Kubernetes (K8s) Documentation

Overview

Kubernetes (K8s) is an open-source container orchestration platform designed to automate the deployment, scaling, and operation of containerized applications.

Control Plane (CP)

The Control Plane is the core management component of a Kubernetes cluster. It makes global decisions about the cluster (e.g., scheduling), and it maintains the desired state of the cluster by managing workloads and directing communication within the system.

Note: By default, the Control Plane does not directly manage or run application containers.

Key Components of the Control Plane:

API Server (kube-apiserver):
Exposes the Kubernetes API and serves as the entry point to the cluster. It handles communication between internal components and external clients.
Scheduler (kube-scheduler):
Assigns work (e.g., pods) to nodes based on resource availability and policies.
Controller Manager (kube-controller-manager):
Runs controllers that regulate the state of the cluster (e.g., Node Controller, Replication Controller, etc.).
etcd:
A consistent, highly-available key-value store that stores all cluster data, configurations, and states. This is the "database" of Kubernetes.

Worker Nodes

Worker nodes are the machines where containerized applications run. Each worker node contains essential components for managing containers.

Key Components of a Worker Node:

Kubelet:
An agent that ensures containers are running as specified in their Pod definitions. It communicates with the Control Plane to receive and execute tasks.
Kube Proxy:
Maintains network rules and handles routing for communication between services within the cluster and with external traffic.

Data Flow

Kubelet and Kube Proxy on each worker node communicate with the API Server in the Control Plane for task execution and resource updates.
The Scheduler assigns pods to nodes based on resource requirements, while the Controller Manager ensures that the cluster state remains consistent.

Administration Tools

kubeadm:
A tool for bootstrapping Kubernetes clusters. It simplifies the setup and configuration of the Control Plane and worker nodes.
kubectl:
The command-line interface (CLI) for interacting with the Kubernetes API. It is used to manage resources, deploy applications, and inspect cluster components.

Kubernetes Version Compatibility

Kubernetes and Container Runtimes:

Kubernetes ≤ 1.23:
Compatible with Docker as the default container runtime.
Kubernetes 1.24 to 1.25:
Docker is not supported as a runtime. These versions require containerd or another Container Runtime Interface (CRI) implementation.
Kubernetes ≥ 1.25:
Docker can be installed on the server but must be used indirectly through containerd or another CRI-compliant runtime. Docker itself is not a supported runtime.

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