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Setting up Cluster Federation with Kubefed

Kubernetes version 1.5 includes a new command line tool called kubefed to help you administrate your federated clusters. kubefed helps you to deploy a new Kubernetes cluster federation control plane, and to add clusters to or remove clusters from an existing federation control plane.

This guide explains how to administer a Kubernetes Cluster Federation using kubefed.

Note: kubefed is an alpha feature in Kubernetes 1.5.

Prerequisites

This guide assumes that you have a running Kubernetes cluster. Please see one of the getting started guides for installation instructions for your platform.

Getting kubefed

Download the client tarball corresponding to Kubernetes version 1.5 or later from the release page, extract the binaries in the tarball to one of the directories in your $PATH and set the executable permission on those binaries.

Note: The URL in the curl command below downloads the binaries for Linux amd64. If you are on a different platform, please use the URL for the binaries appropriate for your platform. You can find the list of available binaries on the release page.

curl -O https://storage.googleapis.com/kubernetes-release/release/v1.5.2/kubernetes-client-linux-amd64.tar.gz
tar -xzvf kubernetes-client-linux-amd64.tar.gz
sudo cp kubernetes/client/bin/kubefed /usr/local/bin
sudo chmod +x /usr/local/bin/kubefed
sudo cp kubernetes/client/bin/kubectl /usr/local/bin
sudo chmod +x /usr/local/bin/kubectl

Choosing a host cluster.

You’ll need to choose one of your Kubernetes clusters to be the host cluster. The host cluster hosts the components that make up your federation control plane. Ensure that you have a kubeconfig entry in your local kubeconfig that corresponds to the host cluster. You can verify that you have the required kubeconfig entry by running:

kubectl config get-contexts

The output should contain an entry corresponding to your host cluster, similar to the following:

CURRENT   NAME                                          CLUSTER                                       AUTHINFO                                      NAMESPACE
          gke_myproject_asia-east1-b_gce-asia-east1     gke_myproject_asia-east1-b_gce-asia-east1     gke_myproject_asia-east1-b_gce-asia-east1

You’ll need to provide the kubeconfig context (called name in the entry above) for your host cluster when you deploy your federation control plane.

Deploying a federation control plane.

To deploy a federation control plane on your host cluster, run kubefed init command. When you use kubefed init, you must provide the following:

The following example command deploys a federation control plane with the name fellowship, a host cluster context rivendell, and the domain suffix example.com:

kubefed init fellowship --host-cluster-context=rivendell  --dns-zone-name="example.com"

The domain suffix specified in --dns-zone-name must be an existing domain that you control, and that is programmable by your DNS provider.

kubefed init sets up the federation control plane in the host cluster and also adds an entry for the federation API server in your local kubeconfig. Note that in the alpha release in Kubernetes 1.5, kubefed init does not automatically set the current context to the newly deployed federation. You can set the current context manually by running:

kubectl config use-context fellowship

where fellowship is the name of your federation.

Adding a cluster to a federation

Once you’ve deployed a federation control plane, you’ll need to make that control plane aware of the clusters it should manage. You can add a cluster to your federation by using the kubefed join command.

To use kubefed join, you’ll need to provide the name of the cluster you want to add to the federation, and the --host-cluster-context for the federation control plane’s host cluster.

The following example command adds the cluster gondor to the federation with host cluster rivendell:

kubefed join gondor --host-cluster-context=rivendell

Note: Kubernetes requires that you manually join clusters to a federation because the federation control plane manages only those clusters that it is responsible for managing. Adding a cluster tells the federation control plane that it is responsible for managing that cluster.

Naming rules and customization

The cluster name you supply to kubefed join must be a valid RFC 1035 label.

Furthermore, federation control plane requires credentials of the joined clusters to operate on them. These credentials are obtained from the local kubeconfig. kubefed join uses the cluster name specified as the argument to look for the cluster’s context in the local kubeconfig. If it fails to find a matching context, it exits with an error.

This might cause issues in cases where context names for each cluster in the federation don’t follow RFC 1035 label naming rules. In such cases, you can specify a cluster name that conforms to the RFC 1035 label naming rules and specify the cluster context using the --cluster-context flag. For example, if context of the cluster your are joining is gondor_needs-no_king, then you can join the cluster by running:

kubefed join gondor --host-cluster-context=rivendell --cluster-context=gondor_needs-no_king

Secret name

Cluster credentials required by the federation control plane as described above are stored as a secret in the host cluster. The name of the secret is also derived from the cluster name.

However, the name of a secret object in Kubernetes should conform to the DNS subdomain name specification described in RFC 1123. If this isn’t the case, you can pass the secret name to kubefed join using the --secret-name flag. For example, if the cluster name is noldor and the secret name is 11kingdom, you can join the cluster by running:

kubefed join noldor --host-cluster-context=rivendell --secret-name=11kingdom

Note: If your cluster name does not conform to the DNS subdomain name specification, all you need to do is supply the secret name via the --secret-name flag. kubefed join automatically creates the secret for you.

Removing a cluster from a federation

To remove a cluster from a federation, run the kubefed unjoin command with the cluster name and the federation’s --host-cluster-context:

kubefed unjoin gondor --host-cluster-context=rivendell

Turning down the federation control plane:

Proper cleanup of federation control plane is not fully implemented in this alpha release of kubefed. However, for the time being, deleting the federation system namespace should remove all the resources except the persistent storage volume dynamically provisioned for the federation control plane’s etcd. You can delete the federation namespace by running the following command:

$ kubectl delete ns federation-system

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