kubectl diff fails on AKS

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/