Kubernetes Cluster Federation with Admiralty

Kubernetes today is a hugely prevalent tool in 2021, and more organizations are increasingly running their applications on multiple clusters of Kubernetes. But these multiple cluster architectures often have a combination of multiple cloud providers, multiple data centres, multiple regions, and multiple zones where the applications are running. So, deploying your application or service on clusters with such diverse resources is a complicated endeavor. This challenge is what the process of federation is intended to help overcome. The fundamental use case of federation is to scale applications on multiple clusters with ease. The process negates the need to perform the deployment step more than once. Instead, you perform one deployment, and the application is deployed on multiple clusters as listed in the federation list.

What Is Kubernetes Cluster Federation?

Essentially, Kubernetes cluster federation is a mechanism to provide one way or one practice to distribute applications and services to multiple clusters. One of the most important things to note is that federation is not about cluster management, federation is about application management. 

Cluster federation is a way of federating your existing clusters as one single curated cluster. So, if you are leveraging Kubernetes clusters in different zones in different countries, you can treat all of them as a single cluster.

In cluster federation, we optimize a host cluster and multiple member clusters. The host cluster comprises all the configurations which pass on all the member clusters. Member clusters are the clusters that share the workloads. It is possible to have a host cluster also share the workload and act as a member cluster, but organizations tend to keep the host clusters separate for simplicity. On the host cluster, it’s important to install the cluster registry and the federated API. Now with the cluster registry, the host will have all the information to connect to the member Clusters. And with the federated API, you require all the controllers running on our host clusters to make sure they reconcile the federated resources. In a nutshell, the host cluster will act as a control plane and propagate and push configuration to the member clusters.

If we talk about federation manifests, we need to discuss two particular points of information: cluster replacement and the configuration override. It is crucial to configure where we want the application deployment and which member clusters the deployment should leverage with cluster replacement. In the configuration overrides, we are able to change any configuration and tailor our application to the cluster if needed. Another feature for federation is the namespace scoped control plane. With the namespace scope control plane, we can use just one namespace to federate the namespace across different clusters. This is quite a powerful mechanism if the intention is to use federation in production but don’t wish to dedicate an entire cluster. Instead, you can just dedicate one namespace.

How to set up Federated Kubernetes Clusters?

KubeFeb v2 is the official Kubernetes cluster federation implementation. It uses a single set of APIs for the coordination of configurations between the host clusters and the member clusters. KubeFeb runs two types of information, one is the type configuration, and the other is the cluster configuration. FederatedTypeConfig stores the type configuration, which tells KubeFed which API types it has to handle. KubeFedCluster and KubeFedConfig comprise the cluster configurations which tell KubeFeb which clusters to target. The host cluster has the KuberFeb control plane. The KuberFeb control plane has KubeFedCluster and KubeFedConfig resources for each member cluster.

Now that we have covered the fundamentals, let’s set up Federated Kubernetes Clusters. To set the federation, a prerequisite is to have KubeFed up and running on the cluster. Once you have the KubeFed control plane up and running, download the kubefedctl command-line tool to communicate with it. You can get the latest release on GitHub. 

Before starting working with multiple clusters, Federated CDRs must be installed in the host cluster to interact with it.