Helm charts

Helm is a command-line automation tool that simplifies the deployment, upgrade, and rollback of applications on Kubernetes. Instead of manually executing numerous individual operations, Helm accepts high-level commands like “install this application” and manages all the necessary steps behind the scenes. It achieves this by reading a set of instructions defined in Helm Charts.

Charts are essentially instruction manuals composed of multiple text files, each serving a specific purpose. One of the key files is the values.yaml file, which defines configuration parameters. These parameters are referenced within template files to generate the final Kubernetes manifest files used to create objects.

Below is an example of a simple Helm chart that deploys a basic "hello-world" application. This chart defines two Kubernetes objects: a Service and a Deployment. Notice how templating is used for values like the container image and replica count, which are later defined in the values.yaml file.

Hello-World Helm Chart Example

Service Definition

apiVersion: v1
kind: Service
metadata:
  name: hello-world
spec:
  type: NodePort
  ports:
    - port: 80
      targetPort: http
      protocol: TCP
      name: http
  selector:
    app: hello-world

Deployment Definition

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-world
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

Default Configuration Values

# values.yaml
replicaCount: 1
image:
  repository: nginx

Templating enables the same chart to be customized based on the configuration provided in values.yaml. When you run the following command, Helm uses the templates alongside the values file to generate the final Kubernetes manifest files:

$ helm install hello-world

Chart Metadata with Chart.yaml

In addition to the values.yaml file, every Helm chart includes a Chart.yaml file. This file contains essential metadata such as the API version, app version, name, description, and chart type.

Consider the following Chart.yaml for the "hello-world" application:

apiVersion: v2
appVersion: "1.16.0"
name: hello-world
description: A web application
type: application

The API version is particularly important. While Helm 2 did not include the API version field, Helm 3 introduced it along with other features like chart dependencies and type fields. Charts built for Helm 3 must have the API version set to v2. Using a Helm 3 chart with API version v2 in Helm 2 may lead to unexpected results. Therefore, always set the API version to v2 when developing a chart for Helm 3.

Comprehensive Example: Deploying WordPress

Consider another example where the Helm chart deploys WordPress. In this case, the appVersion field represents the version of the WordPress application, while the version field tracks the chart version. This distinction helps manage changes in the chart independently of the application version.

Below is the sample Chart.yaml for WordPress:

apiVersion: v2
appVersion: 5.8.1
version: 12.1.27
name: wordpress
description: Web publishing platform for building blogs and websites.
type: application
dependencies:
  - condition: mariadb.enabled
    name: mariadb
    repository: https://charts.bitnami.com/bitnami
    version: 9.x.x
keywords:
  - application
  - blog
  - wordpress
maintainers:
  - email: [email protected]
    name: Bitnami
home: https://github.com/bitnami/charts/tree/master/bitnami/wordpress
icon: https://bitnami.com/assets/stacks/wordpress/img/wordpress-stack-220x234.png

Chart Directory Structure

Helm charts are organized in a standard directory structure. At a minimum, a chart directory includes:

• templates directory: Contains all the template files (e.g., Service and Deployment manifests).
• values.yaml file: Provides the default configuration values.
• Chart.yaml file: Holds metadata about the chart.

Additional files such as a LICENSE or README, and directories like charts (for dependent charts) might also be included.

Installing the WordPress Chart

To install the WordPress chart from the Bitnami repository, execute the following commands:

$ helm repo add bitnami https://charts.bitnami.com/bitnami
$ helm install my-release bitnami/wordpress

This completes the overview of Helm charts. We explored how Helm uses charts to automate Kubernetes deployments with templating and values files, and reviewed the structure and metadata defined in Chart.yaml.

In the next lesson, we will dive deeper into chart dependencies and provide additional details on customizing your Helm charts.

For further reading, consider visiting Helm Documentation.