Direct exchange

How a direct exchange routes by exact match

A RabbitMQ direct exchange delivers a message to the queues whose binding key exactly matches the message's routing key. It's the simplest routing you can control yourself. The classic example is severity-based log routing: info, warning, error.

Say you want one queue that only receives error logs, and another that receives everything. A direct exchange lets each queue subscribe to the keys it cares about.

Declaring the exchange and binding a queue

First declare the exchange, then declare a queue, then bind the queue to the exchange with the routing key you want to receive:

use PhpAmqpLib\Message\AMQPMessage;

// name, type, passive, durable, auto_delete
$channel->exchange_declare('logs_direct', 'direct', false, true, false);

$channel->queue_declare('errors_only', false, true, false, false);
$channel->queue_bind('errors_only', 'logs_direct', 'error');

queue_bind($queue, $exchange, $routingKey) is the line that creates the routing rule: "send messages published to logs_direct with routing key error to the errors_only queue." Unlike the default exchange, here you declare the binding explicitly, so the routing key does not have to equal the queue name.

Publishing with a routing key

The producer publishes to the exchange and sets the routing key to the severity:

$msg = new AMQPMessage('disk is full', [
    'delivery_mode' => AMQPMessage::DELIVERY_MODE_PERSISTENT,
]);

$channel->basic_publish($msg, 'logs_direct', 'error');

This message matches the error binding, so it lands in errors_only. A message published with routing key info does not match that binding, so errors_only never sees it.

Many bindings, same key

A direct exchange isn't limited to one queue per key. You can bind several queues with the same routing key, and all of them receive a copy. You can also bind one queue with several keys:

$channel->queue_bind('important', 'logs_direct', 'error');
$channel->queue_bind('important', 'logs_direct', 'warning');

Now the important queue receives both error and warning messages. This is how you build flexible routing while still matching on exact keys.

Worth knowing: queue_bind is idempotent for a given (queue, exchange, key) triple. Running the same bind twice does not create a second binding or a duplicate copy per message, so a consumer that re-declares its bindings on every startup stays safe.

Common mistake

Publishing with a routing key that no binding matches. A direct exchange has no fallback: if nothing matches, the message is silently dropped. Publishing critical when only error and warning are bound loses the message with no error. Always make sure a binding exists for every key you publish, and remember the match is case-sensitive - Error and error are different keys.

FAQ

What's the difference between the default exchange and a named direct exchange?

They're the same type. The default exchange is just a direct exchange with an automatic queue-name binding you can't change. A named direct exchange lets you define your own bindings, so routing keys and queue names can differ.

Do I declare the exchange in both the producer and the consumer?

Yes, declare it in both. exchange_declare is idempotent - if the exchange already exists with the same settings, the call does nothing. Declaring on both sides means whichever process starts first sets it up.

Can two queues share the same routing key?

Yes. Bind both queues to the exchange with the same key and each gets its own copy of every matching message.