Hidden features of Hadoop MapReduce

Question

What are the hidden features of Hadoop MapReduce that every developer should be aware of?

One hidden feature per answer, please.

Answer 1

Here are some tips and tricks http://allthingshadoop.com/2010/04/28/map-reduce-tips-tricks-your-first-real-cluster/

One item from there specifically that every developer should be aware of:

In your Java code there is a little trick to help the job be “aware” within the cluster of tasks that are not dead but just working hard. During execution of a task there is no built in reporting that the job is running as expected if it is not writing out. So this means that if your tasks are taking up a lot of time doing work it is possible the cluster will see that task as failed (based on the mapred.task.tracker.expiry.interval setting).

Have no fear there is a way to tell cluster that your task is doing just fine. You have 2 ways todo this you can either report the status or increment a counter. Both of these will cause the task tracker to properly know the task is ok and this will get seen by the jobtracker in turn. Both of these options are explained in the JavaDoc http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/mapred/Reporter.html