Producing and consuming messages with no declared queue

Question

Where are the messages stored (in rabbit) when you produce a message and send it without declaring a queue or mentioning it in basic_publish? The code I have to work with looks something like this:

... bunch of setup code (no queue declaring tho)...
channel.exchange_declare(exchange='name', type='direct')
channel.basic_publish(exchange='exch_name', routing_key='rkey', body='message' )
conn.close()

I've looked through the web to my abbilities but haven't found an answer to this. I have a hunch that rabbit creates a queue for as long as this message isn't consumed, and my worries are that this would be quite heavy for the rabbit if it has to declare this queue and then destroy it several (thousand!?) times per minute/hour.

Answer 1

When you publish you (usually) publish to an exchange, as you are doing. The exchange decides what to do with that message. If there is nothing to do with the message it is discarded. If there is something to do with the message then it is routed accordingly.

In your original code snippet where there is not queue declared the message will be discarded.

As you say in your comment there was a queue created by the producer. There are options here that you haven't stated. I will try to run through the possibilities. Usually you would declared the queue in the consumer. However if you wish to make sure that they consumer sees all the messages then the queue must be created by the producer and bound to the exchange by the producer to ensure that every message ends up in this queue. Then when the queue is consumed by the consumer it will see all the messages. Alternatively you can create the queue externally from the code as a non autodelete queue and possibly as a durable queue (this will keep the queue even if you restart RabbitMQ) using the commandline or management gui. You will still need to do a declaration in the producer for the exchange in order to send and a declaration in the consumer to receive but they exchange and queue will already exist and you will just simply be connecting to them.

Queues and exchanges are not persistent they are durable or not, which means they will exist after restarting RabbitMQ. Queues have autodelete so that when they consumer is disconnected from them they will no longer exist.

Messages can be persistent, so that if you send a message to an exchange that will be routed to a queue, the message is not read and the RabbitMQ is restarted the message will still be there upon restart. Even if a message is not persistent if the Queue is not Durable then it will be lost, or if the message is not routed to a queue in the first place.

Make sure that after you create a queue you bind the queue properly to the exchange using the same key that you are using as the routing key for your messages.