Invalid sync source in mongodb config server I have 3 config servers 1 primary and 2 secondaries i have received Waring alerts
in secondary servers as


  Log File Monitoring : key = error, File = /var/log/mongodb/mongod.log
  (mgdb09:log[/var/log/mongodb/mongod.log,@mongodb_error,UTF-8]):
  2018-05-05T05:42:08.913+0900 W REPL     [rsBackgroundSync] Fetcher
  stopped querying remote oplog with error: InvalidSyncSource: sync
  source mgdb08:27019 (last visible optime: { ts: Timestamp
  1525466619000|1, t: 24 }; config version: 1; sync source index: -1;
  primary index: 1) is no longer valid
  
  Log File Monitoring : key = error, File = /var/log/mongodb/mongod.log
  (mgdb07:log[/var/log/mongodb/mongod.log,@mongodb_error,UTF-8]):
  2018-05-05T05:37:19.219+0900 W REPL     [rsBackgroundSync] Fetcher
  stopped querying remote oplog with error: InvalidSyncSource: sync
  source mgdb08:27019 (last visible optime: { ts: Timestamp
  1525466619000|1, t: 24 }; config version: 1; sync source index: -1;
  primary index: 2) is no longer valid
  
  . Log File Monitoring : key = error, File =
  /var/log/mongodb/mongod.log
  (mgdb09:log[/var/log/mongodb/mongod.log,@mongodb_error,UTF-8]):
  2018-05-05T05:42:13.911+0900 I REPL     [SyncSourceFeedback]
  SyncSourceFeedback error sending update to mgdb08:27019:
  InvalidSyncSource: Sync source was cleared. Was mgdb08:27019


and later on one of the secondary became primary 

what cause this kind of issue what action do i need
As I am able to see in your error log file such as error Fetcher stopped querying remote oplog with error: InvalidSyncSource: sync source

As per MongoDB jira documentation here It seems that the common cause for the remote replSetUpdatePosition failing with those error codes NodeNotFound,NotMasterOrSecondary and InvalidReplicaSetConfig would be a change in config. Config changes currently will cause the oplog fetcher to fail because it will detect a change in the config version contained in the oplog query metadata. Subsequently, after exiting the oplog fetcher, we will go back to the sync source resolver which has its own blacklisting logic.

For your further ref here