Why generation fencing?

Without a generation, a runner that blipped offline can reclaim → lose the job → wake up → and still “succeed,” overwriting the winner.

Decision

Every reclaim bumps a generation. Heartbeats, status, and terminal events from an older generation are rejected. The superseded runner learns on the next heartbeat and stops cooperatively.

Consequences

• Final status belongs to the current attempt only.
• Same rule on Redis, NATS, and Kafka — no transport-only honesty gap.
• Pairs with the poison-pill ceiling (how many attempts) — fencing is which attempt may write.

Rejected alternatives

• Trust wall-clock leases alone — late writers still win races.
• Fence only terminal Ack — zombies keep working and emit noisy progress.

Skimmable story: design/fencing.html.