Helm package requirements

Članak
11/01/2023

Helm is a package manager for Kubernetes that helps you manage Kubernetes applications. Helm packages are called charts, and they consist of a few YAML configuration files and some templates that are rendered into Kubernetes manifest files. Charts are reusable by anyone for any environment, which reduces complexity and duplicates.

Registry URL path and imagepullsecrets requirements

When developing a helm package, it's common to keep the container registry server URL in the values. Keeping the container registry server URL in the values is useful for moving artifacts between each environment container registry. Azure Operator Service Manager (AOSM) uses the Network Function Manager (NFM) service to deploy Containerized Network Function (CNF). The Network Function Manager (NFM) contains features to inject container registry server location and imagepullsecrets into the helm values during Network Function (NF) deployment. An imagePullSecret is an authorization token, also known as a secret, that stores Docker credentials that are used for accessing a registry. For example, if you need to deploy an application via Kubernetes deployment, you can define a deployment like the following example:

apiVersion: apps/v1 
kind: Deployment 
metadata: 
  name: nginx-deployment 
  labels: 
    app: nginx 
spec: 
  replicas: 3 
  selector: 
    matchLabels: 
      app: nginx 
  template: 
    metadata: 
      labels: 
        app: nginx 
    spec: 
      {{- if .Values.global.imagePullSecrets }} 
      imagePullSecrets: {{ toYaml .Values.global.imagePullSecrets | nindent 8 }} 
      {{- end }} 
      containers: 
      - name: contosoapp 
        image:{{ .Values.global.registryPath }}/contosoapp:1.14.2 
        ports: 
        - containerPort: 80

values.schema.json is a file that allows you to easily set value requirements and constraints in a single location for Helm charts. In this file, define registryPath and imagePullSecrets as required properties.

{ 
  "$schema": "http://json-schema.org/draft-07/schema#", 
  "title": "StarterSchema", 
  "type": "object", 
  "required": ["global"], 
  "properties": { 
      "global" : {
          "type": "object",
          "properties": {
              “registryPath”: {“type”: “string”}, 
              “imagePullSecrets”: {“type”: “string”}, 
          }
          "required": [ "registryPath", "imagePullSecrets" ], 
      } 
   } 
}

The NFDVersion request payload provides the following values in the registryValuesPaths:

"registryValuesPaths": [ "global.registryPath" ], 
"imagePullSecretsValuesPaths": [ "global.imagePullSecrets" ],

During an NF deployment, the Network Function Operator (NFO) sets the registryPath to the correct Azure Container Registry (ACR) server location. For example, the NFO runs the following equivalent command:

$ helm install --set "global.registryPath=<registryURL>" --set "global.imagePullSecrets[0].name=<secretName>" releasename ./releasepackage

Note

The registryPath is set without any prefix such as https:// or oci://. If a prefix is required in the helm package, publishers need to define this in the package.

values.yaml is a file that contains the default values for a Helm chart. It's a YAML file that defines the default values for a chart. In the values.yaml file, two types of variables must be present; imagePullSecrets and registryPath. Each is described in the table.

global: 
   imagePullSecrets: [] 
   registryPath: “”

Name	Type	Description
imagePullSecrets	String	imagePullSecrets are an array of secret names, which are used to pull container images
registryPath	String	registryPath is the `AzureContainerRegistry` server location

imagePullSecrets and registryPath must be provided in the create NFDVersion onboarding step.

An NFO running in the cluster populates these two variables (imagePullSecrets and registryPath) during a helm release using the helm install –set command.

For more information, see: pull-image-private-registry

Immutability restrictions

Immutability restrictions prevent changes to a file or directory. For example, an immutable file can't be changed or renamed, and a file that allows append operations can't be deleted, modified, or renamed.

Avoid use of mutable tags

Users should avoid using mutable tags such as latest, dev or stable. For example, if deployment.yaml used 'latest' for the .Values.image.tag the deployment would fail.

 image: "{{ .Values.global.registryPath }}/{{ .Values.image.repository }}:{{ .Values.image.tag}}“

Avoid references to external registry

Users should avoid using references to an external registry. For example, if deployment.yaml uses a hardcoded registry path or external registry references it fails validation.

 image: http://myURL/{{ .Values.image.repository }}:{{ .Values.image.tag}}

Recommendations

Splitting the Custom Resource Definitions (CRDs) declaration and usage plus using manual validations are recommended practices. Each is described in the following sections.

Split CRD declaration and usage

We recommend splitting the declaration and usage of CRDs into separate helm charts to support updates. For detailed information see: method-2-separate-charts

Manual validations

Review the images and container specs created to ensure the images have prefix of registryURL and the imagePullSecrets are populated with secretName.

 helm template --set "global.imagePullSecrets[0].name=<secretName>" --set "global.registry.url=<registryURL>" <release-name> <chart-name> --dry-run

 helm install --set "global.imagePullSecrets[0].name=<secretName>" --set "global.registry.url=<registryURL>" <release-name> <chart-name> --dry-run
 kubectl create secret <secretName> regcred --docker-server=<registryURL> --dockerusername=<regusername> --docker-password=<regpassword>

Static image repository and tags

Each helm chart should contain static image repository and tags. Users should set the image repository and tag to static values. The static values can be set by:

By hard-coding them in the image line or,
Setting the Values in values.yaml and not exposing these values in the Network Function Design Version (NFDV).

A Network Function Design Version (NFDV) should map to a static set of helm charts and images. The charts and images are only updated by publishing a new Network Function Design Version (NFDV).

 image: "{{ .Values.global.registryPath }}/contosoapp:1.14.2“

 image: "{{ .Values.global.registryPath }}/{{ .Values.image.repository }}:{{ .Values.image.tag}}“
 
YAML values.yaml
image:
  repository: contosoapp
  tag: 1.14.2

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