Kubernetes Classroom Series – 07/Aug/2021

Helm Chart Creation Command

• Helm includes create command to make it easy for us to create charts
• This command creates a new Nginx chart with name of our choice
• Lets try to install the chart which we have create
• The output from the command is

NAME: myapp
LAST DEPLOYED: Fri Aug  6 19:26:51 2021
NAMESPACE: default
STATUS: deployed
REVISION: 1
NOTES:
1. Get the application URL by running these commands:
  export POD_NAME=$(kubectl get pods --namespace default -l "app.kubernetes.io/name=inventory,app.kubernetes.io/instance=myapp" -o jsonpath="{.items[0].metadata.name}")
  export CONTAINER_PORT=$(kubectl get pod --namespace default $POD_NAME -o jsonpath="{.spec.containers[0].ports[0].containerPort}")
  echo "Visit http://127.0.0.1:8080 to use your application"
  kubectl --namespace default port-forward $POD_NAME 8080:$CONTAINER_PORT

• Now refer the charts.yaml Refer Here for the offical docs
• In this charts.yaml lets try to focus on some name value pairs
  • apiVersion: v2: This tells helm what structure of chart we are using. An apiVersion of v2 is designed for Helm3
  • name: inventory: The name used to identify the chart
  • version: 0.1.0: Charts can have many versions. Helm uses the version information to order and identify charts
• Charts.yaml also contain descriptive information
  • home: URL of chart or projects
  • icon: an image in the form of URL
  • maintainers: contains list of maintainers
  • keywords: can hold list of keyworkds about the project
  • sources: list of URLs for the source code for project or chart
• Refer Here for the sample chart.yaml

Modifying Templates

• Helm is written in Go programming language and Go includes template packages. Helm leverages the text template package as the foundation for its templates Refer Here
• {{ and }} are the opening and closing brackets to enter and exit the template logic
• Sample

product: {{ .Values.product | default "kubernetes" | quote }}

• There are three parts to the template logic sepearted by a |. This is called as pipeline and works exactly in the sameway as pipeline in Unix/Linux Based system. The value or output of a function on the left is passed as a last argument to the next item in pipeline.
• .Values.product This comes for the data passed in when the templates are rendered
• This value is passed as last argument to the default function
• The default is the helm function and output of default is passed to the quoted
• The . at the start .Values.production is considered as root object in the current scope

Developing Templates

• Helm uses the Go text template engine provided as part of standard Go Libarary
• Actions:
  • Logic, control structures and data evaluations are wrapped by {{ and }}. These are called as actions.
  • Anything outside of actions is copied to output
  • When the curly braces are used to start and stop actions they can be accompanies by a – to remove leading or trailing white spaces.

  {{ "Hello" -}}, {{- "World" }}, {{- "of Helm" -}}
  # generated Output Hello,World,of Helm
  

Information Helm Passes to Templates

• When Helm renders a template it passes a single data object to the template with information you can access.
• Inside the template that object is representeed as . (i.e period)
• The properties on .Values are specific to each chart based entirely on values in values.yaml
• What values should be present in values.yaml have no specific structure or schema.
• In addition to values, information about the release can be access as properties of .Release. This information includes
  • .Release.Name: name of the release
  • .Release.Namespace: Contains the namespace the chart is being released to
  • .Release.IsInstall: Set to true when relase is workload being installed
  • .Release.IsUpgrade: Set to true when the release is upgrade or rollback
  • .Release.Service: Lists the Service performing the release. when Helm installs a chart this value would be Helm
• The information in Chart.yaml can alos be found the data object at .Chart
  • .Chart.Name
  • .Chart.Version
  • .Chart.AppVersion
  • .Chart.Annotations
• Note the Names differ as names in Charts.yaml start with lowercase but Start with Uppercase later when they properties of .Chart object
• If you want to pass the custom information from the Chart.yaml to the template, you need to use annotations
• Helm also provides some data about the capabilities of the K8s cluster as properties of .Capabilities.
  • .Capabilities.ApiVersions: Contains the API Versions and resource types available in your cluster
  • .Capabilities.KubeVersion.Version: Full Kubernetes Version
  • .Capabilities.KubeVersion.Major: Contains major K8s version
  • .Capabilities.KubeVersion.Minor: The minor version of K8s being used in cluster
• The final piece of data passed into the template is details about the current template being executed.
  • .Template.Name: Contains the namespaced filepath to the template (inventory/templates/deployment.yaml)
  • .Template.BasePath: Contains the namespaced Path of Templates directory (inventory/templates)