What is a topic exchange and how does routing key pattern matching work?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Design the routing key scheme

   Choose dot-delimited words, e.g. 'order.region.status', ordered from general to specific.

2. Step 2

   Declare a topic exchange

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

3. Step 3

   Bind queues with patterns

   Use '*' for exactly one word and '#' for zero or more words, e.g. 'order.*.created' or 'order.#'.

4. Step 4

   Publish with a concrete key

   Producer publishes with a specific routing key like 'order.eu.created'.

5. Step 5

   Broker matches and routes

   RabbitMQ delivers to every queue whose pattern matches the routing key.

Code Example

const amqp = require('amqplib');

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

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

  const q = await ch.assertQueue('', { exclusive: true });
  // Matches order.eu.created, order.us.created, etc.
  await ch.bindQueue(q.queue, exchange, 'order.*.created');

  ch.publish(exchange, 'order.eu.created', Buffer.from('new EU order'));
  console.log('Routed by pattern order.*.created');

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

run();

Follow-up Questions

• What is the difference between '*' and '#' in topic bindings?
• How can a topic exchange behave like a fanout exchange?
• How can a topic exchange behave like a direct exchange?
• What happens if a routing key matches multiple bindings?
• How would you design routing keys for a logging system?