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Kubernetes Custom Resources: Declarative Configuration

What Are Custom Resources?

See also: Kubernetes Custom Resources - Official Docs

Kubernetes Custom Resource Definitions (CRDs) extend Kubernetes to manage application-specific resources. They let you treat your microfrontends as first-class Kubernetes objects.

Standard Kubernetes Resources: 

kind: Deployment
kind: Service
kind: Ingress

Polyfea Custom Resources: 

kind: MicroFrontendClass
kind: MicroFrontend
kind: WebComponent

The Three Polyfea Resources

1. MicroFrontendClass

Defines shared configuration for a group of microfrontends - like a "tenant" or "application":

apiVersion: polyfea.github.io/v1alpha1
kind: MicroFrontendClass
metadata:
  name: my-app
  namespace: platform
spec:
  baseUri: "/app"
  title: "My Application"
  cspHeader: "default-src 'self'; script-src 'self' 'unsafe-inline'"
  namespacePolicy:
    from: FromNamespaces
    namespaces:
      - team-a
      - team-b

Purpose:

  • Define routing and base URL
  • Set security policies (CSP headers)
  • Configure PWA capabilities
  • Control which namespaces can attach microfrontends

2. MicroFrontend

Describes an individual microfrontend - its location, dependencies, and loading strategy:

apiVersion: polyfea.github.io/v1alpha1
kind: MicroFrontend
metadata:
  name: user-module
  namespace: team-a
spec:
  frontendClass: my-app
  modulePath: "/dist/user.esm.js"
  service:
    name: user-service
    port: 80
  dependsOn:
    - shared-components
  staticPaths:
    - kind: stylesheet
      href: "/dist/styles.css"

Purpose:

  • Point to the JavaScript module and assets
  • Declare dependencies on other microfrontends
  • Configure caching strategy
  • Link to the hosting service

3. WebComponent

Represents individual web components and where they should appear:

apiVersion: polyfea.github.io/v1alpha1
kind: WebComponent
metadata:
  name: user-nav-item
  namespace: team-a
spec:
  element: user-nav-item
  microFrontend: user-module
  displayRules:
    allOf:
    - context-name: navigation
  attributes:
    label: "Profile"
    icon: "person"

Purpose:

  • Map component tags to microfrontends
  • Define where components appear (context areas)
  • Set conditional rendering rules
  • Configure component attributes

How Kubernetes Controllers Work

Controllers continuously reconcile desired state (your YAML) with actual state (running system):

The Polyfea controller watches these resources and:

  1. Validates configurations
  2. Resolves service URLs
  3. Checks namespace policies
  4. Builds metadata for the browser
  5. Updates status conditions

Controller Resolution: Building the Metadata

The Reconciliation Loop

When you create or update resources, the controller:

  1. Validates the Resource
  2. Resolves Service URLs
  3. Builds Context Area Metadata
  4. Updates Status Conditions

Namespace Policy Enforcement

The controller enforces multi-tenancy through namespace policies:

# MicroFrontendClass in platform namespace
namespacePolicy:
  from: FromNamespaces
  namespaces:
    - team-a
    - team-b

When team-c tries to attach a MicroFrontend:

# This will be rejected
apiVersion: polyfea.github.io/v1alpha1
kind: MicroFrontend
metadata:
  name: unauthorized-mfe
  namespace: team-c  # ❌ Not in allowed list
spec:
  frontendClass: my-app

Controller action: 

status:
  phase: Rejected
  rejectionReason: "Namespace not allowed by MicroFrontendClass namespace policy"
  conditions:
    - type: Accepted
      status: "False"
      reason: NamespaceNotAllowed