Template

Templates are used to initialize Namespaces, share common resources across namespaces, and map secrets/ConfigMaps from one namespace to other namespaces.

They can contain pre-defined parameters such as ${namespace}/${tenant}.

Also, you can define custom variables in Template, TemplateInstance and ClusterTemplateInstance. The parameters defined in Templates are overwritten the values defined in TemplateInstance and ClusterTemplateInstance.

Specification

Template Custom Resource (CR) supports three key methods for defining and managing resources: manifests, helm, and resource mapping. Let’s dive into each method, their differences, and their use cases:

1. Manifests

This approach uses raw Kubernetes manifests (YAML files) that specify resources directly in the template.

How It Works

• The template includes the actual YAML specifications of resources like Deployment, Service, ConfigMap, etc.
• These manifests are applied as-is or with minor parameter substitutions (e.g., dynamically populated {tenant} and {namespace} variables wherever added or user defined parameters).

Use Cases

• Best for straightforward and simple resources where you don't need advanced templating logic or dependency management.
• Ideal when the resource definitions are static or have minimal customization needs.

Example

apiVersion: templates.stakater.com/v1alpha1
kind: Template
metadata:
  name: networkpolicy
parameters:
  - name: CIDR_IP
    value: "172.17.0.0/16"
resources:
  manifests:
    - kind: NetworkPolicy
      apiVersion: networking.k8s.io/v1
      metadata:
        name: deny-cross-ns-traffic
      spec:
        podSelector:
          matchLabels:
            role: db
        policyTypes:
        - Ingress
        - Egress
        ingress:
        - from:
          - ipBlock:
              cidr: "${{CIDR_IP}}"
              except:
              - 172.17.1.0/24
          - namespaceSelector:
              matchLabels:
                project: myproject
          - podSelector:
              matchLabels:
                role: frontend
          ports:
          - protocol: TCP
            port: 6379
        egress:
        - to:
          - ipBlock:
              cidr: 10.0.0.0/24
          ports:
          - protocol: TCP
            port: 5978

2. Helm

This method integrates Helm charts into the template, allowing you to leverage Helm's templating capabilities and package management.

How It Works

• The Template references a Helm chart.
• Values for the Helm chart can be passed by the values field.
• The Helm chart generates the necessary Kubernetes resources dynamically at runtime.

Use Cases

• Best for complex resource setups with interdependencies (e.g., a microservice with a Deployment, Service, Ingress, and ConfigMap).
• Useful for resources requiring advanced templating logic or modular packaging.
• Great for managing third-party tools or applications (e.g., deploying Prometheus, Keycloak, or databases).

Example

apiVersion: templates.stakater.com/v1alpha1
kind: Template
metadata:
  name: redis
resources:
  helm:
    releaseName: redis
    chart:
      repository:
        name: redis
        version: 14.6.0
        repoUrl: https://charts.bitnami.com/bitnami
        username:
          key: username
          name: redis-creds
          namespace: namespace-n1
        password:
          key: password
          name: redis-creds
          namespace: namespace-n1
    setValues:
      - name: port
        value: '6379'
        forceString: false
    values: |
      redisPort: 6379

A brief explanation of the fields in the Helm section:

• releaseName: The name of the Helm release.
• chart: The Helm chart details.
  • repository: The Helm repository details.
    • name: The name of the Helm repository.
    • version: The version of the Helm chart.
    • repoUrl: The URL of the Helm repository.
  • username: A reference to the secret containing the username for the Helm repository in case the chart is in a private repository.
    • key: The key in the secret containing the username.
    • name: The name of the secret containing the username.
    • namespace: The namespace of the secret containing the username.
  • password: A reference to the secret containing the password for the Helm repository in case the chart is in a private repository.
    • key: The key in the secret containing the password.
    • name: The name of the secret containing the password.
    • namespace: The namespace of the secret containing the password.
• setValues: The values to set in the Helm chart.
  • name: The name of the value.
  • value: The value to set.
  • forceString: Whether to use --set or --set-string when setting the value. Default is false (use --set).
• values: The values file for the Helm chart.

3. Resource Mapping

This approach maps secrets and ConfigMaps from one tenant's namespace to another tenant's namespace, or within a tenant's namespace.

How It Works

• The template contains mappings to pre-existing resources (secrets and ConfigMaps only).

Use Cases

• Ideal for maintaining consistency across shared resources without duplicating definitions.
• Best when resources already exist.

Example

apiVersion: templates.stakater.com/v1alpha1
kind: Template
metadata:
  name: resource-mapping
resources:
  resourceMappings:
    secrets:
      - name: secret-s1
        namespace: namespace-n1
    configMaps:
      - name: configmap-c1
        namespace: namespace-n2