question: public void nackAll ( PubsubReader < T > reader ) throws IOException { checkState ( this . reader == null , "Cannot nackAll on persisting checkpoint" ) ; List < String > batchYetToAckIds = new ArrayList < > ( Math . min ( notYetReadIds . size ( ) , ACK_BATCH_SIZE ) ) ; for ( String ackId : notYetReadIds ) { batchYetToAckIds . add ( ackId ) ; if ( batchYetToAckIds . size ( ) >= ACK_BATCH_SIZE ) { long nowMsSinceEpoch = now ( reader ) ; reader . nackBatch ( nowMsSinceEpoch , batchYetToAckIds ) ; batchYetToAckIds . clear ( ) ; } } if ( ! batchYetToAckIds . isEmpty ( ) ) { long nowMsSinceEpoch = now ( reader ) ; reader . nackBatch ( nowMsSinceEpoch , batchYetToAckIds ) ; } }
target:  BLOCKING NACK all messages which have been read from Pubsub but not passed downstream. This way Pubsub will send them again promptly.