mongodb官网原文：
Collections to Support Sessions
New in version 3.6.
Starting in MongoDB 3.6, the config database contains the internal collections to support causally consistent sessions for standalones, replica sets, and sharded clusters and retryable writes and transactions for replica sets and sharded clusters.
WARNING
Do not manually modify or drop these collections.
To access these collections for a mongod or mongos instance, connect mongo shell to the instance.
config.system.sessions-
The
system.sessionscollection stores session records that are available to all members of the deployment.When a user creates a session on a
mongodormongosinstance, the record of the session initially exists only in-memory on the instance. Periodically, the instance will sync its cached sessions to thesystem.sessionscollection; at which time, they are visible to all members of the deployment.To view records in the
system.sessionscollection, use$listSessions.WARNING
Do not manually modify or drop the
system.sessionscollection.In a sharded cluster, the
system.sessionscollection is sharded.- When adding a shard to the sharded cluster, if the shard to add already contains its own
system.sessionscollection, MongoDB drops the new shard’ssystem.sessionscollection during the add process. - Starting in version 4.4 (and 4.2.7), MongoDB automatically splits the
system.sessionscollection into at least 1024 chunks and distributes the chunks uniformly across shards in the cluster.
- When adding a shard to the sharded cluster, if the shard to add already contains its own
config.transactions-
The
transactionscollection stores records used to support retryable writes and transactions for replica sets and sharded clusters.WARNING
Do not manually modify or drop the
transactionscollection.ConfigServer中，config库中的
system.sessions集合，是3.6版本新增的内部集合，为支持“因果一致性会话”服务。自4.4版本起，
system.sessions集合自动分片，并至少设置了1024个大块，这些大块均匀地分发至集群中的各个分片。