Helm vs Kustomize

Helm and Kustomize are both popular tools for managing Kubernetes configurations, but they take fundamentally different approaches. Understanding their differences helps you choose the right tool for your use case, and sometimes you might use both together.

Think of Helm like a recipe with variables—you write a template recipe once, then fill in different values (ingredients) for different situations. Kustomize is like having a base recipe and then making modifications (patches) for different variations. Both get you to the same result, but the approach is different.

Helm: Templating Approach

Helm uses a templating approach where you write templates with variables, then provide values files that fill in those variables.

How Helm Works

graph LR Chart[Helm Chart Templates] --> Values[Values File Config] Values --> Render[Rendered YAML] Render --> Deploy[Deploy to K8s] style Chart fill:#e1f5ff style Values fill:#fff4e1 style Render fill:#e8f5e9 style Deploy fill:#f3e5f5

Write templates - Create YAML files with Go template syntax
Define values - Create values files with configuration
Render - Helm fills in template variables
Deploy - Deploy rendered YAML to Kubernetes

Helm Example

Chart Template (templates/deployment.yaml):

apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Values.name }}
spec:
  replicas: {{ .Values.replicas }}
  template:
    spec:
      containers:
      - name: {{ .Values.name }}
        image: {{ .Values.image }}:{{ .Values.tag }}
        ports:
        - containerPort: {{ .Values.port }}

Values File (values.yaml):

name: my-app
replicas: 3
image: nginx
tag: "1.21"
port: 80

Deploy:

helm install my-release ./my-chart

Helm Features

Templating

Go template syntax
Functions and pipelines
Conditional logic
Loops and ranges

Charts

Package applications
Share via repositories
Version management
Dependencies

Releases

Track installed instances
Upgrade and rollback
Release history

Values

Default values
Environment-specific values
Value overrides
Secret management

When to Use Helm

✅ Third-party applications - Installing databases, monitoring tools, etc. ✅ Application packages - Sharing applications as charts ✅ Complex templating - Need for variables, conditionals, loops ✅ Dependencies - Applications with dependencies ✅ Version management - Managing application versions ✅ Chart repositories - Sharing charts publicly or privately

Helm Advantages

Rich ecosystem - Thousands of pre-built charts
Powerful templating - Flexible template system
Dependency management - Handle application dependencies
Release management - Track and manage releases
Mature tooling - Well-established tooling and ecosystem

Helm Disadvantages

Learning curve - Go template syntax can be complex
Template complexity - Can become hard to read
External tool - Requires Helm installation
Overkill for simple cases - May be too much for simple apps

Kustomize: Patching Approach

Kustomize uses a patching approach where you define a base configuration and apply patches for different environments.

How Kustomize Works

graph LR Base[Base Config] --> Overlay[Overlay Patches] Overlay --> Build[Build YAML] Build --> Deploy[Deploy to K8s] style Base fill:#e1f5ff style Overlay fill:#fff4e1 style Build fill:#e8f5e9 style Deploy fill:#f3e5f5

Define base - Create base Kubernetes YAML files
Create overlays - Define patches for different environments
Build - Kustomize applies patches to base
Deploy - Deploy built YAML to Kubernetes

Kustomize Example

Base (base/deployment.yaml):

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 1
  template:
    spec:
      containers:
      - name: my-app
        image: nginx:1.21
        ports:
        - containerPort: 80

Base kustomization (base/kustomization.yaml):

resources:
- deployment.yaml

Production Overlay (overlays/production/kustomization.yaml):

resources:
- ../../base
replicas:
- name: my-app
  count: 5
images:
- name: nginx
  newTag: "1.22"

Deploy:

kubectl apply -k overlays/production

Kustomize Features

Patches

Strategic merge patches
JSON patches
Image transformations
Replica count changes

Composition

Multiple bases
Resource composition
ConfigMap/Secret generation

Built-in kubectl

No external tool needed
kubectl apply -k command
Native integration

Simplicity

Plain YAML files
Easy to understand
Git-friendly

When to Use Kustomize

✅ Environment management - Different configs for dev/staging/prod ✅ GitOps workflows - Native kubectl integration ✅ Simple patching - Modifying existing YAML ✅ Base + overlays - Common base with environment variations ✅ No external tools - Want to use kubectl only ✅ Plain YAML - Prefer plain YAML over templates

Kustomize Advantages

Simple - Easy to learn and use
Native kubectl - Built into kubectl
Git-friendly - Plain YAML files
Environment management - Excellent for overlays
No external tool - No installation needed

Kustomize Disadvantages

Limited templating - No variables or conditionals
No dependencies - Manual dependency management
Fewer third-party resources - Less ecosystem support
Patches can be complex - Complex patches can be hard to read

Side-by-Side Comparison

Simple Application

Helm:

# templates/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Values.name }}
spec:
  replicas: {{ .Values.replicas }}
  # ... template continues

Kustomize:

# base/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 1
  # ... full YAML

Environment-Specific Config

Helm:

# values-dev.yaml
replicas: 1
image: nginx:latest

# values-prod.yaml
replicas: 5
image: nginx:1.21

helm install app ./chart -f values-prod.yaml

Kustomize:

# overlays/production/kustomization.yaml
resources:
- ../../base
replicas:
- name: my-app
  count: 5
images:
- name: nginx
  newTag: "1.21"

kubectl apply -k overlays/production

When to Choose What

Choose Helm When:

Installing third-party applications (databases, monitoring)
Need complex templating (conditionals, loops)
Sharing applications as packages
Managing application dependencies
Need release management and rollback
Working with Helm charts from repositories

Choose Kustomize When:

Managing environment-specific configurations
Prefer plain YAML over templates
Using GitOps workflows
Simple configuration management
Want native kubectl integration
Patching existing YAML files

Using Both Together

You can use Helm and Kustomize together:

# Generate Helm chart
helm template my-app ./chart -f values.yaml > base/manifests.yaml

# Use Kustomize to patch
kubectl apply -k overlays/production

This approach:

Uses Helm for templating
Uses Kustomize for environment management
Combines strengths of both tools

Best Practices

Helm Best Practices

Keep templates simple and readable
Use values files for configuration
Version your charts
Test templates before deploying
Use chart dependencies for complex apps
Document required values

Kustomize Best Practices

Keep base configurations minimal
Use overlays for environment differences
Keep patches focused and small
Use strategic merge patches when possible
Document overlay purposes
Version control all configurations

Migration Considerations

From Helm to Kustomize

Extract values from Helm templates
Create base YAML files
Create overlays for environments
Test thoroughly before switching

From Kustomize to Helm

Identify templating needs
Create Helm chart structure
Convert overlays to values files
Test chart rendering

Key Takeaways

Helm uses templating with variables
Kustomize uses patching with overlays
Helm is better for third-party apps and complex templating
Kustomize is better for environment management and GitOps
Both tools solve configuration management differently
You can use both tools together
Choose based on your specific needs