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Upgrading kubeadm clusters

This page explains how to upgrade a Kubernetes cluster created with kubeadm from version 1.20.x to version 1.21.x, and from version 1.21.x to 1.21.y (where y > x). Skipping MINOR versions when upgrading is unsupported.

To see information about upgrading clusters created using older versions of kubeadm, please refer to following pages instead:

The upgrade workflow at high level is the following:

  1. Upgrade a primary control plane node.
  2. Upgrade additional control plane nodes.
  3. Upgrade worker nodes.

Before you begin

  • Make sure you read the release notes carefully.
  • The cluster should use a static control plane and etcd pods or external etcd.
  • Make sure to back up any important components, such as app-level state stored in a database. kubeadm upgrade does not touch your workloads, only components internal to Kubernetes, but backups are always a best practice.
  • Swap must be disabled.

Additional information

  • Draining nodes before kubelet MINOR version upgrades is required. In the case of control plane nodes, they could be running CoreDNS Pods or other critical workloads.
  • All containers are restarted after upgrade, because the container spec hash value is changed.

Determine which version to upgrade to

Find the latest stable 1.21 version using the OS package manager:

apt update
apt-cache madison kubeadm
# find the latest 1.21 version in the list
# it should look like 1.21.x-00, where x is the latest patch


yum list --showduplicates kubeadm --disableexcludes=kubernetes
# find the latest 1.21 version in the list
# it should look like 1.21.x-0, where x is the latest patch

Upgrading control plane nodes

The upgrade procedure on control plane nodes should be executed one node at a time. Pick a control plane node that you wish to upgrade first. It must have the /etc/kubernetes/admin.conf file.

Call "kubeadm upgrade"

For the first control plane node

  • Upgrade kubeadm:

# replace x in 1.21.x-00 with the latest patch version
apt-mark unhold kubeadm && \
apt-get update && apt-get install -y kubeadm=1.21.x-00 && \
apt-mark hold kubeadm
-
# since apt-get version 1.1 you can also use the following method
apt-get update && \
apt-get install -y --allow-change-held-packages kubeadm=1.21.x-00


# replace x in 1.21.x-0 with the latest patch version
yum install -y kubeadm-1.21.x-0 --disableexcludes=kubernetes

  • Verify that the download works and has the expected version:

    kubeadm version

  • Verify the upgrade plan:

    kubeadm upgrade plan

    This command checks that your cluster can be upgraded, and fetches the versions you can upgrade to. It also shows a table with the component config version states.

Note: kubeadm upgrade also automatically renews the certificates that it manages on this node. To opt-out of certificate renewal the flag --certificate-renewal=false can be used. For more information see the certificate management guide.
Note: If kubeadm upgrade plan shows any component configs that require manual upgrade, users must provide a config file with replacement configs to kubeadm upgrade apply via the --config command line flag. Failing to do so will cause kubeadm upgrade apply to exit with an error and not perform an upgrade.

  • Choose a version to upgrade to, and run the appropriate command. For example:

    # replace x with the patch version you picked for this upgrade
sudo kubeadm upgrade apply v1.21.x

    Once the command finishes you should see:

    [upgrade/successful] SUCCESS! Your cluster was upgraded to "v1.21.x". Enjoy!

[upgrade/kubelet] Now that your control plane is upgraded, please proceed with upgrading your kubelets if you haven't already done so.

  • Manually upgrade your CNI provider plugin.

    Your Container Network Interface (CNI) provider may have its own upgrade instructions to follow. Check the addons page to find your CNI provider and see whether additional upgrade steps are required.

    This step is not required on additional control plane nodes if the CNI provider runs as a DaemonSet.

For the other control plane nodes

Same as the first control plane node but use:

sudo kubeadm upgrade node

instead of:

sudo kubeadm upgrade apply

Also calling kubeadm upgrade plan and upgrading the CNI provider plugin is no longer needed.

Drain the node

  • Prepare the node for maintenance by marking it unschedulable and evicting the workloads:

    # replace <node-to-drain> with the name of your node you are draining
kubectl drain <node-to-drain> --ignore-daemonsets

Upgrade kubelet and kubectl

  • Upgrade the kubelet and kubectl

    # replace x in 1.21.x-00 with the latest patch version
    apt-mark unhold kubelet kubectl && \
    apt-get update && apt-get install -y kubelet=1.21.x-00 kubectl=1.21.x-00 && \
    apt-mark hold kubelet kubectl
    -
    # since apt-get version 1.1 you can also use the following method
    apt-get update && \
    apt-get install -y --allow-change-held-packages kubelet=1.21.x-00 kubectl=1.21.x-00
    

    # replace x in 1.21.x-0 with the latest patch version
    yum install -y kubelet-1.21.x-0 kubectl-1.21.x-0 --disableexcludes=kubernetes

  • Restart the kubelet:

    sudo systemctl daemon-reload
sudo systemctl restart kubelet

Uncordon the node

  • Bring the node back online by marking it schedulable:

    # replace <node-to-drain> with the name of your node
kubectl uncordon <node-to-drain>

Upgrade worker nodes

The upgrade procedure on worker nodes should be executed one node at a time or few nodes at a time, without compromising the minimum required capacity for running your workloads.

Upgrade kubeadm

  • Upgrade kubeadm:

# replace x in 1.21.x-00 with the latest patch version
apt-mark unhold kubeadm && \
apt-get update && apt-get install -y kubeadm=1.21.x-00 && \
apt-mark hold kubeadm
-
# since apt-get version 1.1 you can also use the following method
apt-get update && \
apt-get install -y --allow-change-held-packages kubeadm=1.21.x-00


# replace x in 1.21.x-0 with the latest patch version
yum install -y kubeadm-1.21.x-0 --disableexcludes=kubernetes

Call "kubeadm upgrade"

  • For worker nodes this upgrades the local kubelet configuration:

    sudo kubeadm upgrade node

Drain the node

  • Prepare the node for maintenance by marking it unschedulable and evicting the workloads:

    # replace <node-to-drain> with the name of your node you are draining
kubectl drain <node-to-drain> --ignore-daemonsets

Upgrade kubelet and kubectl

  • Upgrade the kubelet and kubectl:

# replace x in 1.21.x-00 with the latest patch version
apt-mark unhold kubelet kubectl && \
apt-get update && apt-get install -y kubelet=1.21.x-00 kubectl=1.21.x-00 && \
apt-mark hold kubelet kubectl
-
# since apt-get version 1.1 you can also use the following method
apt-get update && \
apt-get install -y --allow-change-held-packages kubelet=1.21.x-00 kubectl=1.21.x-00


# replace x in 1.21.x-0 with the latest patch version
yum install -y kubelet-1.21.x-0 kubectl-1.21.x-0 --disableexcludes=kubernetes

  • Restart the kubelet:

    sudo systemctl daemon-reload
sudo systemctl restart kubelet

Uncordon the node

  • Bring the node back online by marking it schedulable:

    # replace <node-to-drain> with the name of your node
kubectl uncordon <node-to-drain>

Verify the status of the cluster

After the kubelet is upgraded on all nodes verify that all nodes are available again by running the following command from anywhere kubectl can access the cluster:

kubectl get nodes

The STATUS column should show Ready for all your nodes, and the version number should be updated.

Recovering from a failure state

If kubeadm upgrade fails and does not roll back, for example because of an unexpected shutdown during execution, you can run kubeadm upgrade again. This command is idempotent and eventually makes sure that the actual state is the desired state you declare.

To recover from a bad state, you can also run kubeadm upgrade apply --force without changing the version that your cluster is running.

During upgrade kubeadm writes the following backup folders under /etc/kubernetes/tmp:

  • kubeadm-backup-etcd-<date>-<time>
  • kubeadm-backup-manifests-<date>-<time>

kubeadm-backup-etcd contains a backup of the local etcd member data for this control plane Node. In case of an etcd upgrade failure and if the automatic rollback does not work, the contents of this folder can be manually restored in /var/lib/etcd. In case external etcd is used this backup folder will be empty.

kubeadm-backup-manifests contains a backup of the static Pod manifest files for this control plane Node. In case of a upgrade failure and if the automatic rollback does not work, the contents of this folder can be manually restored in /etc/kubernetes/manifests. If for some reason there is no difference between a pre-upgrade and post-upgrade manifest file for a certain component, a backup file for it will not be written.

How it works

kubeadm upgrade apply does the following:

  • Checks that your cluster is in an upgradeable state:
    • The API server is reachable
    • All nodes are in the Ready state
    • The control plane is healthy
  • Enforces the version skew policies.
  • Makes sure the control plane images are available or available to pull to the machine.
  • Generates replacements and/or uses user supplied overwrites if component configs require version upgrades.
  • Upgrades the control plane components or rollbacks if any of them fails to come up.
  • Applies the new CoreDNS and kube-proxy manifests and makes sure that all necessary RBAC rules are created.
  • Creates new certificate and key files of the API server and backs up old files if they're about to expire in 180 days.

kubeadm upgrade node does the following on additional control plane nodes:

  • Fetches the kubeadm ClusterConfiguration from the cluster.
  • Optionally backups the kube-apiserver certificate.
  • Upgrades the static Pod manifests for the control plane components.
  • Upgrades the kubelet configuration for this node.

kubeadm upgrade node does the following on worker nodes:

  • Fetches the kubeadm ClusterConfiguration from the cluster.
  • Upgrades the kubelet configuration for this node.
Last modified September 20, 2021 at 1:27 PM PST : update kubeadm upgrade page to use current branch (a39553d328)