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Bean A needs B while B needs A, so neither constructor can complete first. Treat the exception as an ownership/design signal: extract the shared responsibility, invert a dependency, or communicate through an event. Lazy lookup can defer resolution but does not remove the cycle or its coupling.

A consumer can request one item at a time, but callback-backed sources still need an explicit capacity and overflow policy. Tie early iterator exit and cancellation to resource cleanup.

A bounded queue limits waiting items; workers hold additional active items. task_done releases unfinished-item accounting, while business success and durable retries require separate records.

The destination rejects generations older than its highest accepted token. A lease does not stop a paused worker, and a fence cannot reject an expired generation it has not yet learned is superseded.