You are browsing the Symfony 4 documentation, which changes significantly from Symfony 3.x. If your app doesn't use Symfony 4 yet, browse the Symfony 3.4 documentation.

The Architecture

The Architecture

You are my hero! Who would have thought that you would still be here after the first two parts? Your efforts will be well-rewarded soon. The first two parts didn't look too deeply at the architecture of the framework. Because it makes Symfony stand apart from the framework crowd, let's dive into the architecture now.

Add Logging

A new Symfony app is micro: it's basically just a routing & controller system. But thanks to Flex, installing more features is simple.

Want a logging system? No problem:

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$ composer require logger

This installs and configures (via a recipe) the powerful Monolog library. To use the logger in a controller, add a new argument type-hinted with LoggerInterface:

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use Psr\Log\LoggerInterface;
// ...

public function index($name, LoggerInterface $logger)
{
    $logger->info("Saying hello to $name!");

    // ...
}

That's it! The new log message will be written to var/log/dev.log. Of course, this can be configured by updating one of the config files added by the recipe.

Services & Autowiring

But wait! Something very cool just happened. Symfony read the LoggerInterface type-hint and automatically figured out that it should pass us the Logger object! This is called autowiring.

Every bit of work that's done in a Symfony app is done by an object: the Logger object logs things and the Twig object renders templates. These objects are called services and they are tools that help you build rich features.

To make life awesome, you can ask Symfony to pass you a service by using a type-hint. What other possible classes or interfaces could you use? Find out by running:

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$ php bin/console debug:autowiring
Class/Interface Type Alias Service ID
Psr\Cache\CacheItemPoolInterface alias for "cache.app.recorder"
Psr\Log\LoggerInterface alias for "monolog.logger"
Symfony\Component\EventDispatcher\EventDispatcherInterface alias for "debug.event_dispatcher"
Symfony\Component\HttpFoundation\RequestStack alias for "request_stack"
Symfony\Component\HttpFoundation\Session\SessionInterface alias for "session"
Symfony\Component\Routing\RouterInterface alias for "router.default"

This is just a short summary of the full list! And as you add more packages, this list of tools will grow!

Creating Services

To keep your code organized, you can even create your own services! Suppose you want to generate a random greeting (e.g. "Hello", "Yo", etc). Instead of putting this code directly in your controller, create a new class:

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// src/GreetingGenerator.php
namespace App;

class GreetingGenerator
{
    public function getRandomGreeting()
    {
        $greetings = ['Hey', 'Yo', 'Aloha'];
        $greeting = $greetings[array_rand($greetings)];

        return $greeting;
    }
}

Great! You can use this immediately in your controller:

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use App\GreetingGenerator;
// ...

public function index($name, LoggerInterface $logger, GreetingGenerator $generator)
{
    $greeting = $generator->getRandomGreeting();

    $logger->info("Saying $greeting to $name!");

    // ...
}

That's it! Symfony will instantiate the GreetingGenerator automatically and pass it as an argument. But, could we also move the logger logic to GreetingGenerator? Yes! You can use autowiring inside a service to access other services. The only difference is that it's done in the constructor:

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+ use Psr\Log\LoggerInterface;

class GreetingGenerator
{
+     private $logger;
+
+     public function __construct(LoggerInterface $logger)
+     {
+         $this->logger = $logger;
+     }

    public function getRandomGreeting()
    {
        // ...

 +        $this->logger->info('Using the greeting: '.$greeting);

         return $greeting;
    }
}

Yes! This works too: no configuration, no time wasted. Keep coding!

Twig Extension & Autoconfiguration

Thanks to Symfony's service handling, you can extend Symfony in many ways, like by creating an event subscriber or a security voter for complex authorization rules. Let's add a new filter to Twig called greet. How? Just create a class that extends AbstractExtension:

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// src/Twig/GreetExtension.php
namespace App\Twig;

use App\GreetingGenerator;
use Twig\Extension\AbstractExtension;
use Twig\TwigFilter;

class GreetExtension extends AbstractExtension
{
    private $greetingGenerator;

    public function __construct(GreetingGenerator $greetingGenerator)
    {
        $this->greetingGenerator = $greetingGenerator;
    }

    public function getFilters()
    {
        return [
            new TwigFilter('greet', [$this, 'greetUser']),
        ];
    }

    public function greetUser($name)
    {
        $greeting =  $this->greetingGenerator->getRandomGreeting();

        return "$greeting $name!";
    }
}

After creating just one file, you can use this immediately:

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{# Will print something like "Hey Symfony!" #}
<h1>{{ name|greet }}</h1>

How does this work? Symfony notices that your class extends AbstractExtension and so automatically registers it as a Twig extension. This is called autoconfiguration, and it works for many many things. Just create a class and then extend a base class (or implement an interface). Symfony takes care of the rest.

Blazing Speed: The Cached Container

After seeing how much Symfony handles automatically, you might be wondering: "Doesn't this hurt performance?" Actually, no! Symfony is blazing fast.

How is that possible? The service system is managed by a very important object called the "container". Most frameworks have a container, but Symfony's is unique because it's cached. When you loaded your first page, all of the service information was compiled and saved. This means that the autowiring and autoconfiguration features add no overhead! It also means that you get great errors: Symfony inspects and validates everything when the container is built.

Now you might be wondering what happens when you update a file and the cache needs to rebuild? I like you're thinking! It's smart enough to rebuild on the next page load. But that's really the topic of the next section.

Development Versus Production: Environments

One of a framework's main jobs is to make debugging easy! And our app is full of great tools for this: the web debug toolbar displays at the bottom of the page, errors are big, beautiful & explicit, and any configuration cache is automatically rebuilt whenever needed.

But what about when you deploy to production? We will need to hide those tools and optimize for speed!

This is solved by Symfony's environment system and there are three: dev, prod and test. Based on the environment, Symfony loads different files in the config/ directory:

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config/
├─ services.yaml
├─ ...
└─ packages/
    ├─ framework.yaml
    ├─ ...
    ├─ **dev/**
        ├─ monolog.yaml
        └─ ...
    ├─ **prod/**
        └─ monolog.yaml
    └─ **test/**
        ├─ framework.yaml
        └─ ...
└─ routes/
    ├─ annotations.yaml
    └─ **dev/**
        ├─ twig.yaml
        └─ web_profiler.yaml

This is a powerful idea: by changing one piece of configuration (the environment), your app is transformed from a debugging-friendly experience to one that's optimized for speed.

Oh, how do you change the environment? Change the APP_ENV environment variable from dev to prod:

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# .env
- APP_ENV=dev
+ APP_ENV=prod

But I want to talk more about environment variables next. Change the value back to dev: debugging tools are great when you're working locally.

Environment Variables

Every app contains configuration that's different on each server - like database connection information or passwords. How should these be stored? In files? Or some other way?

Symfony follows the industry best practice by storing server-based configuration as environment variables. This means that Symfony works perfectly with Platform as a Service (PaaS) deployment systems as well as Docker.

But setting environment variables while developing can be a pain. That's why your app automatically loads a .env file, if the APP_ENV environment variable isn't set in the environment. The keys in this file then become environment variables and are read by your app:

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# .env
###> symfony/framework-bundle ###
APP_ENV=dev
APP_SECRET=cc86c7ca937636d5ddf1b754beb22a10
###< symfony/framework-bundle ###

At first, the file doesn't contain much. But as your app grows, you'll add more configuration as you need it. But, actually, it gets much more interesting! Suppose your app needs a database ORM. Let's install the Doctrine ORM:

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$ composer require doctrine

Thanks to a new recipe installed by Flex, look at the .env file again:

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###> symfony/framework-bundle ###
APP_ENV=dev
APP_SECRET=cc86c7ca937636d5ddf1b754beb22a10
###< symfony/framework-bundle ###

+ ###> doctrine/doctrine-bundle ###
+ # ...
+ DATABASE_URL=mysql://db_user:db_password@127.0.0.1:3306/db_name
+ ###< doctrine/doctrine-bundle ###

The new DATABASE_URL environment variable was added automatically and is already referenced by the new doctrine.yaml configuration file. By combining environment variables and Flex, you're using industry best practices without any extra effort.

Keep Going!

Call me crazy, but after reading this part, you should be comfortable with the most important parts of Symfony. Everything in Symfony is designed to get out of your way so you can keep coding and adding features, all with the speed and quality you demand.

That's all for the quick tour. From authentication, to forms, to caching, there is so much more to discover. Ready to dig into these topics now? Look no further - go to the official Symfony Documentation and pick any guide you want.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.