Deliveries a receiver has accepted and not yet acted on. A payload stays pending until the receiver acknowledges it, so work interrupted by a restart is found again; an acknowledged entry keeps only its id, so a replay of it is refused.
Stores a new entry; false, and nothing changes, when an entry with its id was accepted before (acknowledged or not).
The entries accepted and not yet acknowledged, oldest first.
Drops the entry's payload once the receiver has acted on it; its id stays, so a replay is still refused.
Deliveries a receiver has accepted and not yet acted on. A payload stays pending until the receiver acknowledges it, so work interrupted by a restart is found again; an acknowledged entry keeps only its id, so a replay of it is refused.