There is nothing sacred about electrons being called negative charge and protons positive charge.Itâs just a convention. You, can very well call electrons as positive charge and protons as negative charge!
Single Node or Psuedo-Distributed Cluster is the one in which all the essential daemons (like NameNode, DataNode, JobTracker and TaskTracker) run on the same machine. The default replication factor for a single node cluster is 1. A single node cluster is basically used to simulate a full cluster like environment and to test hadoop applications and unlike the Stand-alone mode, HDFS is accessible in this.
The proofs of concept start out with ten or twenty nodes, and then the initial deployment is maybe 50 to 100 nodes. Again, it depends on what they are doing, how much data they need to store. And as the use cases for Hadoop grow, the cluster can grow to 1,000 to 2,000 nodes within a year..