Hadoop has a master-slave architecture (Figure 2), with a unique master host and multiple slave hosts, typ- ically configured as follows. The master host runs two daemons: (1) the JobTracker, which schedules and man- ages all of the tasks belonging to a running job; and (2) the NameNode, which manages the HDFS namespace, and regulates access to files by clients (which are typi- cally the executing tasks). Each slave host runs two daemons: (1) the Task- Tracker, which launches tasks on its host, based on in- structions from the JobTracker; the TaskTracker also keeps track of the progress of each task on its host; and (2) the DataNode, which serves data blocks.
