What is a headers exchange in RabbitMQ?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Declare a headers exchange

   Create an exchange of type 'headers' on the channel.

2. Step 2

   Define binding headers

   Bind a queue with a headers map plus an 'x-match' argument of 'all' or 'any'.

3. Step 3

   Publish with headers

   Producer sets message headers (e.g. format=pdf, region=eu) and any routing key, which is ignored.

4. Step 4

   Broker compares headers

   RabbitMQ matches the message headers against each binding using the x-match rule.

5. Step 5

   Deliver on match

   The message is routed to every queue whose header criteria are satisfied.

Code Example

const amqp = require('amqplib');

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

  await ch.assertExchange(exchange, 'headers', { durable: false });

  const q = await ch.assertQueue('', { exclusive: true });
  await ch.bindQueue(q.queue, exchange, '', {
    'x-match': 'all',
    format: 'pdf',
    region: 'eu'
  });

  // Routing key ignored; headers drive routing
  ch.publish(exchange, '', Buffer.from('quarterly report'), {
    headers: { format: 'pdf', region: 'eu' }
  });
  console.log('Routed by matching headers');

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

run();

Follow-up Questions

• What is the difference between x-match 'all' and 'any'?
• When would you choose a headers exchange over a topic exchange?
• Are header values limited to strings?
• Why is the headers exchange sometimes slower than topic routing?
• How does RabbitMQ ignore the routing key here?