🚢 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 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 cluster's entry point. It handles communication between internal components and external clients.
Scheduler (kube-scheduler)
Assigns workloads (e.g., Pods) to nodes based on resource availability and defined policies.
Controller Manager (kube-controller-manager)
Runs controllers that monitor and regulate the cluster's state, such as the Node Controller and Replication Controller.
etcd
A consistent and highly available key-value store that stores all cluster data, configurations, and state. This is the "database" of Kubernetes.

🧱 Worker Nodes

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

🔧 Key Components of a Worker Node

Kubelet
An agent that ensures containers run as specified in their Pod definitions. It communicates with the Control Plane to execute assigned tasks.
Kube Proxy
Maintains network rules and manages routing for communication within the cluster and with external systems.

🔄 Data Flow

Kubelet and Kube Proxy on each worker node interact with the API Server to perform operations and update resource states.
The Scheduler selects suitable nodes for pod placement based on available resources.
The Controller Manager ensures the actual state of the cluster matches the desired state.

🛠️ Administration Tools

kubeadm
A command-line tool to bootstrap and configure Kubernetes clusters. It streamlines the setup of both the Control Plane and worker nodes.
kubectl
The CLI for interacting with the Kubernetes API. It's used to deploy apps, inspect cluster resources, and manage configurations.

🧩 Kubernetes Version Compatibility

Kubernetes and Container Runtimes

Kubernetes ≤ 1.23
✅ Compatible with Docker as the default container runtime.
Kubernetes 1.24 – 1.25
❌ Docker is not supported directly. Use containerd or another CRI-compliant runtime.
Kubernetes ≥ 1.25
⚠️ Docker may be installed on the system but must be used indirectly through containerd or another supported CRI.

👥 Kubernetes Roles

Control Plane (Manager)
Requires an odd number of nodes for high availability (e.g., 1, 3, 5, ...). This ensures quorum in distributed consensus.
Worker (none)
These nodes run application workloads and do not participate in control decisions.

image pull policy in kubernetes:

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