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.