Concept-Check: The Cluster

Kind: Quiz. One pass over the whole arc — and, really, over the whole course.

The Cluster arc turned the coordinator into a distributed system without touching the scheduler. You framed a byte stream into whole messages with a length-delimited codec; you built a connection actor that multiplexes one socket, tagging results back to the jobs awaiting them; you wrapped it in a RemoteWorker that the scheduler dispatches through exactly like a LocalWorker; and you closed the loop with heartbeat reaping and at-least-once redelivery, made safe by the store's idempotent recording. This check mixes compiler-verified tracing questions with judgment questions spanning framing, the actor, the seam, heartbeats, redelivery, and idempotency — the sentence the whole course pays off.

If a question stings, the fix is upstream: TCP Is a Byte Stream for framing, The Connection Actor for multiplexing and reply-matching, and At-Least-Once, Heartbeats, and Idempotency for reaping and redelivery. And the idempotency half reaches all the way back to the atomic claim and transactional recording in Part IV — that guard is what this arc leans on. Re-read the section, then come back.

Next: Part IX, the wrap-up — where this coordinator plugs into the wider Panoptes harness, and what a production version would add next (real brokers, mTLS, sharding).