I posted an approach to this using streaming, but if the environment variables are available in
standard Java interface, this may work for you.
http://www.mail-archive.com/[email protected]/msg09079.html
You'll have to be able to tolerate some small gaps in the ids.
Michael
Mark Kerzner wrote:
Aaron, although your notes are not a ready solution, but they are a great
help.
Thank you,
Mark
On Tue, Oct 27, 2009 at 11:27 PM, Aaron Kimball <[email protected]> wrote:
There is no in-MapReduce mechanism for cross-task synchronization. You'll
need to use something like Zookeeper for this, or another external
database.
Note that this will greatly complicate your life.
If I were you, I'd try to either redesign my pipeline elsewhere to
eliminate
this need, or maybe get really clever. For example, do your numbers need to
be sequential, or just unique?
If the latter, then take the byte offset into the reducer's current output
file and combine that with the reducer id (e.g.,
<current-byte-offset><zero-padded-reducer-id>) to guarantee that they're
all
building unique sequences. If the former... rethink your pipeline? :)
- Aaron
On Tue, Oct 27, 2009 at 8:55 PM, Mark Kerzner <[email protected]>
wrote:
> Hi,
>
> I need to number all output records consecutively, like, 1,2,3...
>
> This is no problem with one reducer, making recordId an instance variable
> in
> the Reducer class, and setting conf.setNumReduceTasks(1)
>
> However, it is an architectural decision forced by processing need, where
> the reducer becomes a bottleneck. Can I have a global variable for all
> reducers, which would give each the next consecutive recordId? In the
> database scenario, this would be the unique autokey. How to do it in
> MapReduce?
>
> Thank you
>
