Single-machine mode, pseudo-distributed mode, and fully distributed mode for Hadoop

Stand-alone (non-distributed) mode
This mode runs on a single machine without a distributed file system, but directly reads and writes to the local operating system's file system.

Pseudo-distributed operation mode

This mode also runs on a single machine, but uses different Java processes to mimic the various nodes in the distributed operation (Namenode,datanode,jobtracker,tasktracker,secondarynamenode), Notice the difference between these nodes in the distributed operation: from the point of view of distributed storage, nodes in a cluster consist of one namenode and several datanode, and another secondarynamenode as a namenode backup. From the point of view of distributed application, the nodes in the cluster consist of a jobtracker and several tasktracker, Jobtracker is responsible for the task scheduling, Tasktracker is responsible for executing the task in parallel. The Tasktracker must be run on the Datanode, which facilitates the local computation of the data. Jobtracker and Namenode do not need to be on the same machine.

A machine, that is, when the Namenode, and when Datanode, or that is Jobtracker, but also tasktracker. There is no real distributed computing on multiple machines, so it is called "pseudo-distributed".

Fully Distributed Mode

A truly distributed fleet of 3 or more physical or virtual machine components.