Create Distributed Kubernetes Cluster Using Kubeadm on Ubuntu 20.04

Introduction

Kubernetes is an open-source container-orchestration system for automating application deployment, scaling, and management. Originally designed by Google, Kubernetes is now maintained by the Cloud Native Computing Foundation.

In this guide, we will set up a distributed Kubernetes cluster using kubeadm on Ubuntu 20.04.

Kubernetes Cluster Architecture

We will implement a setup with one master node and three worker nodes.

Master Node

The master node runs the Kubernetes control plane processes, including the API server, scheduler, and controllers.

Key components:

• kube-apiserver: Exposes the Kubernetes API.
• kube-scheduler: Assigns pods to nodes.
• kube-controller-manager: Manages controllers like node and replication controllers.

Worker Nodes

Worker nodes contain the kubelet, kube-proxy, and a container runtime to manage and run workloads.

Key components:

• kubelet: Ensures pods are running and configured properly.
• kube-proxy: Maintains network configurations and enables service networking.
• Container runtime: Runs the containers (e.g., Docker).

Infrastructure Details

• Master Node: D2s_v3 (2 vCPUs, 8 GB RAM)
• Worker Node 1: D4s_v3 (4 vCPUs, 16 GB RAM)
• Worker Node 2: D4s_v3 (4 vCPUs, 16 GB RAM)
• Worker Node 3: D4s_v3 (4 vCPUs, 16 GB RAM)

All VMs are on the same private virtual network.

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Step 1: Prepare Workstation

Ensure SSH access to all nodes and configure Ansible to manage them. Set up the Ansible inventory file:

sudo vim /etc/ansible/hosts

Add the following content:

[masters]
master ansible_host=master_ip ansible_user=k8sadmin

[workers]
worker1 ansible_host=worker1_ip ansible_user=k8sadmin
worker2 ansible_host=worker2_ip ansible_user=k8sadmin
worker3 ansible_host=worker3_ip ansible_user=k8sadmin

[all:vars]
ansible_python_interpreter=/usr/bin/python3

Replace master_ip and worker*_ip with your node IPs.

---

Step 2: Create a User with Sudo Privileges

Create an Ansible playbook to add a user with sudo privileges:

vim create-user.yml

Add the following content:

- hosts: all
  become: yes
  tasks:
    - name: Create user kubeuser
      user:
        name: kubeuser
        state: present
        createhome: yes
        shell: /bin/bash
    - name: Grant passwordless sudo
      lineinfile:
        path: /etc/sudoers
        line: 'kubeuser ALL=(ALL) NOPASSWD: ALL'
        validate: 'visudo -cf %s'
    - name: Set up authorized SSH keys
      authorized_key:
        user: kubeuser
        key: "{{ lookup('file', '~/.ssh/id_rsa.pub') }}"

Run the playbook:

ansible-playbook -i hosts create-user.yml

---

Step 3: Install Kubernetes on All Nodes

Create a playbook to install Kubernetes components:

vim install-k8s.yml

Add the following content:

- hosts: all
  become: yes
  tasks:
    - name: Install Docker
      apt:
        name: docker.io
        state: present
        update_cache: true
    - name: Install Kubernetes packages
      apt:
        name:
          - apt-transport-https
          - kubelet=1.19.3-00
          - kubeadm=1.19.3-00
        state: present
        update_cache: true
    - name: Add Kubernetes apt repository
      apt_repository:
        repo: deb https://apt.kubernetes.io/ kubernetes-xenial main
        state: present

- hosts: master
  become: yes
  tasks:
    - name: Install kubectl
      apt:
        name: kubectl=1.19.3-00
        state: present

Run the playbook:

ansible-playbook -i hosts install-k8s.yml

---

Step 4: Configure Master Node

Create a playbook to configure the master node:

vim config-master.yml

Add the following content:

- hosts: master
  become: yes
  tasks:
    - name: Initialize Kubernetes cluster
      shell: kubeadm init --pod-network-cidr=10.244.0.0/16
      args:
        creates: cluster_initialized.txt
    - name: Set up kubeconfig
      copy:
        src: /etc/kubernetes/admin.conf
        dest: /home/kubeuser/.kube/config
        remote_src: yes
        owner: kubeuser
    - name: Install Flannel network
      shell: kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml

Run the playbook:

ansible-playbook -i hosts config-master.yml

---

Step 5: Configure Worker Nodes

Create a playbook to join worker nodes to the cluster:

vim config-workers.yml

Add the following content:

- hosts: master
  become: yes
  tasks:
    - name: Get join command
      shell: kubeadm token create --print-join-command
      register: join_command

- hosts: workers
  become: yes
  tasks:
    - name: Join the cluster
      shell: "{{ hostvars['master'].join_command }}"

Run the playbook:

ansible-playbook -i hosts config-workers.yml

---

Step 6: Verify the Cluster

SSH into the master node and verify:

kubectl get nodes

Expected output:

NAME      STATUS    ROLES     AGE       VERSION
master    Ready     master    1d        v1.19.3
worker1   Ready     <none>    1d        v1.19.3
worker2   Ready     <none>    1d        v1.19.3
worker3   Ready     <none>    1d        v1.19.3

Your Kubernetes cluster is now ready to deploy workloads!