Value Generation¶

Deep dive into Genifest's ValueFrom expression system for dynamic value generation.

Overview¶

ValueFrom expressions are the core of Genifest's dynamic value generation system. They provide multiple ways to generate values for configuration changes, from simple literals to complex pipeline operations.

Core ValueFrom Types¶

DefaultValue¶

The simplest ValueFrom type for literal string values.

valueFrom:
  default:
    value: "literal-string"

Use cases: - Static configuration values - Default fallbacks - Simple string constants

EnvironmentRef¶

Read values from environment variables with optional defaults.

valueFrom:
  envRef:
    name: "DATABASE_URL"
    default: "postgresql://localhost:5432/myapp"  # Optional

Use cases: - Database connection strings - API keys and secrets - Environment-specific configuration - CI/CD pipeline variables

Examples:

# Database host with fallback
changes:
  - keySelector: ".spec.template.spec.containers[0].env[0].value"
    valueFrom:
      envRef:
        name: "DB_HOST"
        default: "localhost"

# Required API key (no default)
changes:
  - keySelector: ".spec.template.spec.containers[0].env[1].value"
    valueFrom:
      envRef:
        name: "API_KEY"

DocumentRef¶

Reference values from other locations within the document:

valueFrom:
  documentRef:
    keySelector: ".metadata.name"

Or in an external document:

valueFrom:
  documentRef:
    fileSelector: outputs.yaml
    documentSelector:
      environment: production
    keySelector: .outputs.deploy.image-name

Use cases: - Consistent naming across document sections - Dynamic field generation based on existing values - Cross-referencing within Kubernetes manifests - Referencing external configuration

Examples:

# Use deployment name as app selector
changes:
  - keySelector: ".spec.selector.matchLabels.app"
    valueFrom:
      documentRef:
        keySelector: ".metadata.name"

# Reference namespace in service account name
changes:
  - keySelector: ".spec.template.spec.serviceAccountName"
    valueFrom:
      template:
        string: "my-app-${namespace}"
        variables:
          - name: "namespace"
            valueFrom:
              documentRef:
                keySelector: ".metadata.namespace"

ArgumentRef¶

Reference variables from the current evaluation context.

valueFrom:
  argRef:
    name: "variable-name"

Use cases: - Function parameters - Pipeline step outputs - Template variables

BasicTemplate¶

Template strings with variable substitution using ${variable} syntax.

valueFrom:
  template:
    string: "Hello ${name}!"
    variables:
      - name: "name"
        valueFrom:
          default:
            value: "World"

Advanced templating:

valueFrom:
  template:
    string: "${app}-${env}-${component}"
    variables:
      - name: "app"
        valueFrom:
          documentRef:
            keySelector: ".metadata.labels.app"
      - name: "env"
        valueFrom:
          envRef:
            name: "ENVIRONMENT"
            default: "dev"
      - name: "component"
        valueFrom:
          default:
            value: "backend"

Advanced ValueFrom Types¶

FunctionCall¶

Call reusable functions with parameters.

valueFrom:
  call:
    function: "get-image-tag"
    args:
      - name: "environment"
        valueFrom:
          envRef:
            name: "DEPLOY_ENV"
            default: "dev"

Functions are defined in the functions section:

functions:
  - name: "get-image-tag"
    params:
      - name: "environment"
        required: true
    valueFrom:
      template:
        string: "myapp:${environment}-latest"
        variables:
          - name: "environment"
            valueFrom:
              argRef:
                name: "environment"

ScriptExec¶

Execute external scripts for dynamic value generation.

valueFrom:
  script:
    exec: "get-commit-hash.sh"
    args:
      - name: "branch"
        valueFrom:
          envRef:
            name: "GIT_BRANCH"
            default: "main"
    env:
      - name: "REPO_PATH"
        valueFrom:
          default:
            value: "/workspace"

FileInclusion¶

Include content from external files with optional transient modifications.

valueFrom:
  file:
    app: "templates"         # Optional subdirectory
    source: "base.yaml"      # Required filename
    changes:                 # Optional transient changes
      - keySelector: ".metadata.name"
        valueFrom:
          template:
            string: "${name}-${env}"
            variables:
              - name: "name"
                valueFrom:
                  documentRef:
                    keySelector: ".metadata.name"
              - name: "env"
                valueFrom:
                  envRef:
                    name: "ENVIRONMENT"

Transient changes allow you to modify included files without persisting changes to disk.

CallPipeline¶

Chain multiple operations together, passing outputs between steps.

valueFrom:
  pipeline:
    - valueFrom:
        envRef:
          name: "BASE_IMAGE"
          default: "nginx"
      output: "base"
    - valueFrom:
        template:
          string: "${base}:${tag}"
          variables:
            - name: "base"
              valueFrom:
                argRef:
                  name: "base"
            - name: "tag"
              valueFrom:
                script:
                  exec: "get-latest-tag.sh"
                  args:
                    - name: "image"
                      valueFrom:
                        argRef:
                          name: "base"

Combining ValueFrom Types¶

ValueFrom expressions can be combined to create sophisticated value generation:

changes:
  - keySelector: ".spec.template.spec.containers[0].image"
    valueFrom:
      pipeline:
        - valueFrom:
            envRef:
              name: "REGISTRY_URL"
              default: "docker.io"
          output: "registry"
        - valueFrom:
            template:
              string: "${registry}/${app}:${tag}"
              variables:
                - name: "registry"
                  valueFrom:
                    argRef:
                      name: "registry"
                - name: "app"
                  valueFrom:
                    documentRef:
                      keySelector: ".metadata.name"
                - name: "tag"
                  valueFrom:
                    call:
                      function: "get-version"
                      args:
                        - name: "environment"
                          valueFrom:
                            envRef:
                              name: "DEPLOY_ENV"

Best Practices¶