Kubeadm

Kubeadm support is added with etcd as datastore

Overview

Kubeadm is a tool built to provide best-practice “fast paths” for creating Kubernetes clusters. ksctl integrates Kubeadm to provision highly-available Kubernetes clusters across your infrastructure.

Features

• External etcd topology: Uses a dedicated etcd cluster for improved reliability
• HA control plane: Supports multiple control plane nodes for high availability
• Seamless cluster expansion: Easily add worker nodes
• Automatic certificate management: Handles all required PKI setup
• Secure token handling: Manages bootstrap tokens with automatic renewal

Architecture

ksctl with Kubeadm creates a cluster with the following components:

• External etcd cluster: Dedicated nodes for the etcd database
• Control plane nodes: Running the Kubernetes control plane components
• Worker nodes: For running your workloads
• Load balancer: Provides a single entry point to the control plane

┌────────────────┐    ┌────────────────┐    ┌────────────────┐
│                │    │                │    │                │
│  Control Plane │    │  Control Plane │    │  Control Plane │
│                │    │                │    │                │
└────────────────┘    └────────────────┘    └────────────────┘
        │                     │                     │
        └─────────────┬───────┴─────────────┬───────┘
                      │                     │
                      ▼                     ▼
           ┌─────────────────────┐  ┌─────────────────┐
           │                     │  │                 │
           │    Load Balancer    │  │  External etcd  │
           │                     │  │                 │
           └─────────────────────┘  └─────────────────┘
                      │
        ┌─────────────┴───────────┐
        │                         │
        ▼                         ▼
┌────────────────┐      ┌────────────────┐
│                │      │                │
│  Worker Node   │ ...  │  Worker Node   │
│                │      │                │
└────────────────┘      └────────────────┘

Technical Details

Kubernetes Versions

ksctl supports multiple Kubernetes versions through Kubeadm. The system automatically fetches available versions from the Kubernetes releases, allowing you to choose your preferred version.

Container Runtime

The default container runtime used is containerd, which is set up and configured automatically during cluster deployment.

Network Configuration

Kubeadm in ksctl configures the following networking defaults:

• Pod subnet: 10.244.0.0/16
• Service subnet: 10.96.0.0/12
• DNS domain: cluster.local

CNI (Container Network Interface)

Bootstrap Process

The bootstrap process follows these steps:

1. Control plane initialization:

   • Generates bootstrap tokens and certificate keys
   • Configures the first control plane node
   • Sets up external etcd connection
   • Initializes the Kubernetes API server

2. Additional control plane nodes:

   • Join additional control plane nodes to form an HA cluster
   • Share certificates securely between nodes

3. Worker node setup:

   • Install required components (containerd, kubelet, etc.)
   • Join worker nodes to the cluster using bootstrap tokens

Cluster Access

Once your cluster is created, ksctl automatically configures and saves your kubeconfig file with appropriate context names based on your cluster details.

Limitations

• All nodes must run Ubuntu-based Linux distributions