Kubernetes Controllers: Reconciliation Loops, the Controller Manager, and Custom Controllers#

Kubernetes is a declarative system. You tell it what you want (a Deployment with 3 replicas), and controllers make it happen. Controllers are the engines that continuously reconcile desired state with actual state. Without controllers, your YAML manifests would be inert data in etcd.

The Controller Pattern#

Every controller follows the same loop:

1. Watch the API server for changes to a specific resource type
2. For each change, compare desired state (spec) to actual state (status)
3. Take action to bring actual state closer to desired state
4. Update status to reflect current actual state
5. Repeat

This is a level-triggered model, not edge-triggered. A controller does not just react to changes – it reconciles the entire state on each pass. If a controller crashes and restarts, it re-reads all objects and converges to the correct state without needing to replay missed events. This makes controllers resilient to transient failures.