Hi Lin, Reduce task starts as soon as output is ready from Mappers. The reduce method does not get called until all Mappers are done. If that's not the case, all operations which are not commutative and associative will yield incorrect result.
Thanks and Regards, Rishi Yadav (o) 408.988.2000x113 || (f) 408.716.2726 InfoObjects Inc || http://www.infoobjects.com *(Big Data Solutions)* *INC 500 Fastest growing company in 2012 || 2011* *Best Place to work in Bay Area 2012 - *SF Business Times and the Silicon Valley / San Jose Business Journal 2041 Mission College Boulevard, #280 || Santa Clara, CA 95054 On Sat, Dec 22, 2012 at 5:25 AM, Lin Ma <[email protected]> wrote: > Hi guys, > > Supposing in a Hadoop job, there are both mappers and reducers. My > question is, reducer tasks cannot begin until all mapper tasks complete? If > so, why designed in this way? > > thanks in advance, > Lin >
