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KRM Functions Specification

apiVersion: v1

Overview

This document specifies a standard for client-side functions that operate on Kubernetes declarative configurations referred to as KRM Functions. This standard enables creating small, interoperable, and language-independent executable programs packaged as containers that can be chained together as part of a configuration management pipeline. The end result of such a pipeline are fully rendered configurations that can then be applied to a control plane (e.g. Using ‘kubectl apply’ for Kubernetes control plane). As such, although this document references Kubernetes Resource Model and API conventions, it is completely decoupled from Kubernetes API machinery and does not depend on any in-cluster components.

This document references terms described in Kubernetes API Conventions.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Use Cases

KRM functions enable shift-left practices (client-side) through:

  • Pre-commit / delivery validation and linting of configuration
    • e.g. Fail if any containers don't have CPU / Memory limits
  • Implementation of abstractions as client actuated APIs
    • e.g. Create a client-side "CRD" for generating configuration checked into git
  • Injection of cross-cutting configuration
    • e.g. T-Shirt size containers by annotating resources with small, medium, large and inject the cpu and memory resources into containers accordingly.
    • e.g. Inject init and side-car containers into resources based off of resource type, annotations, etc.

Performing these on the client rather than the server enables:

  • Configuration to be reviewed prior to being sent to the API server
  • Configuration to be validated as part of the CI/CD pipeline
  • Configuration for resources to validated holistically rather than individually per-resource
    • e.g. ensure the Service.selector and Deployment.spec.template labels match.
    • e.g. MutatingWebHooks are scoped to a single resource instance at a time.
  • Low-level tweaks to the output of high-level abstractions
    • e.g. add an init container to a client "CRD" resource after it was generated.
  • Composition and layering of multiple functions together
    • Compose generation, injection, validation together

Definitions

  • function: A containerized program conforming to the spec described in this document.
  • orchestrator: A program that invokes the function container, passing arguments and processing its output.

Interface

The inter-process communication between the orchestrator and a function works as follows:

  1. Orchestrator runs the function container and provides the input on stdin. The input is a Kubernetes object of kind ResourceList as described below.
  2. Function reads the input from stdin, performs computations, and provides the output as a ResourceList to stdout. The function MAY also emit non-structured error message on stderr.
  3. Orchestrator uses the stdout, stderr, and the exit code of the function as it sees fit following to the semantics described below.

Schema

A function MUST accept input from stdin and MUST output to stdout a Kubernetes object of kind ResourceList with the following OpenAPI schema:

swagger: "2.0"
info:
  title: KRM Functions Specification (ResourceList)
  version: v1
definitions:
  ResourceList:
    type: object
    description: ResourceList is the input/output wire format for KRM functions.
    x-kubernetes-group-version-kind:
      - group: config.kubernetes.io
        kind: ResourceList
        version: v1
      - group: config.kubernetes.io
        kind: ResourceList
        version: v1beta1
    required:
      - items
    properties:
      apiVersion:
        description: apiVersion of ResourceList
        type: string
      kind:
        description: kind of ResourceList i.e. `ResourceList`
        type: string
      items:
        type: array
        description: |
          [input/output]
          Items is a list of Kubernetes objects: 
          https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/api-conventions.md#types-kinds).

          A function will read this field in the input ResourceList and populate
          this field in the output ResourceList.
        items:
          type: object
      functionConfig:
        type: object
        description: |
          [input]
          FunctionConfig is an optional Kubernetes object for passing arguments to a
          function invocation.
      results:
        type: array
        description: |
          [output]
          Results is an optional list that can be used by function to emit results
          for observability and debugging purposes.
        items:
          "$ref": "#/definitions/Result"
  Result:
    type: object
    required:
      - message
    properties:
      message:
        type: string
        description: Message is a human readable message.
      severity:
        type: string
        enum:
          - error
          - warning
          - info
        default: error
        description: |
          Severity is the severity of a result:

          "error": indicates an error result.
          "warning": indicates a warning result.
          "info": indicates an informational result.
      resourceRef:
        type: object
        description: |
          ResourceRef is the metadata for referencing a Kubernetes object
          associated with a result.
        required:
          - apiVersion
          - kind
          - name
        properties:
          apiVersion:
            description:
              APIVersion refers to the `apiVersion` field of the object
              manifest.
            type: string
          kind:
            description: Kind refers to the `kind` field of the object.
            type: string
          namespace:
            description:
              Namespace refers to the `metadata.namespace` field of the object
              manifest.
            type: string
          name:
            description:
              Name refers to the `metadata.name` field of the object manifest.
            type: string
      field:
        type: object
        description: |
          Field is the reference to a field in the object.
          If defined, `ResourceRef` must also be provided.
        required:
          - path
        properties:
          path:
            type: string
            description: |
              Path is the JSON path of the field
              e.g. `spec.template.spec.containers[3].resources.limits.cpu`
          currentValue:
            description: |
              CurrrentValue is the current value of the field.
              Can be any value - string, number, boolean, array or object.
          proposedValue:
            description: |
              PropposedValue is the proposed value of the field to fix an issue.
              Can be any value - string, number, boolean, array or object.
      file:
        type: object
        description: File references a file containing the resource.
        required:
          - path
        properties:
          path:
            type: string
            description: |
              Path is the OS agnostic, slash-delimited, relative path.
              e.g. `some-dir/some-file.yaml`.
          index:
            type: number
            default: 0
            description: Index of the object in a multi-object YAML file.
      tags:
        type: object
        additionalProperties:
          type: string
        description: |
          Tags is an unstructured key value map stored with a result that may be set
          by external tools to store and retrieve arbitrary metadata.
paths: {}

Examples

The following is an example input, where the custom resource of kind FulfillmentCenter is provided as functionConfig. The function will operate on one resource of kind Service.

apiVersion: config.kubernetes.io/v1
kind: ResourceList
functionConfig:
  apiVersion: foo-corp.com/v1
  kind: FulfillmentCenter
  metadata:
    name: staging
  spec:
    address: "100 Main St."
items:
  - apiVersion: v1
    kind: Service
    metadata:
      name: wordpress
      labels:
        app: wordpress
      annotations:
        internal.config.kubernetes.io/index: "0"
        internal.config.kubernetes.io/path: "service.yaml"
    spec: # Example comment
      type: LoadBalancer
      selector:
        app: wordpress
        tier: frontend
      ports:
        - protocol: TCP
          port: 80

The following is an example output containing one result representing a validation error:

apiVersion: config.kubernetes.io/v1
kind: ResourceList
items:
  - apiVersion: v1
    kind: Service
    metadata:
      name: wordpress
      labels:
        app: wordpress
      annotations:
        internal.config.kubernetes.io/index: "0"
        internal.config.kubernetes.io/path: "service.yaml"
    spec: # Example comment
      type: LoadBalancer
      selector:
        app: wordpress
        tier: frontend
      ports:
        - protocol: TCP
          port: 80
results:
  - message: "Invalid type. Expected: integer, given: string"
    severity: error
    resourceRef:
      apiVersion: v1
      kind: Service
      name: wordpress
    field:
      path: spec.ports.0.port
    file:
      path: service.yaml

Serialization Format

A function MUST support YAML as a serialization format for the input and output. A function MUST use utf8 encoding (as YAML is a superset of JSON, JSON will also be supported by any conforming function).

Containerization

A function MUST be implemented as a container.

A function container MUST be capable of running as a non-root user nobody if it does not require access to host filesystem.

stderr

Any non-structured error messages MUST be emitted to stderr. stdout is reserved for ResourceList as described above.

Exit Code

An exit code of zero indicates function execution was successful. A non-zero exit code indicates a failure.

Operations

A function MAY Create, Update, or Delete any number of items in the items field and output the resultant list in the corresponding items field of the output.

A function SHOULD preserve comments when input serialization format is YAML. This allows for human authoring of configuration to coexist with changes made by functions.

Internal Annotations

For orchestration purposes, the orchestrator will use a set of annotations, referred to as internal annotations, on resources in Resources.items. These annotations are not persisted to resource manifests on the filesystem: The orchestrator sets this annotation when reading files from the local filesystem and removes the annotation when writing the output of functions back to the filesystem.

Annotation prefix internal.config.kubernetes.io is reserved for use for internal annotations. In general, a function MUST NOT modify these annotations with the exception of the specific annotations listed below. This enables orchestrators to add additional internal annotations, without requiring changes to existing functions.

internal.config.kubernetes.io/path

Records the slash-delimited, OS-agnostic, relative file path to a resource. The path is relative to a fix location on the filesystem. Different orchestrator implementations can choose different fixed points.

A function SHOULD NOT modify these annotations.

Example:

metadata:
  annotations:
    internal.config.kubernetes.io/path: "relative/file/path.yaml"

internal.config.kubernetes.io/index

Records the index of a Resource in file. In a multi-object YAML file, resources are separated by three dashes (---), and the index represents the position of the Resource starting from zero. When this annotation is not specified, it implies a value of 0.

A function SHOULD NOT modify these annotations.

Example:

metadata:
  annotations:
    internal.config.kubernetes.io/path: "relative/file/path.yaml"
    internal.config.kubernetes.io/index: 2

This represents the third resource in the file.