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Architecture and Concepts

This document explains the foundational concepts behind the Krateo Core Provider. Read this before consulting the how-to guides.

Glossary​

  • CRD (Custom Resource Definition): A Kubernetes resource that defines custom objects and their schemas, enabling users to extend the Kubernetes API.
  • CompositionDefinition: A declarative Krateo resource that acts as a master blueprint for a deployable service. It consumes a Helm chart and uses it to dynamically generate a versioned CRD.
  • Composition: A Custom Resource representing a single live instance of a service defined by a CompositionDefinition.
  • CDC (Composition Dynamic Controller): A dedicated controller deployed by the Core Provider for each CompositionDefinition. It manages the full lifecycle of Helm releases based on Composition resources.
  • Chart Inspector: A service that introspects Helm chart content to provide the CDC with resource metadata for RBAC generation.
  • OCI Registry: A container registry used for storing and distributing Helm charts.
  • values.schema.json: A JSON Schema file included in Helm charts to define and validate the structure of values.yaml.

Architecture​

core-provider Architecture

The Core Provider orchestrates three main actors:

  • Core Provider: Watches CompositionDefinition resources. For each one, it generates a CRD and deploys a CDC.
  • CDC (Composition Dynamic Controller): Watches Composition resources of a specific version and manages Helm releases.
  • Chart Inspector: Provides the CDC with the list of resources it manages, enabling precise RBAC generation.

For a detailed look at the security model, see Security Design.

Lifecycle Workflow​

core-provider State Diagram

The lifecycle of a Krateo service follows these stages:

  1. Registration: A CompositionDefinition is applied to the cluster.
  2. Generation: The Core Provider generates the CRD and spawns the CDC.
  3. Instantiation: A user creates a Composition resource.
  4. Enforcement: The CDC installs the Helm chart and continuously reconciles the state.
  5. Evolution: The service is upgraded following one of the Version Management patterns.

Chart Requirements​

To be compatible with Krateo, a Helm chart must include a values.schema.json file. This file is used to:

  1. Generate the OpenAPI schema for the Composition CRD.
  2. Validate user input during kubectl apply.
  3. Provide metadata for UI generation in the Krateo Control Plane.