In Kubernetes, the terms "controller" and "control loop" are related concepts that play essential roles in managing the desired state of resources in a cluster. However, they serve different purposes and have distinct responsibilities:
Controller:
Definition: A controller in Kubernetes is a higher-level construct that manages the state of a specific type of resource to ensure it matches the desired state defined in a user's declarative configuration.
Responsibility: Controllers are responsible for maintaining the desired state of resources over time. They continuously monitor the cluster's current state, compare it to the desired state specified in resource definitions (such as Deployments, ReplicaSets, StatefulSets, etc.), and take actions to converge the actual state to the desired state.
Examples: Deployment controller, ReplicaSet controller, StatefulSet controller, DaemonSet controller, etc.
Control Loop:
Definition: A control loop is a fundamental architectural pattern within Kubernetes controllers. It's the mechanism through which controllers reconcile the actual state with the desired state of resources.
Responsibility: Control loops are responsible for executing the reconciliation process. They periodically check the state of resources, calculate the differences between the current and desired states, and take action to make them consistent.
Components: A control loop consists of three primary components:
Desired State: The desired state is specified in the resource definition. For example, in a Deployment, it defines the desired number of replicas.
Actual State: The actual state represents the current state of resources within the cluster. For example, the current number of running pods.
Reconciliation Logic: The reconciliation logic is the controller-specific code that determines how to bring the actual state in line with the desired state. It could involve creating, updating, or deleting resources as needed.
In summary, a controller is a higher-level entity responsible for managing a specific type of resource, while a control loop is an internal mechanism used by controllers to ensure that the resource's actual state matches the desired state. Controllers use control loops to achieve their desired state management goals.