kaas-controller

Controller-side logic: Lease election, the partition/group balancer, the assignment writer, and the heartbeat gRPC server.

Everything that runs only on the broker currently holding the kaas-controller Lease (architecture).

Module map: election.rs (the election seam) + kube_election.rs (the Kubernetes Lease implementation; LocalElection is the dev-mode always-elected stub), balancer.rs (partition + consumer-group placement with deterministic smoothing, so recomputes move as little as possible), assignment_writer.rs (atomic assignment.json writes behind the TopicSource / BrokerSource / GroupSource / CrMirror trait seams), heartbeat_server.rs (the bidi gRPC server side of proto/heartbeat.proto), k8s_mirror.rs (the KafkaClusterAssignments debug mirror).

The trait seams are the point: the balancer and writer are pure over their sources, which is what makes tests/controller_failover.rs and tests/stale_controller_race.rs possible without a cluster — the stale-epoch fence (a deposed controller's write rejected by its stale leaseTransitions epoch) is pinned by test, not by hope.

Invariant callers must hold: the assignment file is the only output channel. Nothing in this crate may instruct a broker directly — brokers follow assignment.json, and anything the controller wants to happen must be expressible as an assignment change.

Start reading at balancer.rs, then assignment_writer.rs.