Writing the same code using RxJava Observable

Sample.java

@GET
public Observable<Response> processRequest( @QueryParam("id")String id, @QueryParam("userId")String userId ) {
  return Observable.zip(
    Observable.fromCallable(() -> database.retrieveRecord(id)),
    Observable.fromCallable(() -> userStore.retrieve(userId)),
    Observable.fromCallable(() -> logger.writeEntryFor(id, userId)),
    this::generateAdResponse
  );
}