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), needing no separate tool installation, while Helm is a separate CLI with 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, like 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 — for example, 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.