Hi,

We have a three node replica set cluster with one primary and two secondary nodes, due to slow performance and high query execution time on version 3.6, we have upgraded mongodb version to 4.0.23, but after upgrade when application load is started again then mongodb primary server crashes unexpectedly with high CPU usage to 80% and becomes SECONDARY with mongo02 reaching into unhealthy state.

Also the states of replicaset changes continuously over the time as:

SECONDARY, UNHEALTHY, SECONDARY

SECONDARY, SECONDARY,SECONDARY

SECONDARY, PRIMARY PRIMARY.

Please find one of the state here:

"members" : [ "members" : [ { "_id" : 0, "name" : "PRIMARY:27017", "health" : 1, "state" : 2, "stateStr" : "SECONDARY", "uptime" : 244, "optime" :

{ "ts" : Timestamp(1618811595, 1373), "t" : NumberLong(106) }

, "optimeDate" : ISODate("2021-04-19T05:53:15Z"), "syncingTo" : "SECONDARY02:27017", "syncSourceHost" : "SECONDARY02:27017", "syncSourceId" : 4, "infoMessage" : "", "configVersion" : 19, "self" : true, "lastHeartbeatMessage" : "" }, { "_id" : 1, "name" : "SECONDARY01:27017", "health" : 1, "state" : 1, "stateStr" : "PRIMARY", "uptime" : 186, "optime" :

{ "ts" : Timestamp(1618818925, 1), "t" : NumberLong(141) }

, "optimeDurable" : { "ts" : Timestamp(1618818925, 1), "t" : NumberLong(141) }, "optimeDate" : ISODate("2021-04-19T07:55:25Z"), "optimeDurableDate" : ISODate("2021-04-19T07:55:25Z"), "lastHeartbeat" : ISODate("2021-04-19T07:55:32.430Z"), "lastHeartbeatRecv" : ISODate("2021-04-19T07:55:30.739Z"), "pingMs" : NumberLong(0), "lastHeartbeatMessage" : "", "syncingTo" : "", "syncSourceHost" : "", "syncSourceId" : -1, "infoMessage" : "", "electionTime" : Timestamp(1618818922, 1), "electionDate" : ISODate("2021-04-19T07:55:22Z"), "configVersion" : 19 }, { "_id" : 4, "name" : "SECONDARY02:27017", "health" : 1, "state" : 1, "stateStr" : "PRIMARY", "uptime" : 186, "optime" :

{ "ts" : Timestamp(1618813475, 1405), "t" : NumberLong(106) }

, "optimeDurable" : { "ts" : Timestamp(1618813475, 1405), "t" : NumberLong(106) }, "optimeDate" : ISODate("2021-04-19T06:24:35Z"), "optimeDurableDate" : ISODate("2021-04-19T06:24:35Z"), "lastHeartbeat" : ISODate("2021-04-19T07:55:32.428Z"), "lastHeartbeatRecv" : ISODate("2021-04-19T07:55:32.260Z"), "pingMs" : NumberLong(0), "lastHeartbeatMessage" : "", "syncingTo" : "", "syncSourceHost" : "", "syncSourceId" : -1, "infoMessage" : "", "electionTime" : Timestamp(1618818892, 2), "electionDate" : ISODate("2021-04-19T07:54:52Z"), "configVersion" : 19 }, ],