I am sorry, but I forgot to add one important piece of information. I don't want to write any random N rows to the table. I want to write the *top* N rows - meaning - I want to write the "key" values of the Reducer in descending order. Does this make sense? Sorry for the confusion.
On Wed, Jan 27, 2010 at 11:09 PM, Mridul Muralidharan <[email protected] > wrote: > > A possible solution is to emit only N rows from each mapper and then use 1 > reduce task [*] - if value of N is not very high. > So you end up with utmost m * N rows on reducer instead of full inputset - > and so the limit can be done easier. > > > If you ok with some sort of variance in the number of rows inserted (and if > value of N is very high), you can do more interesting things like N/m' rows > per mapper - and multiple reducers (r) : with assumtion that each reducer > will see atleast N/r rows - and so you can limit to N/r per reducer : > ofcourse, there is a possible error that gets introduced here ... > > > Regards, > Mridul > > [*] Assuming you just want simple limit - nothing else. > Also note, each mapper might want to emit N rows instead of 'tweaks' like > N/m rows, since it is possible that multiple mappers might have less than > N/m rows to emit to begin with ! > > > > Something Something wrote: > >> If I set # of reduce tasks to 1 using setNumReduceTasks(1), would the >> class >> be instantiated only on one machine.. always? I mean if I have a cluster >> of >> say 1 master, 10 workers & 3 zookeepers, is the Reducer class guaranteed >> to >> be instantiated only on 1 machine? >> >> If answer is yes, then I will use static variable as a counter to see how >> may rows have been added to my HBase table so far. In my use case, I want >> to write only N number of rows to a table. Is there a better way to do >> this? Please let me know. Thanks. >> > >
