Crate mz_cluster_controller 

The cluster controller: the single decision-maker for the replica set of every managed cluster.

It is a reconciler. Each tick it reads desired cluster state and live signals through the ClusterControllerCtx boundary, runs a set of pure Strategys, unions their desired contributions, diffs that against the actual replica set, and emits the create/drop and durable-state-write Decisions that close the gap. It holds no in-memory state: the source of truth is always the catalog plus live signals, pulled fresh each tick.

The crate is pure. It depends only on primitive id/shape types and the ClusterControllerCtx trait, never on the adapter or catalog. That boundary is what makes the controller testable against a fake implementation and extractable later without touching controller code.

A tick runs two phases per cluster, update_state then desired_replicas (see ClusterController::reconcile). Every Decision carries the durable state it was derived from, and the apply path transacts it only if that state still holds (compare-and-append). So a create or drop derived from a pre-ALTER snapshot can never reshape the replica set against the config the ALTER has since established. Applies are per cluster, so one cluster’s rejection does not block the others, and commands name explicit replicas, so re-emitting one across a lagging view or a restart is a no-op.

Modules

ctx

The boundary between the controller and its environment.

strategy

The pure strategy interface and the strategy implementations.

Structs

ClusterController

The cluster controller. Holds the (stateless) set of strategies and drives a reconcile tick against a ClusterControllerCtx.