Functions

In this lesson, we explore how functions work in Helm and how they can enhance your templating process. When you combine a template with values defined in a values.yaml file, Helm generates a valid manifest file. However, if a field in values.yaml is not set, the corresponding section in the manifest may be omitted. For example, consider the following template:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Release.Name }}-nginx
spec:
  replicas: {{ .Values.replicaCount }}
  selector:
    matchLabels:
      app: hello-world
  template:
    metadata:
      labels:
        app: hello-world
    spec:
      containers:
        - name: hello-world
          image: {{ .Values.image.repository }}
          ports:
            - name: http
              containerPort: 80
              protocol: TCP

And the corresponding values.yaml file:

replicaCount: 2
image:
  repository: 
  pullPolicy: IfNotPresent
  tag: "1.16.0"

With these definitions, the generated manifest file will be:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-world
spec:
  replicas: 2
  selector:
    matchLabels:
      app: hello-world
  template:
    metadata:
      labels:
        app: hello-world
    spec:
      containers:
        - name: hello-world
          image: nginx
          ports:
            - name: http
              containerPort: 80
              protocol: TCP

To resolve this issue, the chart can define default values. If the user does not provide a value in values.yaml (or via the command line), Helm can fall back to a default specified using a function. In our example, we want the default image to be "nginx".

Helm Functions Overview

Helm functions behave similarly to those in conventional programming languages: they process an input and return a transformed output. For instance, the upper function converts a string to uppercase, and the trim function removes surrounding whitespace. Consider these examples:

upper("helm")  ➞  "HELM"
trim(" helm ")  ➞  "helm"

Applied within Helm templates, the value of a parameter can be transformed on the fly. For example, the snippet:

{{ .Values.image.repository }}

renders as:

image: nginx

However, if you wrap the same value with the upper function:

{{ upper .Values.image.repository }}

the output becomes:

image: NGINX

Helm also provides other useful functions like quote (which surrounds a string with quotes) and replace (which substitutes characters within a string). Here are a few examples:

{{ upper .Values.image.repository }}
{{ quote .Values.image.repository }}
{{ replace "x" "y" .Values.image.repository }}
{{ shuffle .Values.image.repository }}

These string manipulation functions are just a subset of what Helm offers. Additional functions cover areas such as cryptography, date handling, dictionary operations, Kubernetes object management, networking, type conversion, regular expressions, and URL handling. For a complete list of supported functions, refer to the official Helm documentation.

Using the Default Function

Revisiting the earlier issue of a missing image repository value: if the value is not supplied in values.yaml or via the command line, the rendered manifest will omit an image, potentially causing a pod to fail. To ensure that your deployment always includes a valid image, use the default function. By specifying a default value (enclosed in quotes to treat it as a string), Helm will use "nginx" if no repository value is provided.

Consider the updated template snippet:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Release.Name }}-nginx
spec:
  replicas: {{ .Values.replicaCount }}
  selector:
    matchLabels:
      app: hello-world
  template:
    metadata:
      labels:
        app: hello-world
    spec:
      containers:
      - name: hello-world
        image: {{ default "nginx" .Values.image.repository }}
        ports:
        - name: http
          containerPort: 80
          protocol: TCP

The values.yaml file remains:

replicaCount: 2
image:
  repository: ""
  pullPolicy: IfNotPresent
  tag: "1.16.0"

With this configuration, if the image repository is not specified, Helm substitutes the default "nginx", resulting in the following manifest:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-world
spec:
  replicas: 2
  selector:
    matchLabels:
      app: hello-world
  template:
    metadata:
      labels:
        app: hello-world
    spec:
      containers:
      - name: hello-world
        image: nginx
        ports:
        - name: http
          containerPort: 80
          protocol: TCP

That concludes our discussion on functions in Helm. In the next lesson, we will dive deeper into additional Helm templating features.