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I have a project where I send AMQP messages to a RabbitMQ Server. This messages are synchronous messages (I use sendAndReceive method). So, I have a SimpleMessageListenerContainer configured with the RabbitTemplate as MessageListener, and the response queue is fixed (setReplyAddress in the RabbitTemplate).

If I have a multithreading server (Tomcat) where it is possible to send some messages simultaneously, could there be a problem if the response doesn’t arrive in order or the application send a message before a response to another message has arrived?

jandres
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  • Do you have any code to elaborate on? Tomcat threading has nothing to with concurrency of the messaging. You control it. – David Siro Mar 10 '16 at 12:30
  • Sorry, the problem is not Tomcat, it’s only information. The code is working ok, but I have some doubts about how spring amqp will handle synchronous messages if you can send multiple messages simultaneous (from different threads) and the response is always received in the same response queue (I can not change this) – jandres Mar 10 '16 at 12:37
  • Fair enough then, just posted the answer explaining that. – David Siro Mar 11 '16 at 13:00

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The responses are paired with requests using correlationId header. Clients have to set it to unique value for every request and the server has to set the same value the response queue. That way a client can pair the messages even when they arrive in arbitrary order.

From the RabbitMQ tutorial:

In the method presented above we suggest creating a callback queue for every RPC request. That's pretty inefficient, but fortunately there is a better way - let's create a single callback queue per client.

That raises a new issue, having received a response in that queue it's not clear to which request the response belongs. That's when the correlationId property is used. We're going to set it to a unique value for every request. Later, when we receive a message in the callback queue we'll look at this property, and based on that we'll be able to match a response with a request. If we see an unknown correlationId value, we may safely discard the message - it doesn't belong to our requests.

David Siro
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