Wondering if we could also go with a SingleAppKernel class like this:

abstract class SingleAppKernel extends Kernel
{
    use MicroKernelTrait;

    return static function init(): \Closure
    {
        return static function (array $context) {
            return new static($context['APP_ENV'], (bool) $context['APP_DEBUG']);
        };
    }

    public function registerBundles(): iterable
    {
        yield new FrameworkBundle();
    }
}

and then reduce this use case to:

final class App extends SingleAppKernel
{
    #[Route('/')]
    public function __invoke(): Response
    {
        return new Response('Hello World!');
    }
}

return App::init();

?

Not convinced by SingleAppKernel. Could be OK to provide a fallback for registerBundles.

What is the use-case for defining an invokable controller in the kernel? This is a convention for action controllers with a class per controller, I don't think we should apply that convention to other classes. Replacing __invoke with index or controller or whatever is already fully supported.

I'm also not sure about the SingleAppKernel class. To me, the simple micro kernel is a nice gimmick but has a very limited use-case (it requires more work to create and maintain an application using this than it is to use a normal Symfony app). For this specific niche, the existing (with the fix for optional config/ directory) solution seems more than good enough to me.

@wouterj, the __invoke method is already supported. I'm working on a Cloud worker app that only requires one controller and it's nice because the class name already provides the semantics that I need for this small piece of code. The only thing missing was enabling the service arguments capability (which is being added here).

Well, I'm really happy with how small it is now, even though we still have to set up the closure function ourselves:

return static function (array $context) {
    $kernel = new StripeWebhookEventSubscriber($context['APP_ENV'], (bool) $context['APP_DEBUG']);

    return \in_array(\PHP_SAPI, ['cli', 'phpdbg', 'embed'], true) ? new Application($kernel) : $kernel;
};

Any idea to simplify it more? In a nutshell, I'd love to just initialize my kernel/app and run. Random proposals:

return fn () => StripeWebhookEventSubscriber::class; // and then the Runtime does the rest new $kernelClass(...)

return StripeWebhookEventSubscriber::init(); // implement a default in `MicroKernelTrait`

return new StripeWebhookEventSubscriber(); // and set up the env and debug vars using setter through the Runtime?

I've updated the PR description to include my use case as code example

It's ready on my side. Thank you all for your reviews!

Thank you @yceruto.

Not sure why, but after this commit my controllers and commands are no longer loaded. Can be reproduced with a fresh installation with the symfony cli with --version=next.

Oops! Fixed in #57455