controllerruntime

Controllerruntime, in the Kubernetes ecosystem, is a Go library designed to simplify the construction of controllers and operators. It is most commonly accessed as the module sigs.k8s.io/controller-runtime and is a foundational component used by Kubebuilder and Operator SDK to reduce boilerplate in controller development. The library provides reusable building blocks for watching resources, reconciling state, and interacting with the Kubernetes API server.

The core concepts in controllerruntime include a Manager, a Client, a Cache, a Scheme, and the Reconciler and Controller abstractions. The Manager coordinates the lifecycle of controllers, the in-memory cache of objects, and the shared client used to read and write Kubernetes resources. The Scheme maps Go types to Kubernetes API types, enabling serialization and deserialization. A Reconciler implements a reconciliation loop for a given resource, typically via a Reconcile method that accepts a context and a request, and returns a result and an error. Controllers are created and wired to watch specific resource types through a builder pattern, and are then managed by the Manager.

Using controllerruntime, developers implement business logic in the Reconciler, while the library handles event processing, work queues, rate limiting, and concurrency. The framework provides facilities for leader election, health probes, metrics, and event recording, which help operators run reliably in production.

Controllerruntime is widely adopted in Kubernetes operator development and is designed to work well with code generation tools like Kubebuilder. It is a library, not a runtime, and is intended to be extended by application code to implement custom reconciliation behavior.