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REMEDIATION AND WORKAROUNDS

To remediate loss of availability, kill and re-start the config server primary and allow replica set failover to reset the session cache on the new primary.

Setting maxSessions to a higher number than the default of 1 million can delay the onset of this issue.
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请参考：
Jira Ticket: https://jira.mongodb.org/browse/SERVER-42827

解决方案：
升级到4.2.6。下载地址：https://www.mongodb.com/download-center/community

临时解决：
强制重新选举config主节点；
设置更大的maxSessions参数。