In my previous article, HDFS Architecture and Functionality, I’ve described the filesystem of Hadoop. Today, we will be learning about the processing unit of it. There are mainly two mechanisms by which processing takes place in a Hadoop cluster, namely, MapReduce and YARN. In our traditional system, the major focus is on bringing data to the storage unit. In the Hadoop process, the focus is shifted towards bringing the processing power to the data to initiate parallel processing. So, here, we will be going through MapReduce and, in part two, YARN.


As the name suggests, processing mainly takes place in two steps, mapping and reducing. There is a single master (Job tracker) that controls ob execution on multiple slaves (Task tracker). The Job Tracker accepts MapReduce jobs submitted by the client. It pushes a map and reduce tasks out to Task Tracker and also monitors their status. Task trackers’ major function is to run the map and reduce tasks. They also manage and store the intermediate output of the tasks.

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