Understanding how the Front Controller, Kernel and Environments Work together

3.3 version

Understanding how the Front Controller, Kernel and Environments Work together

The section How to Master and Create new Environments explained the basics on how Symfony uses environments to run your application with different configuration settings. This section will explain a bit more in-depth what happens when your application is bootstrapped. To hook into this process, you need to understand three parts that work together:

Note

Usually, you will not need to define your own front controller or AppKernel class as the Symfony Standard Edition provides sensible default implementations.

This documentation section is provided to explain what is going on behind the scenes.

The Front Controller

The front controller is a well-known design pattern; it is a section of code that all requests served by an application run through.

In the Symfony Standard Edition, this role is taken by the app.php and app_dev.php files in the web/ directory. These are the very first PHP scripts executed when a request is processed.

The main purpose of the front controller is to create an instance of the AppKernel (more on that in a second), make it handle the request and return the resulting response to the browser.

Because every request is routed through it, the front controller can be used to perform global initialization prior to setting up the kernel or to decorate the kernel with additional features. Examples include:

  • Configuring the autoloader or adding additional autoloading mechanisms;
  • Adding HTTP level caching by wrapping the kernel with an instance of AppCache;
  • Enabling (or skipping) the ClassCache;
  • Enabling the Debug Component.

The front controller can be chosen by requesting URLs like:

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http://localhost/app_dev.php/some/path/...

As you can see, this URL contains the PHP script to be used as the front controller. You can use that to easily switch the front controller or use a custom one by placing it in the web/ directory (e.g. app_cache.php).

When using Apache and the RewriteRule shipped with the Symfony Standard Edition, you can omit the filename from the URL and the RewriteRule will use app.php as the default one.

Note

Pretty much every other web server should be able to achieve a behavior similar to that of the RewriteRule described above. Check your server documentation for details or see Configuring a Web Server.

Note

Make sure you appropriately secure your front controllers against unauthorized access. For example, you don't want to make a debugging environment available to arbitrary users in your production environment.

Technically, the bin/console script used when running Symfony on the command line is also a front controller, only that is not used for web, but for command line requests.

The Kernel Class

The Kernel is the core of Symfony. It is responsible for setting up all the bundles that make up your application and providing them with the application's configuration. It then creates the service container before serving requests in its handle() method.

There are two methods declared in the KernelInterface that are left unimplemented in Kernel and thus serve as template methods:

registerBundles()
It must return an array of all bundles needed to run the application.
registerContainerConfiguration()
It loads the application configuration.

To fill these (small) blanks, your application needs to subclass the Kernel and implement these methods. The resulting class is conventionally called the AppKernel.

Again, the Symfony Standard Edition provides an AppKernel in the app/ directory. This class uses the name of the environment - which is passed to the Kernel's constructor method and is available via getEnvironment() - to decide which bundles to create. The logic for that is in registerBundles(), a method meant to be extended by you when you start adding bundles to your application.

You are, of course, free to create your own, alternative or additional AppKernel variants. All you need is to adapt your (or add a new) front controller to make use of the new kernel.

Note

The name and location of the AppKernel is not fixed. When putting multiple Kernels into a single application, it might therefore make sense to add additional sub-directories, for example app/admin/AdminKernel.php and app/api/ApiKernel.php. All that matters is that your front controller is able to create an instance of the appropriate kernel.

Having different AppKernels might be useful to enable different front controllers (on potentially different servers) to run parts of your application independently (for example, the admin UI, the front-end UI and database migrations).

Note

There's a lot more the AppKernel can be used for, for example overriding the default directory structure. But odds are high that you don't need to change things like this on the fly by having several AppKernel implementations.

The Environments

As just mentioned, the AppKernel has to implement another method - registerContainerConfiguration(). This method is responsible for loading the application's configuration from the right environment.

Environments have been covered extensively in the previous article, and you probably remember that the Symfony Standard Edition comes with three of them - dev, prod and test.

More technically, these names are nothing more than strings passed from the front controller to the AppKernel's constructor. This name can then be used in the registerContainerConfiguration() method to decide which configuration files to load.

The Symfony Standard Edition's AppKernel class implements this method by simply loading the app/config/config_*environment*.yml file. You are, of course, free to implement this method differently if you need a more sophisticated way of loading your configuration.


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