API server

The API server acts as Kubernetes' central hub. All the other components in Kubernetes communicate by reading, watching, and updating resources in Kubernetes APIs. This central component is used for all of the access and manipulation of information about the current state of the cluster, allowing Kubernetes to be extended and augmented with new features while still maintaining a high degree of consistency.

Kubernetes uses etcd to store the current state of the cluster. An etcd store is used because its design means that it is both resistant to failure and has strong guarantees of its consistency. However, the different components that make up Kubernetes never directly interact with etcd; instead, they communicate with the API server. This is a good design for us, the operator of a cluster, because it allows us to restrict access to etcd only to the API server component, improving security and simplifying management.

While the API server is the component in the Kubernetes architecture that everything else communicates with to access or update the state, it is stateless itself, with all storage being deferred to the backing etcd cluster. This again is an ideal design decision for us as cluster operators since it allows us to deploy multiple instances of the API server (if we wish) to provide high availability.