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Kubernetes Architecture

Understanding Kubernetes Components​

1. Overview​

A Kubernetes cluster is made up of multiple components that work together to deploy, manage, and scale containerized applications.

At a high level, Kubernetes follows a master–worker architecture, where:

  • The Control Plane manages the cluster
  • The Worker Nodes run the applications (Pods)

2. High-Level Kubernetes Architecture​

A Kubernetes cluster consists of:

  • Control Plane — Responsible for decision-making and cluster state
  • Worker Nodes — Responsible for running application workloads
Kubernetes Cluster
│
├── Control Plane
│
└── Worker Nodes (1 or more)

3. Control Plane Components​

The Control Plane manages the overall state of the Kubernetes cluster.

It makes global decisions such as:

  • Scheduling pods
  • Maintaining desired state
  • Handling failures
  • Managing cluster configuration

3.1 kube-apiserver​

kube-apiserver is the central management component of Kubernetes.

Key responsibilities:

  • Exposes the Kubernetes REST/HTTP API
  • Acts as the front-end of the control plane
  • All communication (kubectl, controllers, nodes) goes through it
  • Validates and processes API requests

3.2 etcd​

etcd is a distributed, consistent, and highly available key-value store.

Key responsibilities:

  • Stores the entire cluster state
  • Holds:
    • Pod definitions
    • ConfigMaps
    • Secrets
    • Node information
  • Acts as the single source of truth

3.3 kube-scheduler​

The kube-scheduler decides where Pods should run.

Key responsibilities:

  • Watches for Pods without assigned nodes
  • Selects the best node based on:
    • CPU & memory availability
    • Affinity / anti-affinity rules
    • Taints and tolerations
  • Assigns the Pod to a node

3.4 kube-controller-manager​

The kube-controller-manager runs multiple controllers.

Each controller continuously monitors the cluster and ensures the desired state matches the actual state.

Common controllers:

  • Node Controller
  • ReplicaSet Controller
  • Deployment Controller
  • Job Controller
  • Endpoint Controller

3.5 cloud-controller-manager​

This component integrates Kubernetes with cloud providers.

Responsibilities:

  • Manages cloud-specific resources:
    • Load balancers
    • Volumes
    • Node lifecycle
  • Allows Kubernetes to run consistently across clouds

4. Worker Node Components​

Worker Nodes are the machines where application workloads run.

Each node contains components required to:

  • Run Pods
  • Communicate with the control plane
  • Handle networking and container execution

4.1 kubelet​

kubelet is the node agent.

Key responsibilities:

  • Communicates with kube-apiserver
  • Ensures containers defined in Pods are running
  • Reports node and pod status
  • Performs health checks

4.2 kube-proxy​

kube-proxy handles networking for Services.

Key responsibilities:

  • Maintains network rules on nodes
  • Enables Service-to-Pod communication
  • Implements load balancing using:
    • iptables or IPVS

4.3 Container Runtime​

The container runtime is responsible for running containers.

Examples:

  • containerd
  • CRI-O

Responsibilities:

  • Pull container images
  • Create and manage containers
  • Handle container lifecycle