Why do we need routing key in RabbitMQ?

Question

Why do we need routing key to route messages from exchange to queue? Can't we simply use the queue name to route the message? Also, in case of publishing to multiple queues, we can use multiple queue names. Can anyone point out the scenario where we actually need routing key and queue name won't be suffice?

Solution 1

There are several types of exchanges. The fanout exchange ignores the routing key and sends messages to all queues. But pretty much all other exchange types use the routing key to determine which queue, if any, will receive a message.

The tutorials on the RabbitMQ website describes several usecases where different exchange types are useful and where the routing key is relevant.

For instance, tutorial 5 demonstrates how to use a topic exchange to route log messages to different queues depending on the log level of each message.

If you want to target multiple queues, you need to bind them to a fanout exchange and use that exchange in your publisher.

You can't specify multiple queue names in your publisher. In AMQP, you do not publish a message to queues, you publish a message to an exchange. It's the exchange responsability to determine the relevant queues. It's possible that a message is routed to no queue at all and just dropped.

Solution 2

Decoupling queue names from applications is useful for flexibility.

• You could establish multiple queues to consume the same message, but queues can't have the same name.

• In some cases, message's originator doesn't know the names of queues. (like when you have randomly generated queue names when horizontally scaling a server)

• An exchange may be routing messages for more than just one type of consumer. Then you would need some wildcards in your routing keys to route messages to concerned consumers.