Container Building Workflow

Container Building Workflow

The location of the files and classes related to the Dependency Injection component depends on the application, library or framework in which you want to use the container. Looking at how the container is configured and built in the Symfony full-stack Framework will help you see how this all fits together, whether you are using the full-stack framework or looking to use the service container in another application.

The full-stack framework uses the HttpKernel component to manage the loading of the service container configuration from the application and bundles and also handles the compilation and caching. Even if you are not using HttpKernel, it should give you an idea of one way of organizing configuration in a modular application.

Working with a Cached Container

Before building it, the kernel checks to see if a cached version of the container exists. The HttpKernel has a debug setting and if this is false, the cached version is used if it exists. If debug is true then the kernel checks to see if configuration is fresh and if it is, the cached version of the container is used. If not then the container is built from the application-level configuration and the bundles's extension configuration.

Read Dumping the Configuration for Performance for more details.

Application-level Configuration

Application level config is loaded from the app/config directory. Multiple files are loaded which are then merged when the extensions are processed. This allows for different configuration for different environments e.g. dev, prod.

These files contain parameters and services that are loaded directly into the container as per Setting Up the Container with Configuration Files. They also contain configuration that is processed by extensions as per Managing Configuration with Extensions. These are considered to be bundle configuration since each bundle contains an Extension class.

Bundle-level Configuration with Extensions

By convention, each bundle contains an Extension class which is in the bundle's DependencyInjection directory. These are registered with the ContainerBuilder when the kernel is booted. When the ContainerBuilder is compiled, the application-level configuration relevant to the bundle's extension is passed to the Extension which also usually loads its own config file(s), typically from the bundle's Resources/config directory. The application-level config is usually processed with a Configuration object also stored in the bundle's DependencyInjection directory.

Compiler Passes to Allow Interaction between Bundles

Compiler passes are used to allow interaction between different bundles as they cannot affect each other's configuration in the extension classes. One of the main uses is to process tagged services, allowing bundles to register services to be picked up by other bundles, such as Monolog loggers, Twig extensions and Data Collectors for the Web Profiler. Compiler passes are usually placed in the bundle's DependencyInjection/Compiler directory.

Compilation and Caching

After the compilation process has loaded the services from the configuration, extensions and the compiler passes, it is dumped so that the cache can be used next time. The dumped version is then used during subsequent requests as it is more efficient.


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