In this article we have tried to summaries, how a MapReduce program executes in Hadoop environment.

MapReduce 1 Execution Sequence:

MapReduce execution starts with below command.

Step 1: $ hadoop jar <jar> [mainClass] args...

This command starts the MapReduce execution in Clients JVM. creates the Job.

Step 2: JobTracker.getNewJobId()

Client Asks JobTracker for a new JobId.

Step 3: JobClient.SubmitJob() / JobClient.runJob()

• Checking the input and output specifications of the job. Computing the InputSplits for the job. Setup the requisite accounting information for the DistributedCache of the job, if necessary.
• Copying the job’s jar and configuration to the JobTracker file-system, in a folder names as the JobId assigned with very high replication factor(default 10).
• Submitting the job to the JobTracker and optionally monitoring it’s status.
• JobTracker puts the job into an internal queue from where JobScheduler will pick it up and Initialize

Step 4: Initialize the Job

• Creates an object of the Job. Encapsulates its Tasks.
• Retrieve the InputSplit and create one map task for each split.
• Creates the reduces task the number of reduce task depends on the number defined in the driver code(default 1).
• Creates a Job Setup task to setup the job before map tasks run.
• Creates a Job Cleanup task to run after the reducer task run.