java - Turn multiple Cassandra partitions into Akka stream -

i'm trying develop source turns set of cassandra partitions akka stream (i execute multiple cassandra queries through bound statement, 1 each partition , mapconcat result iterable of dtos):

source<result<journalentry>, notused> src = source.unfoldasync(querybag, s -> {      boundstatement bound = getboundstatement(querybag);      // out data cassandra:     future<result<journalentry>> page =          future.apply(() -> journalmapper.map(session.execute(bound)), ec);      future<optional<pair<querybag,result<journalentry>>>> next = page.map(r -> {         optional<pair<querybag,result<journalentry>>> opt;         if (r.isexhausted()) {             opt = optional.empty();         } else {             opt = optional.of(pair.apply(querybag.incrementpartitionid(), r));         }         return opt;     }, ec);     return futureconverters.tojava(next); });  source<journalentry, notused> concat = src.mapconcat(res -> () -> res.iterator());  // materialize concat , consume stream 

is safe practice? in particular, i'm concerned generating iterable through mapconcat((res) -> () -> res.iterator()), multiple calls of iterator() method of resulting iterable same instance of iterator.

besides, sensible approach iterate across different cassandra partitions or more appropriate use graphstage?