Info
A control loop is a non-terminating loop that aims to bring the state of the system closer to the desired state.
The control loop is implemented by Kubernetes - Controller, which are the building blocks of the Kubernetes - Control Plane.
Desired (Spec) VS Current (Status) State
A Kubernetes controller reads both the Status and the Spec stanza, of the Custom Resource Definition and pushes the Status closer to the Spec, one step at a time.
Idempotency
The Idempotent nature of the controller is required in an environment such as Kubernetes, as many actors act potentially on your resource and the high Volatility of the environment.
Robustness
Handling Errors
As a general rule, when an error occurs during the control loop, the err is immediately returned and no further actions are taken. The controller schedules a retry of the reconciliation of the Custom Resource Definition, following an Exponential Backoff mechanism.