question: private void retire ( ) throws IOException { long nowMsSinceEpoch = now ( ) ; while ( true ) { List < String > ackIds = ackedIds . poll ( ) ; if ( ackIds == null ) { return ; } numAcked . add ( nowMsSinceEpoch , ackIds . size ( ) ) ; for ( String ackId : ackIds ) { inFlight . remove ( ackId ) ; safeToAckIds . remove ( ackId ) ; } } }
target:  Messages which have been ACKed (via the checkpoint finalize) are no longer in flight. This is only used for flow control and stats.