controller-runtime is the most popular SDK to write controllers for individual Kubernetes clusters. But the Kubernetes landscape is changing quickly: multi-cluster is becoming ubiquitous (e.g. through Cluster API), with clusters joining and leaving dynamically. controller-runtime has had no direct support, making writing uniform multi-cluster controllers hard and fracturing the emerging ecosystem.

This talk explores how to build controllers that reconcile resources across a dynamic fleet of Kubernetes clusters. A key change is the ability to plug in a dynamic cluster provider that registers new Kubernetes clusters from a specific source. While implementation internals are briefly discussed, focus is on a hands-on walkthrough for writing your own cluster provider, event handlers and reconciler functions.

We discuss a simplistic cluster provider implementation for “kind” clusters as an example and extrapolate from that how more complex providers could look like (e.g. for CAPI or kcp).