[rabbitmq-discuss] Message delivered even if routing key doen't match

Tue May 11 00:08:04 BST 2010

Alex,

> I'm starting to write a prototype with RabbitMQ but I've some problem to
> understand the concept of "routing key", below there's an excerpt of the
> producer:
>
>      String exchangeName = "myExchange";
>      channel.exchangeDeclare(exchangeName, "direct");
>      String queueName = "myQueue";
>      channel.queueDeclare(queueName);
>      String routingKey = "testRoute";
>      channel.queueBind(queueName, exchangeName, routingKey);
>
> the consumer's code is the the same except it defines a routingKey = "xyz"

(See the answer on routing algorithms, below, first.)

All of exchange.declare, queue.declare, and queue.bind are idempotent 
given the same arguments; in this case though you're giving two 
different arguments to queue.bind, so you end up with two bindings from 
a single exchange to a single queue.

Hence messages to the exchange with a routing key of "xyz" OR a routing 
key of "testRoute" will be delivered to the queue.

> Why does the consumer gets the messages even if the routing key that it
> defines is different from the one defined by the producer?
>
> Is there a document that describes the routing algorithm based on the
> routing key?

In the case above, what you're supplying is not a routing key but a 
/binding key/.

Individual messages have routing keys; routing involves comparing the 
message's /routing key/ with each binding's /binding key/ to see which 
queues (via the bindings) should be delivered the message.

For a direct exchange, the algorithm is (informally) "deliver via all 
bindings with a binding key equal to the message's routing key".  A 
fanout exchange ignores binding keys and routing keys, and delivers via 
all bindings.  A topic exchange does a kind of wildcard matching on the 
routing key, with binding keys giving patterns.

Wikipedia is not bad on this:
http://en.wikipedia.org/wiki/Advanced_Message_Queuing_Protocol#Exchange_types_and_the_effect_of_bindings

Michael.