How to Join a Worker Node to a Kubernetes Cluster

Once you’ve set up your Kubernetes control plane (master node), the next step is to add worker nodes to your cluster. Worker nodes are where your applications and workloads will run. In this guide, we’ll walk you through the process of joining a worker node to your Kubernetes cluster.

Prerequisites

Control Plane Node:

A fully initialized Kubernetes control plane (using kubeadm init).
The kubeadm join command from the control plane output.

Worker Node:

Ubuntu installed (same version as the control plane).
Docker or containerd installed and configured.
kubeadm, kubelet, and kubectl installed (same version as the control plane).

Step 1: Prepare the Worker Node

Before joining the worker node, ensure it meets the following requirements:

1. Disable Swap

Kubernetes requires swap to be disabled:

sudo swapoff -a
sudo sed -i '/ swap / s/^\(.*\)$/#\1/g' /etc/fstab

2. Install Container Runtime

Install Docker or containerd (same as the control plane). For example, to install containerd:

sudo apt update
sudo apt install -y containerd
sudo systemctl enable --now containerd

3. Install Kubernetes Tools

Install kubeadm, kubelet, and kubectl (ensure the versions match the control plane):

sudo apt update
sudo apt install -y kubeadm kubelet kubectl
sudo apt-mark hold kubeadm kubelet kubectl

Step 2: Join the Worker Node

Use the kubeadm join command generated during the control plane initialization.

1. Retrieve the Join Command

On the control plane node, retrieve the kubeadm join command:

kubeadm token create --print-join-command

This will output a command like:

kubeadm join <control-plane-ip>:6443 --token <token> --discovery-token-ca-cert-hash sha256:<hash>

2. Run the Join Command on the Worker Node

On the worker node, run the kubeadm join command:

sudo kubeadm join <control-plane-ip>:6443 --token <token> --discovery-token-ca-cert-hash sha256:<hash>

Example:

sudo kubeadm join 192.168.1.100:6443 --token abcdef.0123456789abcdef \
  --discovery-token-ca-cert-hash sha256:1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef

Step 3: Verify the Worker Node

After joining the worker node, verify its status from the control plane.

1. Check Node Status

On the control plane node, run:

kubectl get nodes

The worker node should appear in the list with a NotReady status initially. After a few minutes, it should transition to Ready.

2. Check Pods on the Worker Node

Verify that the CNI plugin (e.g., Flannel, Calico) is running on the worker node:

kubectl get pods -n kube-system -o wide

You should see CNI-related pods (e.g., kube-flannel-ds-*) running on the worker node.

Troubleshooting

If the worker node doesn’t join successfully, check the following:

1. Token Expired

If the token has expired, generate a new one on the control plane:

kubeadm token create --print-join-command

2. Network Issues

Ensure the worker node can reach the control plane on port 6443.
Check firewall rules to allow traffic between nodes.

3. CNI Plugin Not Installed

If the worker node stays in NotReady state, ensure the CNI plugin is installed:

kubectl apply -f https://raw.githubusercontent.com/flannel-io/flannel/master/Documentation/kube-flannel.yml

4. Kubelet Logs

Check the kubelet logs on the worker node for errors:

sudo journalctl -u kubelet -f

Conclusion

Joining a worker node to a Kubernetes cluster is a simple process using the kubeadm join command. By ensuring your worker node meets the prerequisites and running the join command, you can scale your cluster to handle more workloads.

Next Steps:

Add more worker nodes to scale your cluster.
Deploy applications and monitor their performance using tools like kubectl and kubectl top.

For more details, refer to the official Kubernetes documentation. Happy clustering! 🚀