Where does RabbitMQ store the messages When consumers are not available

When no consumers are available to consume messages, RabbitMQ stores the messages in queues. The storage mechanism and location depend on the configuration of RabbitMQ and the nature of the queues (whether they are durable or transient). Here's a detailed explanation:

1. In-Memory Storage

By default, RabbitMQ stores messages in memory. If the messages cannot be immediately delivered to consumers, they will be held in memory until a consumer becomes available or until the messages are persisted to disk if they are part of a durable queue.

2. Durable Queues and Persistent Messages

For ensuring message durability and persistence, RabbitMQ supports durable queues and persistent messages:

• Durable Queues: A queue is marked as durable when it is declared. This means the queue itself will survive broker restarts. To declare a durable queue, the durable parameter is set to true.

  channel.assertQueue('my-durable-queue', { durable: true });
  

• Persistent Messages: Messages are marked as persistent by setting the delivery mode to 2. This ensures that the messages will be written to disk and survive broker restarts.

  channel.sendToQueue('my-durable-queue', Buffer.from('Hello, World!'), { persistent: true });
  

3. Disk-Based Storage

For durable queues and persistent messages, RabbitMQ uses disk-based storage to ensure messages are not lost even if the broker restarts or crashes. RabbitMQ writes these messages to the disk, typically in a directory specified by the configuration file.

4. Configuration for Storage Location

The location where RabbitMQ stores its data (including persistent messages) is determined by the configuration settings. This is usually defined in the RabbitMQ configuration file (rabbitmq.conf or rabbitmq.config) and can be overridden by environment variables.

• Default Location: By default, RabbitMQ stores data in the /var/lib/rabbitmq/ directory on Unix-based systems and in a similar appropriate location on Windows.

• Configuration Example: The data directory can be specified in the configuration file.

  # rabbitmq.conf
  # Set the data directory location
  mnesia_base = /var/lib/rabbitmq/mnesia
  

Example Workflow

Here’s a simple workflow explaining how RabbitMQ handles messages with no consumers available:

1. Message Publication:

   • A producer sends a message to a queue.
   • If the queue is marked as durable and the message is marked as persistent, RabbitMQ writes the message to disk.

2. Message Storage:

   • RabbitMQ stores the message in memory initially.
   • For durable queues and persistent messages, RabbitMQ also writes the message to disk for persistence.

3. No Consumer Available:

   • If no consumer is available, RabbitMQ keeps the message in the queue (in memory and/or on disk).
   • The message remains in the queue until a consumer becomes available to consume it.

4. Consumer Availability:

   • Once a consumer becomes available, RabbitMQ delivers the message from the queue to the consumer.
   • The message is then acknowledged by the consumer (if using acknowledgment), and removed from the queue.