Apache Hadoop  is designed to have Master Slave architecture.

Master: Namenode, JobTracker

Slave: {DataNode, TaskTraker}, …..  {DataNode, TaskTraker}

HDFS is one primary components of Hadoop cluster and HDFS is designed to have Master-slave architecture.

Master: NameNode

Slave: {Datanode}…..{Datanode}

-     The Master (NameNode) manages the file system namespace operations like opening, closing, and renaming files and directories and determines the mapping of blocks to DataNodes along with regulating access to files by clients

-     Slaves (DataNodes) are responsible for serving read and write requests from the file system’s clients along with perform block creation, deletion, and replication upon instruction from the Master (NameNode).

 

Map/Reduce is also primary component of Hadoop and it also have Master-slave architecture

Master: JobTracker

Slaves: {tasktraker}……{Tasktraker}

 -     Master {Jobtracker} is the point of interaction between users and the map/reduce framework. When a map/reduce job is submitted, Jobtracker puts it in a queue of pending jobs and executes them on a first-come/first-served basis and then manages the assignment of map and reduce tasks to the tasktrackers.

-     Slaves {tasktracker} execute tasks upon instruction from the Master {Jobtracker} and also handle data motion between the map and reduce phases.