How to use Helm templates?

Helm templates allow for dynamic generation of Kubernetes manifests based on provided values and functions.

Main components of templating:

• Templates: YAML manifest files with embedded Go template syntax.
• Values: YAML files or command-line parameters containing data to fill templates.
• Functions: Built-in or custom Go template functions for processing values and generating strings.

Example of a basic deployment.yaml template:

apiVersion: apps/v1  
kind: Deployment  
metadata:  
  name: {{ .Release.Name }}-{{ .Chart.Name }}  
spec:  
  replicas: {{ .Values.replicaCount }}  
  selector:  
    matchLabels:  
      app: {{ .Chart.Name }}  
  template:  
    metadata:  
      labels:  
        app: {{ .Chart.Name }}  
    spec:  
      containers:  
        - name: {{ .Chart.Name }}  
          image: "{{ .Values.image.repository }}:{{ .Values.image.tag }}"  
          ports:  
            - containerPort: {{ .Values.service.port }}

Example of a values.yaml file:

replicaCount: 1  
image:  
  repository: nginx  
  tag: latest  
service:  
  port: 80

Using helpers for reusable code blocks, usually in _helpers.tpl:

{{/*  
Common labels  
*/}}  
{{- define "mychart.labels" -}}  
helm.sh/chart: {{ include "mychart.chart" . }}  
{{ include "mychart.selectorLabels" . }}  
{{- if .Chart.AppVersion }}  
app.kubernetes.io/version: {{ .Chart.AppVersion | quote }}  
{{- end }}  
app.kubernetes.io/instance: {{ .Release.Name }}  
app.kubernetes.io/managed-by: {{ .Release.Service }}  
{{- end }}  

{{/*  
Selector labels  
*/}}  
{{- define "mychart.selectorLabels" -}}  
app.kubernetes.io/name: {{ include "mychart.name" . }}  
app.kubernetes.io/instance: {{ .Release.Name }}  
{{- end }}

Calling a helper in a template:

labels:  
  {{ include "mychart.labels" . | nindent 4 }}

Main functions of the Go templating engine, often used in Helm:

Function	Description	Example
include	Inserts the content of another template.	{{ include "my.helper" . }}
nindent	Indents multi-line output.	`{{ .Values.multiline
quote	Wraps a string in quotes.	`{{ .Values.someString
default	Sets a default value.	`{{ .Values.replicas
required	Fails if a value is missing.	{{ required "A database password is required!" .Values.database.password }}
toJson	Converts data to a JSON string.	`{{ .Values.config
toYaml	Converts data to a YAML string.	`{{ .Values.config

Managing spaces: - after {{ and before }} trims whitespace.

{{- range .Values.items }}  
- name: {{ .name }}  
  value: {{ .value | quote }}  
{{- end -}}

Branching logic and loops:

{{- if .Values.ingress.enabled }}  
apiVersion: networking.k8s.io/v1  
kind: Ingress  
...  
{{- end }}  

{{- range $key, $value := .Values.configMapData }}  
  {{ $key }}: {{ $value | quote }}  
{{- end }}

Template testing:

helm lint mychart  # Basic syntax check  
helm template mychart --values mychart/values.yaml  # Render template with values