Simple Schema

kro follows a different approach for defining your API schema and shapes. It leverages a human-friendly and readable syntax that is OpenAPI specification compatible. Here's a comprehensive example:

apiVersion: kro.run/v1alpha1
kind: ResourceGraphDefinition
metadata:
  name: web-application
spec:
  schema:
    apiVersion: v1alpha1
    kind: WebApplication
    spec:
      # Basic types
      name: string | required=true description="My Name"
      replicas: integer | default=1 minimum=1 maximum=100
      image: string | required=true

      # Structured type
      ingress:
        enabled: boolean | default=false
        host: string | default="example.com"
        path: string | default="/"

      # Array type
      ports: "[]integer"

      # Map type
      env: "map[string]mytype"

    # Custom Types
    types:
      myType:
        value1: string | required=true
        value2: integer | default=42

    status:
      # Status fields with auto-inferred types
      availableReplicas: ${deployment.status.availableReplicas}
      serviceEndpoint: ${service.status.loadBalancer.ingress[0].hostname}

Type Definitions

Basic Types

kro supports these foundational types:

• string: Text values
• integer: Whole numbers
• boolean: True/False values
• float: Decimal numbers

For example:

name: string
age: integer
enabled: boolean
price: float

Structure Types

You can create complex objects by nesting fields. Each field can use any type, including other structures:

# Simple structure
address:
  street: string
  city: string
  zipcode: string

# Nested structures
user:
  name: string
  address: # Nested object
    street: string
    city: string
  contacts: "[]string" # Array of strings

Array Types

Arrays are denoted using [] syntax:

• Basic arrays: []string, []integer, []boolean

Examples:

tags: []string
ports: []integer

Map Types

Maps are key-value pairs denoted as map[keyType]valueType:

• map[string]string: String to string mapping
• map[string]integer: String to integer mapping

Examples:

labels: "map[string]string"
metrics: "map[string]float"

Custom Types

Custom types are specified in the separate types section. They provide a map of names to type specifications that follow the simple schema.

Example:

schema:
  types:
    Person:
      name: string
      age: integer
  spec:
    people: '[]Person | required=true`

Validation and Documentation

Fields can have multiple markers for validation and documentation:

name: string | required=true default="app" description="Application name"
replicas: integer | default=3 minimum=1 maximum=10
mode: string | enum="debug,info,warn,error" default="info"

Supported Markers

• required=true: Field must be provided
• default=value: Default value if not specified
• description="...": Field documentation
• enum="value1,value2": Allowed values
• minimum=value: Minimum value for numbers
• maximum=value: Maximum value for numbers

Multiple markers can be combined using the | separator.

For example:

name: string | required=true default="app" description="Application name"
replicas: integer | default=3 minimum=1 maximum=10
price: float | minimum=0.01 maximum=999.99
mode: string | enum="debug,info,warn,error" default="info"

Status Fields

Status fields use CEL expressions to reference values from resources. kro automatically:

• Infers the correct types from the expressions
• Validates that referenced resources exist
• Updates values when the underlying resources change

status:
  # Types are inferred from the referenced fields
  availableReplicas: ${deployment.status.availableReplicas}
  endpoint: ${service.status.loadBalancer.ingress[0].hostname}

Default Status Fields

kro automatically injects two fields to every instance's status:

1. Conditions

An array of condition objects tracking the instance's state:

status:
  conditions:
    - type: string # e.g., "Ready", "Progressing"
      status: string # "True", "False", "Unknown"
      lastTransitionTime: string
      reason: string
      message: string

Common condition types:

• Ready: Instance is fully reconciled
• Progressing: Working towards desired state
• Degraded: Operational but not optimal
• Error: Reconciliation error occurred

2. State

A high-level summary of the instance's status:

status:
  state: string # Ready, Progressing, Degraded, Unknown, Deleting

tip

conditions and state are reserved words. If defined in your schema, kro will override them with its own values.

Additional Printer Columns

You can define additionalPrinterColumns for the created CRD through the ResourceGraphDefinition by setting them on spec.schema.additionalPrinterColumns.

schema:
  spec:
    image: string | default="nginx"
  status:
      availableReplicas: ${deployment.status.availableReplicas}
  additionalPrinterColumns:
    - jsonPath: .status.availableReplicas
      name: Available replicas
      type: integer
    - jsonPath: .spec.image
      name: Image
      type: string