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1.2. Basic Concepts

Introducing Karmada​

Karmada (Kubernetes Armada) is a CNCF incubating project -- an open source system for multi-cluster Kubernetes orchestration.

Architecture Overview​

MCM structure

Control Plane Components​

karmada-apiserver​

  • Acts as the front end of the control plane
  • A REST endpoint that serves the Karmada and Kubernetes APIs
  • Compatible with existing tools such as kubectl

karmada-controller-manager​

  • Runs the controllers that watch and process Karmada objects
  • Talks to the member clusters' API servers to create resources
  • The main controllers:
    • Cluster Controller: manages the cluster lifecycle
    • Policy Controller: processes PropagationPolicy
    • Binding Controller: turns a ResourceBinding into Work
    • Execution Controller: deploys Work to the member clusters

karmada-scheduler​

  • Assigns resources to suitable member clusters
  • Evaluates cluster constraints and available resources
  • Selects and binds the best cluster

karmada-webhook​

  • The validating and mutating webhooks for API requests
  • Mutating webhook: alters objects and sets defaults
  • Validating webhook: rejects requests that violate policy

karmada-agent​

  • Deployed on Pull mode clusters
  • Registers the cluster with the control plane
  • Synchronises manifests and reports status

Resource Propagation Flow​

Resource propagation flow

The Core Resource Types​

Resource Template​

Existing Kubernetes resource definitions are used as they are.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    spec:
      containers:
      - name: nginx
        image: nginx:1.21

PropagationPolicy​

Defines which clusters a resource is deployed to.

apiVersion: policy.karmada.io/v1alpha1
kind: PropagationPolicy
metadata:
  name: nginx-propagation
spec:
  resourceSelectors:
    - apiVersion: apps/v1
      kind: Deployment
      name: nginx-deployment
  placement:
    clusterAffinity:
      clusterNames:
        - cluster-a
        - cluster-b

OverridePolicy​

Changes resource settings per cluster.

apiVersion: policy.karmada.io/v1alpha1
kind: OverridePolicy
metadata:
  name: nginx-override
spec:
  resourceSelectors:
    - apiVersion: apps/v1
      kind: Deployment
      name: nginx-deployment
  overrideRules:
    - targetCluster:
        clusterNames:
          - cluster-b
      overriders:
        plaintext:
          - path: "/spec/replicas"
            operator: replace
            value: 5

Cluster Registration Modes​

Push Mode​

  • The control plane reaches the member cluster directly
  • The cluster credentials are held by the control plane
  • Simple to set up, but network reachability is required

Pull Mode​

  • karmada-agent is deployed on the member cluster
  • The agent connects to the control plane and receives work
  • Suited to registering clusters behind a firewall

Next Steps​

📖 Note: for more on Karmada, see the official Karmada documentation.