Hadoop: The Definitive Guide, 2nd Edition

Chapter 6. How MapReduce Works

In this chapter, we look at how MapReduce in Hadoop works in detail. This knowledge provides a good foundation for writing more advanced MapReduce programs, which we will cover in the following two chapters.

Anatomy of a MapReduce Job Run

You can run a MapReduce job with a single line of code: JobClient.runJob(conf). It’s very short, but it conceals a great deal of processing behind the scenes. This section uncovers the steps Hadoop takes to run a job.

The whole process is illustrated in Figure 6-1. At the highest level, there are four independent entities:

• The client, which submits the MapReduce job.

• The jobtracker, which coordinates the job run. The jobtracker is a Java application whose main class is JobTracker.

• The tasktrackers, which run the tasks that the job has been split into. Tasktrackers are Java applications whose main class is TaskTracker.

• The distributed filesystem (normally HDFS, covered in Chapter 3), which is used for sharing job files between the other entities.

Figure 6-1. How Hadoop runs a MapReduce job

Job Submission

The runJob() method on JobClient is a convenience method that creates a new JobClient instance and calls submitJob() on it (step 1 in Figure 6-1). Having submitted the job, runJob() polls the job’s progress once a second and reports the progress to the console if it has changed since the last report. When the job is complete, ...