Using riak_pipe for Map Reduce task may return inconsistent results when one of cluster node is down. 

Using riak_pipe for Map Reduce task may return inconsistent results(for example empty list,however all data are available).
1. We must generate constant hash for the output reduce pase taking account of which nodes are up.
2. We must return an error If can't send result of reduce(prereduce phase to the output)  because some node are failed during map reduce job and we can't send result to it.

See http://lists.basho.com/pipermail/riak-users_lists.basho.com/2013-January/010870.html