kubectl diff fails on AKS - kubernetes

Question

I'd like to diff a Kubernetes YAML template against the actual deployed ressources. This should be possible using kubectl diff. However, on my Kubernetes cluster in Azure, I get the following error:
Error from server (InternalError): Internal error occurred: admission webhook "aks-webhook-admission-controller.azmk8s.io" does not support dry run
Is there something I can enable on AKS to let this work or is there some other way of achieving the diff?

Answer 1

As a workaround you can use standard GNU/Linux diff command in the following way:
diff -uN <(kubectl get pods nginx-pod -o yaml) example_pod.yaml
I know this is not a solution but just workaround but I think it still can be considered as full-fledged replacement tool.
Thanks, but that doesn't work for me, because it's not just one pod
I'm interested in, it's a whole Helm release with deployment,
services, jobs, etc. – dploeger
But anyway you won't compare everything at once, will you ?
You can use it for any resource you like, not only for Pods. Just substitute Pod by any other resource you like.
Anyway, under the hood kubectl diff uses diff command
In kubectl diff --help you can read:
KUBECTL_EXTERNAL_DIFF environment variable can be used to select your
own diff command. By default, the "diff" command available in your
path will be run with "-u" (unified diff) and "-N" (treat absent files
as empty) options.
The real problem in your case is that you cannot use for some reason --dry-run on your AKS Cluster, which is question to AKS users/experts. Maybe it can be enabled somehow but unfortunately I have no idea how.
Basically kubectl diff compares already deployed resource, which we can get by:
kubectl get resource-type resource-name -o yaml
with the result of:
kubectl apply -f nginx.yaml --dry-run --output yaml
and not with actual content of your yaml file (simple cat nginx.yaml would be ok for that purpose).
You can additionally use:
kubectl get all -l "app.kubernetes.io/instance=<helm_release_name>" -o yaml
to get yamls of all resources belonging to specific helm release.
As you can read in man diff it has following options:
--from-file=FILE1
compare FILE1 to all operands; FILE1 can be a directory
--to-file=FILE2
compare all operands to FILE2; FILE2 can be a directory
so we are not limited to comparing single files but also files located in specific directory. Only we can't use these two options together.
So the full diff command for comparing all resources belonging to specific helm release currently deployed on our kubernetes cluster with yaml files from a specific directory may look like this:
diff -uN <(kubectl get all -l "app.kubernetes.io/instance=<helm_release_name>" -o yaml) --to-file=directory_containing_yamls/