Batches and Locks

Claiming one job at a time is correct but slow: every claim is a round trip. @bclaim claims a whole batch in a single move — claim_batch/4 and the BatchConsumer shape it by a min_size and a timeout, and @gbclaim keeps a batch inside one group's lane — so throughput rises without loosening the lease discipline that protects each job. A batch is not a looser claim; it is many claims made at once.

The word lock on this bus is not a separate primitive — the lock is the lease. A claimed job is locked to its worker for the duration of its lease; extendLock renews one, extendLocks renews a whole batch together, releaseLock frees it, and a stalled-job sweep reclaims the locks of a worker that died. That pairing is what makes batches safe: the batch holds many locks and renews them as one, so a slow run keeps its whole batch, and yet the partitioned finish settles each job on its own verdict — these acked, those returned to retry — so one poison job in a batch of fifty never drags the other forty-nine back.

A batch has to hold two things that pull against each other: efficiency wants the jobs treated as one, correctness wants them treated as many. Treat them purely as one — claim, run, ack the lump — and a single bad job fails the whole batch, forcing fifty retries for one fault. Treat them purely as many — a round trip each — and the throughput win is gone. The bus keeps both by splitting the join along the right seam: claimed together, locked together, settled apart.

The lease is where "together" lives. @bclaim takes the batch atomically, and extendLocks renews every lease in it in one move, so a batch that runs long does not lose a job to an expired lock halfway through. The verdict is where "apart" lives. EchoMQ.BatchFinish carries a per-member result, so the finish acks the jobs that succeeded and returns the ones that failed to retry, each on its own attempts clock. The stalled-job sweep backs the whole thing: if the worker dies holding the batch, its locks are reclaimed and the jobs return to be claimed again. Bulk where bulk is safe, individual where individuality matters.

The batch rule — a batch is claimed together and settled apart: many locks renewed as one, verdicts rendered per item. @bclaim takes many in one round trip and extendLocks renews their leases together, but the partitioned finish acks and retries each on its own — so one poison job never drags its batch down, and a dead worker's locks are reclaimed for the whole batch.