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 <markkerz...@gmail.com> 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 >
