What's the difference between Helm and Kustomize?
Quick Answer
Helm uses **templating** — placeholders in YAML files filled in with values at render time, packaged as a versioned chart with its own release-tracking mechanism. Kustomize uses **overlays and patches** — you keep a plain, valid base set of Kubernetes YAML (no template placeholders at all) and layer environment-specific patches on top of it declaratively, without a templating language or a separate packaging/release concept. Kustomize is also built directly into `kubectl` (`kubectl apply -k`), requiring no separate tool installation, while Helm is a separate CLI and has its own release-state model.
Detailed Answer
Helm's approach: templating
# templates/deployment.yaml -- NOT valid standalone YAML, has template syntax
apiVersion: apps/v1
kind: Deployment
metadata:
name: {{ .Release.Name }}-web
spec:
replicas: {{ .Values.replicaCount }}
A Helm template file, by itself, isn't valid Kubernetes YAML — it only becomes valid once the templating engine substitutes in actual values. This gives Helm a lot of expressive power (conditionals, loops, functions, reusable snippets via _helpers.tpl), at the cost of templates being harder to read/lint directly (you're reading a mix of YAML and Go template syntax, not plain YAML) and occasionally producing subtly invalid YAML if template logic has bugs (e.g., incorrect indentation introduced by a template conditional).
Kustomize's approach: overlays and patches on valid YAML
base/
├── deployment.yaml # PLAIN, valid, complete Kubernetes YAML -- no placeholders at all
└── kustomization.yaml
overlays/
└── production/
├── kustomization.yaml
└── replica-patch.yaml # a small patch, applied ON TOP of the base
# base/deployment.yaml -- perfectly valid, standalone Kubernetes YAML
apiVersion: apps/v1
kind: Deployment
metadata:
name: web
spec:
replicas: 2
# overlays/production/replica-patch.yaml -- a patch, not a template
apiVersion: apps/v1
kind: Deployment
metadata:
name: web
spec:
replicas: 10
# overlays/production/kustomization.yaml
resources:
- ../../base
patchesStrategicMerge:
- replica-patch.yaml
kubectl apply -k overlays/production/
The base/ directory is always plain, valid, directly-readable YAML — there's no templating syntax to parse mentally. Each environment's overlay declares which base to start from and what specific patches to layer on top, and Kustomize merges them at apply time — no separate templating language to learn, and the base manifests can be validated/linted as ordinary Kubernetes YAML on their own.
Key differences, side by side
| Helm | Kustomize | |
|---|---|---|
| Mechanism | Templating (placeholders filled at render time) | Overlays/patches on plain, valid base YAML |
| Base manifests | Not valid YAML on their own (contain template syntax) | Always valid, standalone YAML |
| Built into kubectl | No — separate CLI | Yes (kubectl apply -k), also available as a standalone CLI |
| Release/version tracking | Yes — built-in release history, helm rollback | No native concept of a "release" — you track and roll back via your own git history/CI process instead |
| Packaging and distribution ecosystem | Rich — Artifact Hub, versioned charts, dependency management | Minimal — no equivalent packaging/distribution convention |
| Learning curve for the templating/patching approach itself | Steeper (Go template syntax, functions, _helpers.tpl) | Generally considered gentler (patches are just YAML) |
When each tends to be preferred
Helm tends to be preferred when you need to install and manage complex third-party software (databases, ingress controllers, monitoring stacks) via a rich ecosystem of pre-built charts, or when built-in release/rollback tracking is genuinely valuable for your own applications. Kustomize tends to be preferred for organizations that want to keep manifests as plain, directly-readable YAML with no templating layer at all, relying on GitOps tooling and git history itself for change tracking and rollback rather than a separate release-tracking mechanism.
They aren't mutually exclusive
It's common to use both together — e.g., using Kustomize to manage environment-specific overlays for your own application's plain manifests, while still using Helm to install third-party charts (a monitoring stack, an ingress controller) that are only distributed as Helm charts in the first place.