Helm Development - Named Templates

Step-01: Introduction

• Create Named Template
• Call the named template using template action
• Pass Root Object dot (.) to template action provided if we are using Helm builtin objects in our named template
• For template call use pipelines and see if it works
• Replace template call with special purpose function include in combination with pipelines and test it

Step-02: Create a Named Template

• File Location: deployment.yaml
• Define the below named template in deployment.yaml

{{/* Common Labels */}}
{{- define "helmbasics.labels"}}
    app: nginx
{{- end }}

Step-03: Call the named template using template action

apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Release.Name }}-{{ .Chart.Name }}-deployment 
  labels:
  {{- template "helmbasics.labels" }}

Step-04: Test the output with template action

# Change to Chart Directory 
cd helmbasics

# Helm Template Command
helm template myap101 .

# Helm Install with dry-run command
helm install myapp101 . --dry-run

# Helm Release
helm install myapp101 . --atomic
kubectl get deploy
kubectl describe deploy <DEPLOYMENT-NAME>
helm uninstall myapp101

Step-05: Add one Builtin Object Chart.Name to labels

{{/* Common Labels */}}
{{- define "helmbasics.labels"}}
    app: nginx
    chartname: {{ .Chart.Name }}
{{- end }}

Step-06: Test the output with template action

# Change to Chart Directory 
cd helmbasics

# Helm Template Command
helm template myap101 .

# Helm Install with dry-run command
helm install myapp101 . --dry-run
Observation:
1. Chart name filed should be empty
2. Chart Name was not in the scope for our defined template.
3. When a named template (created with define) is rendered, it will receive the scope passed in by the template call. 
4. No scope was passed in, so within the template we cannot access anything in "."
5. This is easy to fix. We simply pass a scope to the template

Step-07: Pass scope to the template call

• Add dot "." (Root Object or period) at the end of template call to pass scope to template call

apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Release.Name }}-{{ .Chart.Name }}-deployment # Action Element
  labels:
  {{- template "helmbasics.labels" . }}

Step-08: Test the output with template action when scope passed to template call

# Change to Chart Directory 
cd helmbasics

# Helm Template Command
helm template myap101 .

# Helm Install with dry-run command
helm install myapp101 . --dry-run
Observation:
Chart Name should be displayed

Step-09: Pipe an Upper function to template

apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Release.Name }}-{{ .Chart.Name }}-deployment # Action Element
  labels:
  {{- template "helmbasics.labels" . | upper }}

Step-10: Test the output when template action + pipe + upper function

# Change to Chart Directory 
cd helmbasics

# Helm Template Command
helm template myap101 .

# Helm Install with dry-run command
helm install myapp101 . --dry-run
Observation:
1. Should fail with error. What is the reason for failure ?
2. Template is an action, and not a function, there is no way to pass the output of a template call to other functions; 

Step-11: Replace Template action with Special Purpose include function

apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Release.Name }}-{{ .Chart.Name }}-deployment # Action Element
  labels:
  {{- include "helmbasics.labels" . | upper }}

Step-10: Test the output include function

# Change to Chart Directory 
cd helmbasics

# Helm Template Command
helm template myap101 .

# Helm Install with dry-run command
helm install myapp101 . --dry-run
Observation:
1. Call include "helmbasics.labels" -- should be successful
2. Should show all labels in upper case

Step-11: Underscoe file (_helpers.tpl)

• Move the named template helmbasics.labels to _helpers.tpl file
• But files whose name begins with an underscore (_) are assumed to not have a kubernetes manifest inside.
• These files are not rendered to Kubernetes object definitions, but are available everywhere within other chart templates for use.
• These files are used to store partials and helpers.

{{/* Common Labels */}}
{{- define "helmbasics.labels"}}
    app: nginx
    chartname: {{ .Chart.Name }}
{{- end }}

Step-12: Test the output after moving named template to _helpers.tpl

# Change to Chart Directory 
cd helmbasics

# Helm Template Command
helm template myap101 .

# Helm Install with dry-run command
helm install myapp101 . --dry-run
Observation:
1. call include "helmbasics.labels" -- should be successful
2. Should show all labels in upper case