At work the other day, I was reading about Hadoop’s 5 daemons. The information wasn’t quite clicking, so I drew a picture to cement the concepts into my mind. (I’ve checked that all information regarding Hadoop in this blogpost is publicly available.)

In words: Hadoop is comprised of five separate daemons. Each of these daemon runs in its own JVM. The following 3 Daemons run on Master nodes:

• NameNode - This daemon stores and maintains the metadata for HDFS.
• Secondary NameNode - Performs housekeeping functions for the NameNode.
• JobTracker - Manages MapReduce jobs, distributes individual tasks to machines running the Task Tracker.

The following 2 Daemons run on Slave nodes:

• DataNode – Stores actual HDFS data blocks.
• TaskTracker - Responsible for instantiating and monitoring individual Map and Reduce tasks.