Symfony supports the concept of just a single DI container for the entire application. If the configuration file becomes too big it can be split into separate files with includes, but it does not support isolating separate groups of services in separate containers. This is true despite the fact that the documentation explains how to create containers from scratch with configuration files.

It may be desirable in some applications to provide sub-containers that contain services of just one type for scenarios where container injection is desirable or required, but one does not want to inject the main container with all services. This is already possible by defining a ContainerInterface service that loads a custom configuration file as shown in the aforementioned documentation section. However, even though only services defined in the sub-container can be retrieved from that container, some of the services in the sub-container may have dependencies on services in the main container. For this use-case, it is important to establish a parent/child relationship between the sub-container and the main Symfony container in which the child container defers to the parent for dependencies it cannot fulfil.

Zend Framework already supports separate containers for different groupings of services, such as controllers and forms, but it's a little less trivial in Symfony due to the compiler steps. We cannot simply override get() to defer to the parent container for missing service definitions because compilation will fail before we can ever make a get() call. The compiler must be made aware of the parent container and either copy in or link to the definitions it needs from the parent according to the dependency graph.