Document DI Kernel infrastructure for HTTP-less apps

bundles.rst

@@ -62,6 +62,13 @@ Start by creating a new class called ``AcmeBlogBundle``::
     {
     }
 
+.. tip::
+
+    If your bundle is designed for an application that does not use the
+    HttpKernel component (e.g. a console tool or worker), you can extend
+    :class:`Symfony\\Component\\DependencyInjection\\Kernel\\AbstractBundle`
+    instead. See :ref:`building-http-less-applications` for more details.
+
 .. warning::
 
     If your bundle must be compatible with previous Symfony versions you have to

configuration/front_controllers_and_kernel.rst

@@ -242,6 +242,70 @@ includes the following:
     You can change the cache directory location and name. For more information
     read the article :doc:`/configuration/override_dir_structure`.
 
+.. _building-http-less-applications:
+
+Building HTTP-less Applications
+-------------------------------
+
+.. versionadded:: 8.1
+
+    The DependencyInjection Kernel infrastructure was introduced in Symfony 8.1.
+
+If your application does not handle HTTP requests (e.g. a console tool, a
+message consumer or a background worker), you can build a kernel that uses the
+full Symfony dependency injection infrastructure without requiring the HttpKernel
+component.
+
+The DependencyInjection component provides a set of classes in the
+``Symfony\Component\DependencyInjection\Kernel`` namespace that mirror the
+traditional HttpKernel-based classes:
+
+=================================================  ================================================================
+HTTP-less (DependencyInjection)                    HTTP (HttpKernel)
+=================================================  ================================================================
+``DependencyInjection\Kernel\KernelInterface``     ``HttpKernel\KernelInterface``
+``DependencyInjection\Kernel\AbstractKernel``      ``HttpKernel\Kernel``
+``DependencyInjection\Kernel\KernelTrait``         ``FrameworkBundle\Kernel\MicroKernelTrait``
+``DependencyInjection\Kernel\AbstractBundle``      ``HttpKernel\Bundle\AbstractBundle``
+``DependencyInjection\Kernel\BundleInterface``     ``HttpKernel\Bundle\BundleInterface``
+=================================================  ================================================================
+
+To create an HTTP-less kernel, extend
+:class:`Symfony\\Component\\DependencyInjection\\Kernel\\AbstractKernel` and use the
+:class:`Symfony\\Component\\DependencyInjection\\Kernel\\KernelTrait`::
+
+    // src/Kernel.php
+    namespace App;
+
+    use Symfony\Component\DependencyInjection\Kernel\AbstractKernel; // new in 8.1
+    use Symfony\Component\DependencyInjection\Kernel\KernelTrait; // new in 8.1
+
+    class Kernel extends AbstractKernel
+    {
+        use KernelTrait;
+
+        // The KernelTrait provides a default configureContainer() method
+        // that loads config/packages/*.{php,yaml} and config/services.{yaml,php}.
+        // Override it only if you need a custom configuration setup.
+    }
+
+The ``KernelTrait`` provides the same container lifecycle as ``MicroKernelTrait``
+(building, compiling, caching the service container) but without any HTTP-related
+logic. It follows the same conventions: ``config/bundles.php`` for bundle
+registration, ``config/packages/`` for configuration and ``config/services.yaml``
+(or ``.php``) for service definitions.
+
+Code that only needs to interact with the kernel for dependency injection
+purposes (bundles, container, environment) can now type-hint
+``Symfony\Component\DependencyInjection\Kernel\KernelInterface``
+instead of ``Symfony\Component\HttpKernel\KernelInterface``.
+
+.. tip::
+
+    The ``getLogDir()`` method is nullable on ``KernelTrait``. To disable
+    the log directory (e.g. for a console-only application), set the
+    ``APP_LOG_DIR`` environment variable to ``false``.
+
 .. _`front controller`: https://en.wikipedia.org/wiki/Front_Controller_pattern
 .. _`decorate`: https://en.wikipedia.org/wiki/Decorator_pattern
 .. _Debug component: https://github.com/symfony/debug