Dynamic AMQP Routing Key (AmqpStamp)

Let's repeat the new exchange setup for the async_priority_high transport: we want this to deliver to the same direct exchange, but then use a different routing key to route messages to a different queue.

Change the exchange to messages, set the type to direct, then use default_publish_routing_key to automatically attach a routing key called high to each message.

Below, for the messages_high queue, this tells Messenger that we want this queue to be created and bound to the exchange. That's cool, but we now need that binding to have a routing key. Set binding_keys to [high].

How can we trigger Messenger to create that new queue and add the new binding? Just perform any operation that uses this transport... like uploading a photo! Ok, go check out the RabbitMQ manager - start with Exchanges.

Yep, we still have just one messages exchange... but now it has two bindings! If you send a message to this exchange with a high routing key, it will be sent to message_high.

Click "Queues" to see... nice - a new messages_high queue with one message waiting inside.

And... we're done! This new setup has the same end-result: each transport ultimately delivers messages to a different queue. Let's go consume the waiting messages: consume async_priority_high then async.

php bin/console messenger:consume -vv async_priority_high async

And it consumes them in the correct order: handling AddPonkaToImage first because that's in the high priority queue and then moving to messages in the other queue.

By the way, when we consume from the async transport, for example, behind-the-scenes, it means that Messenger is reading messages from any queue that's configured for that transport. In our app, each transport has config for only one queue, but you could configure multiple queues under a transport and even set different binding keys for each one. But when you consume that transport, you'll be consuming messages from every queue you've configured.

Dynamic Routing Keys

So, let's back up and look at the whole flow. When we dispatch an AddPonkaToImage object, our Messenger routing config always routes this to the async_priority_high transport. This causes the message to be sent to the messages exchange with a routing key set to high... and the binding logic means that it will ultimately be delivered to the messages_high queue.

Due to the way that Messenger's routing works - the fact that you route a class to a transport - every message class will always be delivered to the same queue. But what if you did want to control this dynamically? What if, at the moment you dispatch a message, you needed to send that message to a different transport than normal? Maybe you decide that this particular AddPonkaToImage message is not important and should be routed to async.

Well... that's just not possible with Messenger: each class is always routed to a specific transport. But this end-result is possible... if you know how to leverage routing keys.

Here's the trick: what if we could publish an AddPonkaToImage object... but tell Messenger that when it sends it to the exchange, it should use the normal routing key instead of high? Yea, the message would technically still be routed to the async_priority_high transport... but it would ultimately end up in the messages_normal queue. That would do it!

Is that possible? Totally! Open up ImagePostController and find where we dispatch the message. After the DelayStamp, add a new AmqpStamp - but be careful not to choose AmqpReceivedStamp - that's something different... and isn't useful for us. This stamp accepts a few arguments and the first one - gasp! - is the routing key to use! Pass this normal.

Let's try it! Stop the worker so we can see what happens internally. Then, upload a photo, go to the RabbitMQ manager, click on queues... refresh until you see the message in the right queue... we have to wait for the delay... and there it is! It ended up in messages_normal.

What else can you Customize on an Amqp Message?

By the way, if you look inside this AmqpStamp class, the second and third arguments are for something called $flags and $attributes. These are a bit more advanced, but might just come in handy. I'll hit Shift+Shift to open a file called Connection.php - make sure to open the one in the AmqpExt directory. Now search for a method called publishOnExchange().

When a message is sent to RabbitMQ, this is the low-level method that actually does that sending. Those $flags and $attributes from the stamp are used here! Passed as the third and fourth arguments to some $exchange->publish() method. Hold Cmd or Ctrl and click to jump into that method.

Oh! This jumps us to a "stub" - a "fake" method & declaration... because this class - called AMQPExchange is not something you'll find in your vendor/ directory. Nope, this class comes from the AMQP PHP extension that we installed earlier.

So, if you find that you really need to control something about how a message is published through this extension, you can do that with the $flags and $attributes. The docs above this do a nice job of showing you the options.

And... that's it for AMQP and RabbitMQ! Sure, there's more to learn about RabbitMQ - it's a huge topic on its own - but you now have a firm grasp of its most important concepts and how they work. And unless you need to do some pretty advanced stuff, you understand plenty to work with Messenger.

Next, up until now we've been sending messages from our Symfony app and consuming them from that same app. But, that's not always the case. One of the powers of a "message broker" like RabbitMQ is the ability to send messages from one system and handle them in a totally different system... maybe on a totally different server or written in a totally different language. Craziness!

But if we're going to use Messenger to send messages to a queue that will then be handled by a totally different app... we probably need to encode those messages as JSON... instead of the PHP serialized format we're using now.