I saw one of my ES 5.2.1 client node (client-2) disappeared from the cluster. After check the log on client-2 machine I found this one:
[2017-02-21T05:55:57,248][INFO ][o.e.c.s.ClusterService ] [client-2] removed {{data-1}{WsYYmepdQ2S-WVCiWfqrUg}{wmsFDIoMRnCaFhvLncJx1Q}{data-1}{10.0.1.6:9300},}, reason: zen-disco-receive(from master [master {master-0}{CQ6JKDZTQeq5qktPNG6aEg}{o47uxtuRSeaJNJKR6HGebQ}{master-0}{10.0.0.4:9300} committed version [44]])
Then there was NO any logs until I manually re-start the client-2 machine.
Seems the client-2 node lost for several days since 2.21.
What happened per above log?