Zen discovery is the discovery module used by Elasticsearch. Since Elasticsearch is a distributed system, you can think of this module as a glue that keeps the cluster together. Cluster management and failure detection are handled automatically by Elasticsearch.
In the configuration file, there is a discovery section dedicated to zen discovery. One of the settings in the discovery section is discovery.zen.ping.unicast.hosts. This setting is a list of other hosts Elasticsearch is running, so that the node can join the existing nodes to form a cluster. When we start the elasticsearch2 instance, this instance will first try to ping the hosts in discovery.zen.ping.unicast.hosts. It will scan the ports 9300 to 9305 and find elasticsearch1 running at 192.168.0.1:9300. (Note that 9300 is the port for internal communication, 9200 is the HTTP server.) To join...