High Priority Transports

The two messages that we we're sending to the async transport are AddPonkaToImage and DeletePhotoFile, which handles deleting the physical file from the filesystem. And... that second one isn't something the user actually notices or cares about - it's just housekeeping. If it happened 5 minutes from now or 10 days from now, the user wouldn't care.

This creates an interesting situation. Our worker handles things in a first-in-first-out basis: if we send 5 messages to the transport, the worker will handle them in the order in which they were received. This means that if a bunch of images are deleted and then someone uploads a new photo... the worker will process all of those delete messages before finally adding Ponka to the photo. And that... isn't ideal.

The truth is that AddPonkaToImage messages should have a higher priority in our system than DeletePhotoFile: we always want AddPonkaToImage to be handled before any DeletePhotoFile messages... even if they were added first.

Creating the "high" Priority Transport

So... can we set a priority on messages? Not exactly. It turns out that in the queueing world, this is solved by creating multiple queues and giving each of those a priority. In Symfony Messenger, that translates to multiple transports.

Below the async transport, create a new transport called, how about, async_priority_high. Let's use the same DSN as before, which in our case is using doctrine. Below, add options, then queue_name set to high. The name high isn't important - we could use anything. The queue_name option is specific to the Doctrine transport and ultimately controls the value of a column in the table, which operates like a category and allows us to have multiple "queues" of messages inside the same table. And also, for any transport, you can configure these options as query parameters on the DSN or under this options key.

At this point we have three queues - which are all stored in the same table in the database, but with different queue_name values. And now that we have this new transport, we can route AddPonkaToImage to async_priority_high.

Consuming Prioritized Transports

If we stopped now... all we've really done is make it possible to send these two different message classes to two different queues. But there's nothing special about async_priority_high. Sure, I put the word "high" in its name, but it's no different than async.

The real magic comes from the worker. Find your terminal where the worker is running and hit Control+C to stop it. If you just run messenger:consume without any arguments and you have more than one transport, it asks you which transport you want to consume:

php bin/console messenger:consume

Meaning, which transport do you want to receive messages from. But actually, you can read messages from multiple transports at once and tell the worker which should be read first. Check this out: I'll say async_priority_high, async.

This tells the worker: first ask async_priority_high if it has any messages. If it doesn't, then go check the async transport.

We should be able to see this in action. I'll refresh the page, delete a bunch of images here as fast as I can and then upload a couple of photos. Check the terminal output:

It's handles DeletePhotoFile then... AddPonkaToImage, another AddPonkaToImage, another AddPonkaToImage and... yea! It goes back to handling the lower-priority DeletePhotoFile.

So, in the beginning - before we uploaded - it did consume a few DeletePhotoFile messages. But as soon as it saw a message on that async_priority_high transport, it consumed all of those until it was empty. When it was, it then returned to consuming messages from async.

Basically, each time the worker looks for the next message, it checks the highest priority transport first and only asks the next transport - or transports - if it's empty.

And... that's it! Create a new transport for however many different priority "levels" you need, then tell the worker command which order to process them. Oh, and instead of using this interactive way of doing things, you can run:

php bin/console messenger:consume async_priority_high async

Perfect. Next, let's talk about one option we can use to make it easier to develop while using queues... because always needing to remember to run the worker command while coding can be a pain.