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Creating Highly Available Clusters with kubeadm

This page explains two different approaches to setting up a highly available Kubernetes cluster using kubeadm:

Before proceeding, you should carefully consider which approach best meets the needs of your applications and environment. This comparison topic outlines the advantages and disadvantages of each.

Your clusters must run Kubernetes version 1.12 or later. You should also be aware that setting up HA clusters with kubeadm is still experimental and will be further simplified in future versions. You might encounter issues with upgrading your clusters, for example. We encourage you to try either approach, and provide us with feedback in the kubeadm issue tracker.

Note that the alpha feature gate HighAvailability is deprecated in v1.12 and removed in v1.13.

See also The HA upgrade documentation.

Caution: This page does not address running your cluster on a cloud provider. In a cloud environment, neither approach documented here works with Service objects of type LoadBalancer, or with dynamic PersistentVolumes.

Before you begin

For both methods you need this infrastructure:

For the external etcd cluster only, you also need:

Note: The following examples run Calico as the Pod networking provider. If you run another networking provider, make sure to replace any default values as needed.

First steps for both methods

Note: All commands on any control plane or etcd node should be run as root.

Create load balancer for kube-apiserver

Note: There are many configurations for load balancers. The following example is only one option. Your cluster requirements may need a different configuration.

  1. Create a kube-apiserver load balancer with a name that resolves to DNS.

    • In a cloud environment you should place your control plane nodes behind a TCP forwarding load balancer. This load balancer distributes traffic to all healthy control plane nodes in its target list. The health check for an apiserver is a TCP check on the port the kube-apiserver listens on (default value :6443).

    • It is not recommended to use an IP address directly in a cloud environment.

    • The load balancer must be able to communicate with all control plane nodes on the apiserver port. It must also allow incoming traffic on its listening port.

    • HAProxy can be used as a load balancer.

    • Make sure the address of the load balancer always matches the address of kubeadm’s ControlPlaneEndpoint.

  2. Add the first control plane nodes to the load balancer and test the connection:

    nc -v LOAD_BALANCER_IP PORT
    • A connection refused error is expected because the apiserver is not yet running. A timeout, however, means the load balancer cannot communicate with the control plane node. If a timeout occurs, reconfigure the load balancer to communicate with the control plane node.

  3. Add the remaining control plane nodes to the load balancer target group.

Configure SSH

SSH is required if you want to control all nodes from a single machine.

  1. Enable ssh-agent on your main device that has access to all other nodes in the system:

    eval $(ssh-agent)

  2. Add your SSH identity to the session:

    ssh-add ~/.ssh/path_to_private_key

  3. SSH between nodes to check that the connection is working correctly.

    • When you SSH to any node, make sure to add the -A flag:

      ssh -A 10.0.0.7

    • When using sudo on any node, make sure to preserve the environment so SSH forwarding works:

      sudo -E -s

Stacked control plane and etcd nodes

Steps for the first control plane node

  1. On the first control plane node, create a configuration file called kubeadm-config.yaml:

    apiVersion: kubeadm.k8s.io/v1beta1
kind: ClusterConfiguration
kubernetesVersion: stable
apiServer:
  certSANs:
  - "LOAD_BALANCER_DNS"
controlPlaneEndpoint: "LOAD_BALANCER_DNS:LOAD_BALANCER_PORT"
    • kubernetesVersion should be set to the Kubernetes version to use. This example uses stable.
    • controlPlaneEndpoint should match the address or DNS and port of the load balancer.
    • It’s recommended that the versions of kubeadm, kubelet, kubectl and Kubernetes match.

  2. Make sure that the node is in a clean state:

    sudo kubeadm init --config=kubeadm-config.yaml

    You should see something like:

    ...
You can now join any number of machines by running the following on each node
as root:
    
kubeadm join 192.168.0.200:6443 --token j04n3m.octy8zely83cy2ts --discovery-token-ca-cert-hash    sha256:84938d2a22203a8e56a787ec0c6ddad7bc7dbd52ebabc62fd5f4dbea72b14d1f

  3. Copy this output to a text file. You will need it later to join other control plane nodes to the cluster.

  4. Apply the Weave CNI plugin:

    kubectl apply -f "https://cloud.weave.works/k8s/net?k8s-version=$(kubectl version | base64 | tr -d '\n')"

  5. Type the following and watch the pods of the components get started:

    kubectl get pod -n kube-system -w
    • It’s recommended that you join new control plane nodes only after the first node has finished initializing.

  6. Copy the certificate files from the first control plane node to the rest:

    In the following example, replace CONTROL_PLANE_IPS with the IP addresses of the other control plane nodes.

    USER=ubuntu # customizable
CONTROL_PLANE_IPS="10.0.0.7 10.0.0.8"
for host in ${CONTROL_PLANE_IPS}; do
    scp /etc/kubernetes/pki/ca.crt "${USER}"@$host:
    scp /etc/kubernetes/pki/ca.key "${USER}"@$host:
    scp /etc/kubernetes/pki/sa.key "${USER}"@$host:
    scp /etc/kubernetes/pki/sa.pub "${USER}"@$host:
    scp /etc/kubernetes/pki/front-proxy-ca.crt "${USER}"@$host:
    scp /etc/kubernetes/pki/front-proxy-ca.key "${USER}"@$host:
    scp /etc/kubernetes/pki/etcd/ca.crt "${USER}"@$host:etcd-ca.crt
    scp /etc/kubernetes/pki/etcd/ca.key "${USER}"@$host:etcd-ca.key
    scp /etc/kubernetes/admin.conf "${USER}"@$host:
done
Caution: Copy only the certificates in the above list. kubeadm will take care of generating the rest of the certificates with the required SANs for the joining control-plane instances. If you copy all the certificates by mistake, the creation of additional nodes could fail due to a lack of required SANs.

Steps for the rest of the control plane nodes

  1. Move the files created by the previous step where scp was used:

    USER=ubuntu # customizable
mkdir -p /etc/kubernetes/pki/etcd
mv /home/${USER}/ca.crt /etc/kubernetes/pki/
mv /home/${USER}/ca.key /etc/kubernetes/pki/
mv /home/${USER}/sa.pub /etc/kubernetes/pki/
mv /home/${USER}/sa.key /etc/kubernetes/pki/
mv /home/${USER}/front-proxy-ca.crt /etc/kubernetes/pki/
mv /home/${USER}/front-proxy-ca.key /etc/kubernetes/pki/
mv /home/${USER}/etcd-ca.crt /etc/kubernetes/pki/etcd/ca.crt
mv /home/${USER}/etcd-ca.key /etc/kubernetes/pki/etcd/ca.key
mv /home/${USER}/admin.conf /etc/kubernetes/admin.conf

    This process writes all the requested files in the /etc/kubernetes folder.

  2. Start kubeadm join on this node using the join command that was previously given to you by kubeadm init on the first node. It should look something like this:

    sudo kubeadm join 192.168.0.200:6443 --token j04n3m.octy8zely83cy2ts --discovery-token-ca-cert-hash sha256:84938d2a22203a8e56a787ec0c6ddad7bc7dbd52ebabc62fd5f4dbea72b14d1f --experimental-control-plane
    • Notice the addition of the --experimental-control-plane flag. This flag automates joining this control plane node to the cluster.

  3. Type the following and watch the pods of the components get started:

    kubectl get pod -n kube-system -w

  4. Repeat these steps for the rest of the control plane nodes.

External etcd nodes

Set up the etcd cluster

Set up the first control plane node

  1. Copy the following files from any node from the etcd cluster to this node:

    export CONTROL_PLANE="ubuntu@10.0.0.7"
+scp /etc/kubernetes/pki/etcd/ca.crt "${CONTROL_PLANE}":
+scp /etc/kubernetes/pki/apiserver-etcd-client.crt "${CONTROL_PLANE}":
+scp /etc/kubernetes/pki/apiserver-etcd-client.key "${CONTROL_PLANE}":
    • Replace the value of CONTROL_PLANE with the user@host of this machine.

  2. Create a file called kubeadm-config.yaml with the following contents:

    apiVersion: kubeadm.k8s.io/v1beta1
kind: ClusterConfiguration
kubernetesVersion: stable
apiServer:
  certSANs:
  - "LOAD_BALANCER_DNS"
controlPlaneEndpoint: "LOAD_BALANCER_DNS:LOAD_BALANCER_PORT"
etcd:
    external:
        endpoints:
        - https://ETCD_0_IP:2379
        - https://ETCD_1_IP:2379
        - https://ETCD_2_IP:2379
        caFile: /etc/kubernetes/pki/etcd/ca.crt
        certFile: /etc/kubernetes/pki/apiserver-etcd-client.crt
        keyFile: /etc/kubernetes/pki/apiserver-etcd-client.key

    • The difference between stacked etcd and external etcd here is that we are using the external field for etcd in the kubeadm config. In the case of the stacked etcd topology this is managed automatically.

    • Replace the following variables in the template with the appropriate values for your cluster:

      • LOAD_BALANCER_DNS
      • LOAD_BALANCER_PORT
      • ETCD_0_IP
      • ETCD_1_IP
      • ETCD_2_IP

  3. Run kubeadm init --config kubeadm-config.yaml on this node.

  4. Write the join command that is returned to a text file for later use.

  5. Apply the Weave CNI plugin:

    kubectl apply -f "https://cloud.weave.works/k8s/net?k8s-version=$(kubectl version | base64 | tr -d '\n')"

Steps for the rest of the control plane nodes

To add the rest of the control plane nodes, follow these instructions. The steps are the same as for the stacked etcd setup, with the exception that a local etcd member is not created.

To summarize:

Common tasks after bootstrapping control plane

Install a pod network

Follow these instructions to install the pod network. Make sure this corresponds to whichever pod CIDR you provided in the master configuration file.

Install workers

Each worker node can now be joined to the cluster with the command returned from any of the kubeadm init commands. The flag --experimental-control-plane should not be added to worker nodes.

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