Mirrored queues for redundancy in RabbitMQ - Commands & Configuration

Introduction

Sometimes messages in a queue can be lost if the server crashes. Mirrored queues copy messages to multiple servers to keep them safe. This way, if one server fails, another has the same messages ready.

When you want to make sure messages are not lost if one RabbitMQ server goes down

When running RabbitMQ in a cluster and you want queues to be available on multiple nodes

When your application depends on reliable message delivery even during server failures

When you want to balance load and have failover for message processing

When you need high availability for critical messaging systems

Config File - rabbitmq.conf

rabbitmq.conf

cluster_formation.peer_discovery_backend = rabbit_peer_discovery_classic_config

# Define policies for mirrored queues
# This policy mirrors all queues to all nodes in the cluster

policies = [{"ha-all", ".*", {"ha-mode", "all"}, {"priority", 0}}]

This configuration file sets up RabbitMQ to discover cluster nodes using classic config. The policies section defines a policy named ha-all that matches all queues (using .*) and sets ha-mode to all, which means every queue is mirrored to all nodes in the cluster. The priority sets the policy application order.

Commands

This command creates a policy named 'ha-all' that matches all queues and sets them to be mirrored on all nodes. It ensures redundancy for every queue.

Terminal

rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"all"}'

Expected Output

Setting policy "ha-all" for pattern ".*" to "{"ha-mode":"all"}" ...

This command lists all policies currently set in RabbitMQ so you can verify the 'ha-all' policy is active.

Terminal

rabbitmqctl list_policies

Expected Output

Name    Pattern    Definition          Priority

ha-all  .*         {"ha-mode":"all"} 0

This command shows all queues and the policies applied to them, confirming that the mirrored queue policy is in effect.

Terminal

rabbitmqctl list_queues name policy

Expected Output

Name          Policy

my-queue      ha-all

another-queue ha-all

Key Concept

If you remember nothing else from this pattern, remember: Mirrored queues copy messages to multiple servers to keep them safe and available during failures.

Common Mistakes

Setting ha-mode to 'exactly' without specifying the number of nodes

RabbitMQ requires a number with 'exactly' mode, otherwise the policy is invalid and queues won't mirror.

Use 'ha-mode' set to 'all' for all nodes or 'exactly' with a number like {"ha-mode":"exactly", "ha-params":2}.

Not applying the policy before creating queues

Queues created before the policy is set will not be mirrored automatically.

Set the policy first, then create queues to ensure they are mirrored.

Using incorrect JSON syntax in the policy command

RabbitMQ rejects policies with invalid JSON, so the command fails silently or errors out.

Use single quotes around the JSON and double quotes inside, like '{"ha-mode":"all"}'.

Summary

Set a policy with rabbitmqctl to mirror queues across all nodes for redundancy.

Verify the policy is active using rabbitmqctl list_policies.

Check queues have the policy applied with rabbitmqctl list_queues.