Rabbit Mq，在发布者之前启动消费者

Question

I have created two separate application for publisher and consumer that send and receives message from the common queue. I have started consumer application first and the exchange was not declared yet (It has been declared and bind to queue on publisher) that's why I got the error.

So my question is,

1. Is it a good idea to declare and bind exchange to queue on consumer?
2. In case of declaring exchange in consumer, should consumer know about the exchange properties and exchange type. Here in my case, consumer only knows the exchange,queue,route to receive message from the particular queue.

Answer 1

First it is important to note that starting a consumer and consuming from an existing queue should not give you any error even it the queue is not bound to any exchange. It is not necessary for a queue to be bound to an exchange, it can exist on its own.

To answer your questions: This depends on your use case. It may be OK for the consumer to create the queue, create the exchange and then bind the queue to the exchange. This allows the consumer to have control which messages are routed to the queue and can be consumed by him. If the consumer is the party that should execute this control, this is fine. But if the use case indicates that another party but the consumer shall control the routing, this other party shall create the exchange and the binding.

Consider a topology where there are only simple exchanges bound to queues. In such a topology there would be only bindings from exchanges to queues but none from exchanges to exchanges. Such a topology can be created by the consumer.

But consider a different topology with two levels of exchanges. The exchanges on the lower level are bound to queues, this is similar to the first topology. But above this lower level there is a higher level of exchanges which are only bound to exchanges on the lower level. The exchanges on the higer level distribute messages based on rules that are not related to concrete consumers. In fact, the two levels of exchanges could exist without any queues and consumers. The creation of the exchanges and bindings in this topology can not be done by a consumer.

A consumer could become a part of the second topology by declaring a queue for himself, binding this queue to the exchanges on the lower level he is interested in, and consume from the queue. The consumer would not create any exchanges, he would just bind his queue to them.

So to sum up: In trivial scenarios, it does not matter who declares exchanges, queues and bindings, as long as everything is done in the right order. But on more complex scenarios, the responsibility should by spread between the RabbitMQ admin, produces and consumers.