All About services.yaml

When Symfony first boots up, it needs to get the full list of all of the services that should be in the container. That includes the service ID, its class name, and all of its constructor arguments. The first and biggest source of services are bundles. If you run

php bin/console debug:container

the vast majority of these services come from bundles. The second place the container gets services from is our code. And to learn about our services, Symfony reads services.yaml.

The Special _defaults Section

At the moment that Symfony starts parsing the first line of this file, nothing in our src/ directory has been registered as a service in the container. This is really important. Adding our classes to the container is, in fact, the job of this file! And the way it does it is pretty amazing. Let's take a tour!

Notice that the config is under a services key. Like parameters, this is a special key. And, like its name suggests, anything under this is meant to configure services.

The first sub-key under this is _defaults. _defaults is a magic key that allows us to define some default options that will be added to all services that are registered in this file. So every service that we register below will automatically have autowire: true and autoconfigure: true.

Let's look at an example. The most basic thing you can do under the services key is... register a service! That's what we're doing at the bottom. This tells the container that there should be an App\Service\MixRepository service in the container and we specified one option: bind.

Services can actually have a bunch of options, including autowire and autoconfigure. So it would be totally legal to say, autowire: true and autoconfigure: true right here. This would work just fine. But thanks to the _defaults section, those aren't needed! The _defaults says:

Unless it's been overridden on a specific service, set autowire and autoconfigure to true for all services in this file.

And what does autowire do? Simple! It tells Symfony's container:

Hey! Please try to guess my constructor arguments by looking at their type-hints.

This feature is pretty awesome... which is why it's automatically turned on for all of our services. The other option - autoconfigure - is more subtle and we'll talk about it later.

Service Auto-Registration

All right, by the time we get to the _defaults line, we've established some default configuration... but we haven't actually registered any services yet. That's the job of the next section... and it's the key to everything. This special syntax says

Please look inside the src/ directory and automatically register all PHP classes as a service... except for these three things.

This is why, immediately after we created the MixRepository class, it was already in the container! And thanks to the _defaults section, any services registered by this will automatically have autowire: true and autoconfigure: true. That's some serious team work! This mechanism is called "Service Auto-Registration".

But remember, every service in the container needs to have a unique ID. If you look back at debug:container, most of the service IDs are snake case. Let me zoom out a bit so it's easier to see. Better! So, for example, the Twig service has the snake case twig ID. But if you scroll up to the top of this list, our MixRepository ID is... the full class name.

Yep! When you use Service Auto-Registration, it uses the class name as both the class and the service ID. This is done for simplicity... but also for autowiring. When we try to autowire MixRepository into our controller or anywhere else, to figure out which service to pass us, Symfony will look for a service whose ID exactly matches App\Service\MixRepository. So Service Auto-Registration not only registers our classes as services, it does it in a way that makes them autowireable. That's awesome!

Auto-Registration of Non-Services?

Anyway, after this section here, every class in src/ is now registered as a service in the container. Except, well... we don't want every class in src/ to be a service.

There are really two types of classes in your app: "Service classes" that do work, and "model classes" - sometimes called "DTOs" - whose job is mostly to hold data - like a Product class with name and price properties. We want the container to handle instantiating our services. But for model classes, we will create them whenever we need them - like with $product = new Product(). So, these will not be services in the container.

In the next tutorial, we'll create Doctrine entity classes, which are model classes for the database. These will live in the src/Entity/ directory... and since they're not meant to be services, that directory is excluded. So we register everything in the src/ directory as a service, except for these three things.

But.. fun fact! This exclude key is not that important. Heck, you could delete it and everything would still work! If you accidentally register something as a service that isn't meant to be a service, no worries! Since you'll never try to autowire and use that class like a service, Symfony will realize it's not being used and remove it from the container. Dang, that is smart!

Custom Service Configuration

So everything in src/ is automatically registered as a service without us needing to do anything or touch this file.

But... occasionally, you'll need to add extra config to a specific service. That's what happened with MixRepository thanks to its non-autowireable $isDebug argument.

To fix that, at the bottom of this file, we're registering a new service whose ID and class is App\Service\MixRepository. This will actually override the service that was created during Service Auto-Registration, since both IDs will match App\Service\MixRepository. So, we're defining a brand new service.

But thanks to _defaults, it automatically has autowire: true and autoconfigure: true. Then we add the additional bind option.

So the only thing we need to put at the bottom of this file are services that need additional configuration to work. And... there's actually a cooler way to fix non-autowireable arguments that I'll show you next.

All Configuration Files are Equals!

But before we get to that, I want to mention one more thing: this file, services.yaml, is loaded via the same system that loads all of the files in config/packages/. In fact, there's no technical difference between this file and say... framework.yaml. That's right! If we wanted to, we could copy and delete the contents of services.yaml, paste them into framework.yaml, and everything would work exactly the same.

Except that... we would need to, y'know, just correct these paths since we're one directory deeper. Watch! I'll move this around real quick and... this still works just fine! Cool! Let's put that back the way it was and... there we go.

The only reason we have a service.yaml file is for organization. It feels good to have one file to "configure your services". The truly important thing is that all of this config lives under the services key. In fact, near the top of this file, you'll notice there's an empty parameters key.

In cache.yaml, we created a parameters key there to register a new parameter. It's really up to us to decide where we want to define this parameter. We can do it in cache.yaml or, to keep all parameters in one spot, we could copy this and move it over to services.yaml.

In cache.yaml, I'll also grab the when@dev, delete that, and paste it into services.yaml. On a technical level, that makes no difference and our app still works. But I like this better. Services and parameters are a global idea in your app... so it's nice to organize them all in one file.

All right, the only reason we wrote any code at the bottom of services.yaml was to tell the container what to pass to the non-autowireable $isDebug argument. But what if I told you there's a more automatic way to solve these problematic arguments? That's next.