Hadoop: Common Library
Introduction
Hadoop Common or (Common Utilities) includes the Java library, java files and java scripts that are needed for all other components present in a Hadoop cluster to work together. These utilities are used by the HDFS, YARN and MapReduce components for running the cluster.
In particular it is responsible for how data is stored in the HDFS and how MapReduce should handle the data stored by the HDFS in chunks. It is also responsible for the communications between the MapReduce and YARN frameworks for job scheduling and resource management.