RFC: Implement some kind of partialApply for the kustomize-controller

[cwrau]

(Copied from fluxcd/kustomize-controller#1241 and fluxcd/kustomize-controller#1231)

Original issue;

@cwrau

Often one installs dependent configuration at the same time, think postgres-operator and the application with the postgresql.

With flux this is, currently, not possible, as the kustomize-controller fails with

Warning  ReconciliationFailed  3m23s  kustomize-controller  postgresql/discourse/discourse-db dry-run failed: no matches for kind "postgresql" in version "acid.zalan.do/v1"

The current solutions would be to

1. create the resources step-by-step, splitting up normal PRs to multiple:
   1. install postgres-operator
   2. apply application
2. apply the intermediate resources by hand every time

Both of these solutions have the disadvantage to not work with a full recovery with an existing repo.

In my opinion, an option with partialApply: true would be amazing, which would just apply everything that's possible and ignore what's not working, but still failing at the end.

And during the next reconciliation trying again, which,, in time,, would result in a complete working cluster.

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@stefanprodan

There is spec.dependsOn made for this, see here how to use it: fluxcd/flux2-kustomize-helm-example#infrastructure

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@cwrau

There is spec.dependsOn made for this

But what if I don't have anything that can use depensOn or the dependee can't be depended upon?

Imagine the following scenario;

• have application.yaml with plain k8s resources, including a custom postgresql resource
• have operator.yaml with a flux HelmRelease with the operator

that won't work with the dependsOn (or need the solution below)

, see here how to use it: fluxcd/flux2-kustomize-helm-example#infrastructure

And in this example, I'd need to have multiple kustomizations, which,, in my use-case,, really only serve the purpose for ordering

Original PR;

@stefanprodan

This is not something that I would be in favor of, we have dependsOn which allows you to properly apply things in order.

This kind of change would require a RFC, you need to find a Flux maintainer to sponsor your proposal, see here the RFC process fluxcd/flux2@main/rfcs.