Q/A

Changes description

This PR introduces very similar changes to this one: #34737, which was closed due to the lack of the feedback.
I'd like to continue this topic, share the missing feedback and discuss it further if needed.

I have introduced the possibility to configure exchange-to-exchange bindings in amqp transport. This feature uses \AMQPExchange::bind() method already provided by the stub in php-amqp/php-amqp: https://github.com/php-amqp/php-amqp/blob/bb7611220e341039a7f5d72e606ca1e16eda4642/stubs/AMQPExchange.php#L22

Example messenger.yaml:

framework:
    messenger:
        transports:
            some_transport:
                dsn: 'amqp://'
                options:
                    exchange:
                        type: topic
                        name: some_exchange
                        bindings: # added configuration
                            another_exchange:
                                binding_keys:
                                    - key1
                                    - key2
                                binding_arguments:
                                    x-match: all

With the above configuration, the Connection class creates some_exchange and binds it to another_exchange using ['key1', 'key2'] keys and [x-match =>'all'] arguments.

Reasoning

Binding an exchange to an exchange feature can be used to create more complex RabbitMQ topologies. It is also briefly described here: https://www.cloudamqp.com/blog/exchange-to-exchange-binding-in-rabbitmq.html

A real-world example could be kind of RabbitMQ publisher/subscriber pattern implementation between microservices, that could be visualized as follows:

In the above example (all the exchanges in this example are of the topic type):

App `Foo` publishes events to its own exchange AND subscribes to the events that app `Bar` publishes on its exchange.
App `Bar` publishes events to its own exchange AND subscribes to the events that app `Foo` publishes on its exchange.
App `...` subscribes to the events that apps `Foo` and `Bar` publish on their exchanges.

This approach might have few advantages, such as easier maintainability, dependency management and monitoring, or better separation between microservices.

My thoughts

I feel that the fact, that https://github.com/php-amqp/php-amqp has \AMQPExchange::bind() implemented is already sufficient reason to have this supported in symfony/amqp-messenger. I am aware this feature might be rarely used, but it's already there in the extension, and having the ability to use it within amqp-messenger seems reasonable to me.