Issue in kubectl scale dry-run output

[KubeHawk]

What happened:

When I ran:

kubectl scale deployment -n test my-pod --replicas 3  --dry-run=client -o yaml

or

kubectl scale deployment -n test my-pod --replicas 3  --dry-run=client -o yaml

the generated manifest did match the number of replicas desired:

spec:
  progressDeadlineSeconds: 600
  replicas: 1

What you expected to happen:

The generated YAML should include the spec.replicas expected after the apply at least in --dry-run=server because kubectl sends a PATCH request to the API server and the API server returns the patched object:

spec:
  progressDeadlineSeconds: 600
  replicas: 3

How to reproduce it (as minimally and precisely as possible):

1. Create a namespace:

kubectl create ns test

2. Create a deployment in that namespace:

kubectl create deployment my-pod --image=nginx -n test

3. Generate the new deployment manifest with --dry-run=client:

kubectl scale deployment -n test my-pod --replicas=3  --dry-run=client -o yaml

or

kubectl scale deployment -n test my-pod --replicas=3  --dry-run=server -o yaml

4. Inspect the output:
   The generated YAML does not include the right number of replicas.

Anything else we need to know?:

I faced this issue while preparing for the CKAD with kubectl v1.34.1

Environment:

• Kubernetes client and server versions (use kubectl version): v1.34.1 - v1.34.1
• Cloud provider or hardware configuration: Local
• OS (e.g: cat /etc/os-release): Ubuntu 24.04

[brianpursley]

This isn't just the dry-run output, but the output of the kubectl scale command in general.

It gets the resource info before the scale occurs, performs a patch to update the replicas, but it never updates the original resource info, so when it is printed it is out of date.

Example:

~ % kubectl scale deployment example --replicas 3 -o json | jq '.spec | {replicas}'
{
  "replicas": 1
}

If I run it again, I now see 3 even though this time I have change it to 4. I am always seeing what it was before the scale happened.

~ % kubectl scale deployment example --replicas 4 -o json | jq '.spec | {replicas}'
{
  "replicas": 3
}

[KubeHawk]

Is it the normal way kubectl scale works ?

[brianpursley]

It seems reasonable to expect the output of any command to show the updated YAML, whether you are using dry-run or not.

By the way, I see kubernetes/kubernetes#135513 was just merged to fix what appears to be a similar (but not identical) problem with kubectl apply.

[KubeHawk]

Thanks for the pointer. The fix in kubernetes/kubernetes#135513 is a great precedent here.

That PR ensures kubectl apply --dry-run=client outputs the post-mutation merged object, rather than stale server state, by applying the patch locally before printing. The issue with kubectl scale appears conceptually similar: the PATCH is performed, but the printed object is still the pre-scale version.

It seems reasonable for kubectl scale to follow the same pattern and print the patched object (or simulated patched object for dry-run), for consistency across mutating kubectl commands.

[ardaguclu]

This sounds like a bug
/triage accepted
/priority backlog

[jomn123]

Hi @brianpursley @ardaguclu,

I'd like to work on this issue if it's still available.

Creating the PR for review.
Thanks!

[ardaguclu]

@brianpursley I tried kubectl scale deploy/test-deploy --replicas=2 --dry-run=server and it scaled to 2 replicas. I think, there is another complementary bug.

• dry-run client does not print the new object
• dry-run server is not taken account and execution actually scales the object (at least what I saw)
• dry-run server does not print the new object

[jomn123]

@ardaguclu i created a PR releated to this issue kubernetes/kubernetes#135902