The MR master handles it. Mapreduce workers can request shards (work) that have data local to the host it is running on. For large mapreduces this basically w

The MR master handles it. Mapreduce workers can request shards (work) that have data local to the host it is running on. For large mapreduces this basically works because both the data and the tasks are in most of the cluster, and for small mapreduces it doesn't matter.