server side dryRun should be used before apply

[redbaron]

One of the problem with kubectl apply is that it can apply some resources before encountering problem with another.

Many of errors can be caught dryRun (https://kubernetes.io/blog/2019/01/14/apiserver-dry-run-and-kubectl-diff/) , which has an added benefit of invoking validating admission controllers.

kapp apply should make a single dryRun apply and if comes clean proceed to the actual apply.

[cppforlife]

that sounds like a good idea. ill check it out shortly.

[100mik]

We will need to make the value of a flag like this available as a part of :

1. ResourceImplOpts
   So that we can pass the DryRun option to api-machinery for Create(), Update(), Patch() and Delete() API calls.
2. AddOrUpdateChangeOpts
   So that kapp does not wait for changes to appear and does not attempt to do a Get() API call for the resource.

[ringerc]

Take some care with this - I've seen situations where a dry-run will fail, but apply succeeds. Usually due to a prior incomplete or broken apply. It'll be important to support a flag to skip the dry-run.