MongoDB——4.4版本因果一致性會話
mongodb官網原文：
Collections to Support Sessions
New in version 3.6.
Starting in MongoDB 3.6, theconfigdatabase contains theinternalcollections to supportcausally consistent sessionsfor standalones, replica sets, and sharded clusters and retryable writes andtransactionsfor replica sets and sharded clusters.
WARNING
Do not manually modify or drop these collections.
To access these collections for amongodormongosinstance, connectmongoshell 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 supportretryable writesandtransactionsfor replica sets and sharded clusters.WARNING
Do not manually modify or drop the
transactionscollection.ConfigServer中，config庫中的
system.sessions集合，是3.6版本新增的內部集合，為支援“因果一致性會話”服務。自4.4版本起，
system.sessions集合自動分片，並至少設定了1024個大塊，這些大塊均勻地分發至叢集中的各個分片。