ automatically promotes a replica to master, and consumers transparently switch to the new master (though a brief reconnect might be needed).
Run this consumer example:
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.DeliverCallback;
public class Consumer {
private final static String QUEUE_NAME = "my_mirrored_queue";
public static void main(String[] argv) throws Exception {
ConnectionFactory factory = new ConnectionFactory();
factory.setHost("localhost"); // Connect to any cluster node
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();
channel.queueDeclare(QUEUE_NAME, true, false, false, null);
System.out.println(" [*] Waiting for messages. To exit press CTRL+C");
DeliverCallback deliverCallback = (consumerTag, delivery) -> {
String message = new String(delivery.getBody(), "UTF-8");
System.out.println(" [x] Received '" + message + "'");
};
// Basic consume with auto-acknowledgement
channel.basicConsume(QUEUE_NAME, true, deliverCallback, consumerTag -> {});
}
}Failover & Replica Sync
When a master node fails, RabbitMQ elects a new master from the available replicas. This new master takes over, and message processing continues without data loss.
If a node with a replica rejoins the cluster, or a new node is added, the replica will synchronize its contents with the current master. This ensures all messages are consistent across the mirrored queue instances.
Quick Check: Mirrored Queues
You've learned about the importance and mechanics of mirrored queues. Let's test your understanding.
Recap: Mirrored Queues for HA
We've explored mirrored queues, a crucial feature for ensuring high availability and data durability in RabbitMQ clusters.
- Mirrored queues replicate messages across master and replica nodes.
- They are configured using policies, allowing flexible control over mirroring behavior.
- Client applications (producers/consumers) interact with mirrored queues transparently.
- In case of a master node failure, a replica is promoted, ensuring continuous service and message safety.
This mechanism is vital for building robust, fault-tolerant messaging systems.
Frequently asked questions
Is the “Mirrored Queues for HA” lesson free?
Yes — the full text of “Mirrored Queues for HA” 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 “Mirrored Queues for HA”?
Implement mirrored queues to replicate messages across multiple cluster nodes. Achieve high availability for your queues, ensuring messages survive node failures. 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 “Mirrored Queues for HA” 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.