question: public static < T > ParallelFlux < T > from ( Publisher < ? extends T > source , int parallelism , int prefetch , Supplier < Queue < T > > queueSupplier ) { if ( parallelism <= 0 ) { throw new IllegalArgumentException ( "parallelism > 0 required but it was " + parallelism ) ; } if ( prefetch <= 0 ) { throw new IllegalArgumentException ( "prefetch > 0 required but it was " + prefetch ) ; } Objects . requireNonNull ( queueSupplier , "queueSupplier" ) ; Objects . requireNonNull ( source , "queueSupplier" ) ; return onAssembly ( new ParallelUnorderedSource < > ( source , parallelism , prefetch , queueSupplier ) ) ; }
target:  Take a Publisher and prepare to consume it on parallallism number of 'rails' and round-robin fashion and use custom prefetch amount and queue for dealing with the source Publisher's values.