Read Concern 读取关注"available"

A query with read concern "available" returns data from the instance with no guarantee that the data has been written to a majority of the replica set members (i.e. may be rolled back).读取关注为"available"的查询从实例返回数据，但不能保证数据已写入大多数副本集成员（即可能被回滚）。

For a sharded cluster, "available" read concern provides greater tolerance for partitions since it does not wait to ensure consistency guarantees. 对于分片集群，"available"读取关注为分区提供了更大的容忍度，因为它不需要等待以确保一致性保证。That is, read concern "available" does not contact the shard's primary nor the config servers for updated metadata. 也就是说，读取关注"available"不会联系分片的主服务器，也不会联系配置服务器以获取更新的元数据。However, this means that a query with "available" read concern may return orphaned documents if the shard is undergoing chunk migrations.然而，这意味着，如果分片正在进行块迁移，则具有"available"读取关注的查询可能会返回孤立文档。

For unsharded collections (including collections in a standalone deployment or a replica set deployment), "local" and "available" read concerns behave identically.对于未排序的集合（包括独立部署或副本集部署中的集合），"local"和"available"读取关注的行为相同。

Regardless of the read concern level, the most recent data on a node may not reflect the most recent version of the data in the system.无论读取关注级别如何，节点上的最新数据都可能无法反映系统中数据的最新版本。

Example实例

Consider the following timeline of a write operation Write ₀ to a three member replica set:请考虑以下写入操作的时间线将₀写入三成员副本集：

Note

For simplification, the example assumes:为了简化，该示例假设：

• All writes prior to Write ₀ have been successfully replicated to all members.写入₀之前的所有写入操作都已成功复制到所有成员。
• Write _prev is the previous write before Write ₀.写入_prev是写入₀之前的上一次写入。
• No other writes have occured after Write ₀.写入₀之后没有发生其他写入操作。

Time	Event	Most Recent Write最近的写入	Most Recent w: "majority" write
t 0	Primary applies Write 主要应用写入0	Primary: Write 0 Secondary 1: Write prev Secondary 2: Write prev	Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev
t 1	Secondary 1 applies write 0	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write prev	Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev
t 2	Secondary 2 applies write 0	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev
t 3	Primary is aware of successful replication to Secondary 1 and sends acknowledgement to client主服务器知道已成功复制到辅助服务器1，并向客户端发送确认	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write 0 Secondary 1: Write prev Secondary 2: Write prev
t 4	Primary is aware of successful replication to Secondary 2主服务器知道已成功复制到辅助服务器2	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write 0 Secondary 1: Write prev Secondary 2: Write prev
t 5	Secondary 1 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: "majority" write辅助1收到通知（通过常规复制机制）更新其最近w:“多数”写入的快照	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write prev
t 6	Secondary 2 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: "majority" write辅助2收到通知（通过常规复制机制）更新其最近w:“多数”写入的快照	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0

Then, the following tables summarizes the state of the data that a read operation with "available" read concern would see at time T.然后，下表总结了具有"available"读取关注的读取操作在时间T时会看到的数据状态。

Read Target	Time T	State of Data数据状态
Primary	After t 0	Data reflects Write 0.
Secondary 1	Before t 1	Data reflects Write prev
Secondary 1	After t 1	Data reflects Write 0
Secondary 2	Before t 2	Data reflects Write prev
Secondary 2	After t 2	Data reflects Write 0