[Feature request] Support modification of pre-existing resources from CompositeController

[gavin-nia]

I'm not sure exactly how to express this feature request, but I have a use case like the following:

1. We have an existing deployment that is created by some other system.
2. We'd like to modify some parts of the spec of that existing deployment from our CompositeController.

This is conceptually similar to things like HorizontalPodAutoscaler which can modify "spec.replicas" of deployments, but doesn't take ownership of those resources.

I've used CompositeController to partially achieve what I want (created a new CRD and parent resource, and made the existing deployment a child of the parent via label selectors). The problem with this approach is that when I delete the parent, it will also delete the child which is not what I want to happen (would be quite a big problem if someone does this by accident).

Happy to hear if you've got any ideas on this. I know we can specify "orphan" mode when deleting the resource, but it is tricky to remember about that (and we would ideally like to have a mix of owned and non-owned child objects).

[grzesuav]

It is reasonable thing to request.

[rolandkool]

What about using the Open Policy Agent to modify the deployment before it gets submitted to the Kubernetes API?

[grzesuav]

@rolandkool I think @gavin-nia want to updates those fields based on some logic in its own controller.