How MapReduce Works

1. Write a MapReduce Java program and bundle it in a JAR file. You can have a look in my previous post how to create a MapReduce program in Java using Eclipse  and bundle a JAR file “First Example Project using Eclipse“.
2. Client submit the job to the JobTracker by running the JAR file ($ hadoop jar ….). Actually the driver program (WordCountDriver.java) act as a client which will submit the job by calling “JobClient.runJob(conf);“. The program can run on any node (as a separate JVM) in the Hadoop cluster or outside cluster. In our example, we are running the client program on the same machine where JobTracker is running usually NameNode. The job submission steps includes: