MapReduce Work Flow

Here are some of the key concepts:

• Job – A Job in the context of Hadoop MapReduce is the unit of work to be performed as requested by the client / user. The information associated with the Job includes the data to be processed (input data), MapReduce logic / program / algorithm, and any other relevant configuration information necessary to execute the Job.
• Task – It divides a Job into multiple sub-jobs known as Tasks. These tasks can be run independent of each other on various nodes across the cluster. There are primarily two types of Tasks – Map Tasks and Reduce Tasks.
• JobTracker – Just like the storage (HDFS), the computation also works in a master-slave / master-worker fashion. A JobTracker node acts as the Master and is responsible for scheduling / executing Tasks on appropriate nodes, coordinating the execution of tasks, sending the information for the execution of tasks, getting the results back after the execution of each task, re-executing the failed Tasks, and monitors / maintains the overall progress of the Job. Since a Job consists of multiple Tasks, a Job’s progress depends on the status / progress of Tasks associated with it. There is only one JobTracker node per Hadoop Cluster.
• TaskTracker – A TaskTracker node acts as the Slave and is responsible for executing a Task assigned to it by the JobTracker. There is no restriction on the number of TaskTracker nodes that can exist in a Hadoop Cluster. TaskTracker receives the information necessary for execution of a Task from JobTracker, Executes the Task, and Sends the Results back to JobTracker.
• Map() – Map Task in Map Reduce is performed using the Map() function. This part of the MapReduce is responsible for processing one or more chunks of data and producing the output results.
• Reduce() – The next part / component / stage of the MapReduce programming model is the Reduce() function. This part of the MapReduce is responsible for consolidating the results produced by each of the Map() functions/tasks.
• Data Locality – It tries to place the data and the compute as close as possible. First, it tries to put the compute on the same node where data resides, if that cannot be done (due to reasons like compute on that node is down, compute on that node is performing some other computation, etc.), then it tries to put the compute on the node nearest to the respective data node(s) which contains the data to be processed. This feature of MapReduce is “Data Locality”.

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