zookeeper
ZooKeeper is a high-performance coordination service for distributed applications. - https://zookeeper.apache.org/doc/current/
Examples
The four letter words
"ZooKeeper responds to a small set of commands. Each command is composed of four letters. You issue the commands to ZooKeeper via telnet or nc, at the client port." - https://zookeeper.apache.org/doc/r3.4.8/zookeeperAdmin.html#sc_zkCommands
Lists brief details for the server and connected clients
echo 'stat' | nc localhost 2181
View a list of variables that could be used for monitoring the health of the cluster
echo 'mntr' | nc localhost 2181
List full details for the server
echo 'srvr' | nc localhost 2181