Using Post-Renderers

1. Understanding Post-Renderers

Concept	Detail
What	External binary that receives rendered YAML on stdin, returns modified YAML on stdout
When	After Helm renders, before resources sent to Kubernetes API
Why	Apply cluster-specific patches without forking the chart

Flag	Use
`--post-renderer ./kustomize.sh`	Path to executable
`--post-renderer-args arg1 --post-renderer-args arg2`	Pass arguments

#!/usr/bin/env bash
set -euo pipefail
cat </dev/stdin > base/all.yaml
kustomize build .

Layout	Files
Working dir	`kustomization.yaml` + `patches/` + `kustomize.sh`
kustomization.yaml	Lists `base/all.yaml` + patches

# kustomization.yaml
resources: [base/all.yaml]
patches:
  - path: patches/add-sidecar.yaml
    target: { kind: Deployment, name: api }

# kustomization.yaml
resources: [base/all.yaml]
commonLabels:
  cluster: us-east-1
commonAnnotations:
  managed-by: gitops

Use case	Tool
Inject sidecars	Kustomize patch / yq
Set image registry	Kustomize `images:` transformer
Strip fields	yq `del()`
Add namespace	Kustomize `namespace:`

Pattern	Implementation
Single wrapper	Shell script pipes through multiple tools
Example	`cat \| yq '...' \| kustomize build .`

helm template api ./chart \
  --post-renderer ./kustomize.sh \
  --debug > final.yaml
diff <(helm template api ./chart) final.yaml

Behavior	Detail
Non-zero exit	Helm aborts; stderr surfaced to user
Invalid YAML	Helm rejects before sending to API
Debug	Run wrapper standalone with sample input

helm upgrade --install api ./chart \
  -f values-prod.yaml \
  --post-renderer ./overlays/$ENV/kustomize.sh \
  --atomic --timeout 10m