I have a question concerning the third RabbitMQ tutorial. I am trying to implement something similar, except there is no guarantee the consumer(s) would be running at the time the producer sends a message to the exchange.
So, I have my producer which publishes the messages to a fanout exchange:
$channel->exchange_declare('my_exchange', 'fanout', false, false, false);
$channel->basic_publish('my_message', 'my_exchange');
In my publishers, I declare queues, which I then bind to the exchange:
list($queueName,, ) = $channel->queue_declare("", false, false, true, false);
$channel->queue_bind($queueName, 'my_exchange');
And this is where my problem has it's root. The tutorial says:
The messages will be lost if no queue is bound to the exchange yet, but that's okay for us; if no consumer is listening yet we can safely discard the message.
Is there a way to somehow preserve those messages, so when a consumer starts, it would access the previously sent messages? The only way I figured out how to do it is to declare the same queue in my producer and my publisher, but it kind of defeats the purpose of having an exchange and separate queues for different consumers.
The queues need to exist, doesn't matter really who/what creates them: it can be producer (althoug I would strongly discourage this), consumer, some third admin app that just creates queus via rest api, rabbitmqctl... If you want to consume the queue(s) later, just make sure that they're durable and that TTL for messages is long enough (also durable messages if needed). But beware that your queue(s) don't get into flow state.
The only way I figured out how to do it is to declare the same queue in my producer and my publisher, but it kind of defeats the purpose of having an exchange and separate queues for different consumers.
First - I think you meant to say in my producer and my subscriber :)
Second, separate queues for consumers (or queue per consumer) is just in this example. Bare in mind that this is for a fanout exchange, and each consumer decalres an exclusive queue - when the consumer disconnects, the queue is gone. And that's why that's okay for us, because we're simply broadcasting and who wants the broadcast (the messages) needs to get it. Fanout exchange just puts messages to all the queues bound to it, that's it.
It's perfectly ok to have multiple consumers consuming from same queue (look at tutorial 2).
So you just need to consider your use case. Of course it doesn't make sense to create fanout exchange and pre-setup the queus for the consumers... Perhaps you need just some routing keys or something else.
In this example (so tutorial 3) it's ment that there is a brodcast of messages, and if no one get's them, not a big (or small) deal. If anyone wants them, they need to get them. It's like a tv channel - regardless if someone is watching or not, the signal goes on.