What is a fanout exchange and how does it broadcast messages?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Declare a fanout exchange

   Create an exchange of type 'fanout' on the channel; no routing key configuration is needed.

2. Step 2

   Declare queues

   Each consumer declares its own queue, often exclusive or auto-delete for transient subscribers.

3. Step 3

   Bind queues to the exchange

   Bind every queue to the fanout exchange; the routing key argument is ignored.

4. Step 4

   Publish a message

   Producer publishes to the exchange with any (or empty) routing key.

5. Step 5

   RabbitMQ fans out

   The broker copies the message to every bound queue so all consumers receive it.

Code Example

const amqp = require('amqplib');

async function publish() {
  const conn = await amqp.connect('amqp://localhost');
  const ch = await conn.createChannel();
  const exchange = 'logs';

  await ch.assertExchange(exchange, 'fanout', { durable: false });
  ch.publish(exchange, '', Buffer.from('system event'));
  console.log('Broadcast sent to all bound queues');

  await ch.close();
  await conn.close();
}

publish();

Follow-up Questions

• How does a fanout exchange differ from a direct exchange?
• When would you choose fanout over topic exchange?
• How do temporary (exclusive) queues help with fanout pub/sub?
• What happens to a message if no queues are bound to a fanout exchange?
• How would you scale consumers of a fanout exchange?