MongoDB——4.4版本因果一致性会话

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.sessions collection stores session records that are available to all members of the deployment.

When a user creates a session on a mongod or mongos instance, the record of the session initially exists only in-memory on the instance. Periodically, the instance will sync its cached sessions to the system.sessions collection; at which time, they are visible to all members of the deployment.

To view records in the system.sessions collection, use $listSessions.

WARNING

Do not manually modify or drop the system.sessions collection.

In a sharded cluster, the system.sessions collection is sharded.

• When adding a shard to the sharded cluster, if the shard to add already contains its own system.sessions collection, MongoDB drops the new shard’s system.sessions collection during the add process.
• Starting in version 4.4 (and 4.2.7), MongoDB automatically splits the system.sessions collection into at least 1024 chunks and distributes the chunks uniformly across shards in the cluster.

config.transactions

The transactions collection stores records used to support retryable writes and transactions for replica sets and sharded clusters.

WARNING

Do not manually modify or drop the transactions collection.

 

 

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。