Kustomize Patch Not Applying — Fix

# Always preview the final output before applying
kubectl kustomize ./overlays/production
 
# Diff against what's running
kubectl kustomize ./overlays/production | kubectl diff -f -
 
# If patch isn't showing up → the patch is silently not matching

# base/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app        # ← exact name
 
# overlays/production/patch-deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app-prod   # ← WRONG — doesn't match base name, patch is ignored
spec:
  replicas: 3

# Fix: name must exactly match
metadata:
  name: my-app        # ← must match base exactly

# kustomization.yaml
patches:
- target:
    group: apps
    version: v1
    kind: Deployment
    name: my-app
  patch: |-
    - op: replace
      path: /spec/replicas
      value: 3

# Check the exact apiVersion of your resource:
kubectl api-resources | grep Deployment
# group=apps, version=v1 → correct
 
# Common mistake: using "group: apps/v1" instead of separate group + version
# Wrong:
group: apps/v1    # ← this is not valid, group and version are separate fields
# Right:
group: apps
version: v1

# If the field doesn't exist yet, use add not replace:
- op: add
  path: /spec/template/spec/containers/0/env/-
  value:
    name: ENV_VAR
    value: "production"
 
# Replace only works if the path already exists in the base resource

# kustomization.yaml — patch file must be listed
patches:
- path: patch-deployment.yaml   # ← must be listed here
 
resources:
- ../../base
 
# Common mistake: file exists in the directory but isn't referenced in kustomization.yaml

# Verify what Kustomize sees
kubectl kustomize . --load-restrictor=none 2>&1
# Errors about missing files will appear here

# base resource has no namespace:
metadata:
  name: my-app
 
# patch must also have no namespace (or both must match):
metadata:
  name: my-app
  namespace: production   # ← won't match base with no namespace

# kustomization.yaml
namespace: production   # Applied to all resources uniformly

# Build and check specific field
kubectl kustomize ./overlays/production | grep -A3 "replicas:"
 
# Build and apply dry-run
kubectl kustomize ./overlays/production | kubectl apply --dry-run=client -f -
 
# Check what kustomize built vs what's running
kubectl kustomize ./overlays/production > /tmp/desired.yaml
kubectl get deployment my-app -o yaml > /tmp/actual.yaml
diff /tmp/desired.yaml /tmp/actual.yaml