3.13 Kubernetes Control Plane Component Communication
Kubernetes Control Plane components communicate through APIs and internal networks to manage cluster state and orchestrate containerized workloads.
Kubernetes Control Plane Component Communication is the specific set of rules governing how the API server, etcd, scheduler, and controller manager exchange information with one another, describing the hub-and-spoke pattern in which every component other than etcd communicates exclusively through the API server, never directly with any peer component.
The Hub-and-Spoke Pattern
API Server as the Only Shared Contact Point
Control plane component communication is architected as a hub-and-spoke pattern: the API server sits at the hub, and every other component, the scheduler, the controller manager, and etcd, communicates only with the API server, never establishing a direct connection to one another.
No Peer-to-Peer Channel Between Scheduler and Controllers
A direct consequence of this pattern is that the scheduler has no direct communication channel to the controller manager, and vice versa; if one needs to observe something the other has done, it must do so by watching the shared state exposed through the API server, not through any private channel between them.
etcd's Exception, Not Its Inclusion in the Hub
Reachable Only by the API Server
etcd is the one component with a direct, exclusive relationship to the API server rather than being reached through it; no other control plane component communicates with etcd at all, meaning etcd sits below the hub rather than beside it as another spoke.
Why This Pattern Was Chosen
Simplifying the Communication Surface
By restricting all inter-component communication to pass through a single hub, the architecture avoids the combinatorial complexity of components needing to know about and directly integrate with every other component individually, since each only ever needs to understand how to talk to the API server.
Enabling Independent Component Evolution
This pattern also allows the scheduler, controller manager, and any custom controllers to be modified, replaced, or scaled independently, since none of them depend on a stable direct interface to any other specific component, only on the stable, shared API server interface.
Watch as the Primary Communication Verb
Push-Based Updates Rather Than Polling
The dominant form of communication in this pattern is not request-response but watch: components establish long-lived watch connections to the API server and receive a stream of change events, rather than repeatedly issuing individual requests to check for updates.