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Events & Event Bus


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Messenger is a "message bus". And it turns out that a "message" is a pretty generic term in computer science. In fact, there are three types of messages you'll commonly hear about.

Messages: Commands, Events & Queries

The first type of message is a "command". And that is the type we've been creating so far: we create message classes that sound like a command: AddPonkaToImage or DeleteImagePost and whose handlers do some action. When you create message classes & handlers that look like this, you're using Messenger as a "command bus". And one of the, sort of, "rules" of commands is that each command should have exactly one handler. That's the "command" design pattern.

The second type of message is an "event". If you create an "event" class and pass it to Messenger, then you're using Messenger as an "event" bus. The difference between what a "command" class looks like and what an "event" class looks like is subtle: it comes down to naming conventions and what you're ultimately trying to accomplish. An event is dispatched after something happens and can have zero to many handlers. Don't worry, we'll see what this looks like soon.

The third type of message is a "query" and we'll talk about those later. For now, let's focus on understanding events and how they're different from commands... because... it can be super confusing. And Messenger, being a generic "message bus" works perfectly with either.

Creating a Second Bus

Before we create our first event, I'll close a few things and then open config/packages/messenger.yaml. If our app leverages both commands and events, it's totally ok to use just one bus to handle all of that. But, in the interest of making our life a bit more difficult and learning more, let's continue to use our existing bus only as a command bus and create a new bus to only use with events.

To do that, under the buses: key, add a new one called, how about, event.bus. Set this to ~ which is null... just because we don't have any other configuration that we need to put here yet. This will cause a new MessageBus service to be added to the container.

32 lines | config/packages/messenger.yaml
// ... lines 4 - 7
event.bus: ~
// ... lines 9 - 32

So far, whenever we needed the message bus - like in ImagePostController - we autowired it by using the MessageBusInterface type-hint. The question now is: how can we get access to the new message bus service?

Find your terminal and run:

php bin/console debug:autowiring

... which... explodes! My bad:

Invalid configuration for path framework.messenger: you must specify default_bus

Copy the name of the default bus. Once you define more than one bus, you need a default_bus key set to your "main" bus. This tells Symfony which MessageBus service to pass us when we use the MessageBusInterface type-hint.

34 lines | config/packages/messenger.yaml
default_bus: messenger.bus.default
// ... lines 6 - 9
event.bus: ~
// ... lines 11 - 34

Try the debug:autowiring command again... and search for "mess".

php bin/console debug:autowiring

Ah, now we have two entries! This tells me that if we use the MessageBusInterface type-hint, we'll get the messenger.bus.default service. Ignore the debug.traced part - that's just Symfony adding some debug tools. But now, if you use the MessageBusInterface type-hint and you name the argument $eventBus, it will pass you the new event bus service!

This is a new feature in Symfony 4.2 where you can autowire things by a combination of the type-hint and argument name. Symfony took the name of our bus - event.bus - and made it possible to use the $eventBus argument name.

Differences Between Buses

Great! We now know how to get the event bus! But.. what's the difference between these two buses? Do they behave differently? The answer is... no!

A bus is nothing more than a set of middleware. If you have two bus objects that have the same middleware... well then... those message buses effectively are identical! So, other than the fact that, so far, we've only added our AuditMiddleware to the first bus, these buses will work and act identically. That's why, even though I've created one service to handle commands and another service to handle events... ah... we really could send all our commands and events to just one service.

Next, let's create an event, learn what it looks like, why we might use them, and how they're different than commands.