of a collection of controllers. Per Kubernetes, "controllers are control loops that watch the state of your cluster, then make or request changes where needed."

In Istio, we use the term a bit more liberally. Istio controllers watch more than just the state of *a* cluster -- many are reading from multiple clusters, or even external sources (files and XDS). Generally, Kubernetes controllers are then writing state back to the cluster; Istio does have a few of these controllers, but most of...