Flux Kustomize PostBuild Quick Fix - Post-Build Configuration

Flux Kustomize post-build substitutions replace variables in manifests after kustomize build. Incorrect substitution syntax or missing variables cause reconciliation failures. This guide covers the fix.

Quick Fix

Wrong

spec:
  postBuild:
    substitute:
      image_tag: v1.0.0
      replicas: "3"

The issue: variable names with underscores may not match references in the kustomize manifests. PostBuild substitution uses ${variable_name} syntax, and manifests may use different variable naming.

Right

spec:
  postBuild:
    substitute:
      image_tag: "v1.0.0"
      replicas: "3"
    substituteFrom:
    - kind: ConfigMap
      name: build-vars
    - kind: Secret
      name: build-secrets

With the kustomize manifest referencing:

apiVersion: apps/v1
kind: Deployment
spec:
  replicas: ${replicas}
  template:
    spec:
      containers:
      - image: myapp:${image_tag}

# Expected output after applying the fix
# PostBuild substitutes ${image_tag} → v1.0.0
# PostBuild substitutes ${replicas} → 3
# Variables also sourced from ConfigMap and Secret
# Kustomization applies with substituted values

Prevention

• Use ${variable_name} syntax in kustomize manifests for substitution
• Quote values in substitute to preserve string types
• Use substituteFrom for bulk variable loading from ConfigMaps/Secrets
• Ensure variable names match exactly between substitute and manifest references
• Test substitutions with flux build kustomization <name> --dry-run

DodaTech Tools

Doda Browser's substitution inspector shows resolved variable values. DodaZIP archives substitution configurations for audit. Durga Antivirus Pro validates substitution values for injection.

FAQ