Read Concern "available"¶
New in version 3.6.版本3.6中的新功能。
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).
Read concern “available” is the default for reads against secondaries if the reads are not associated with causally consistent sessions.
For a sharded cluster, "available" read concern provides greater tolerance for partitions since it does not wait to ensure consistency guarantees. That is, read concern "available" does not contact the shard’s primary nor the config servers for updated metadata. However, this means that a query with "available" read concern may return orphaned documents if the shard is undergoing chunk migrations.
For unsharded collections (including collections in a standalone deployment or a replica set deployment), "local" and "available" read concerns behave identically.
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.
See also参阅
Availability¶
Read concern available is unavailable for use
with causally consistent sessions and transactions.
Example示例¶
Consider the following timeline of a write operation Write0 to a three member replica set:
Note
For simplification, the example assumes:
- All writes prior to Write0 have been successfully replicated to all members.
- Writeprev is the previous write before Write0.
- No other writes have occured after Write0.
| Time | Event | Most Recent Write | Most Recent w: “majority” write |
|---|---|---|---|
| t0 | Primary applies Write0 | Primary: Write0
Secondary1: Writeprev
Secondary2: Writeprev |
Primary: Writeprev
Secondary1: Writeprev
Secondary2: Writeprev |
| t1 | Secondary1 applies write0 | Primary: Write0
Secondary1: Write0
Secondary2: Writeprev |
Primary: Writeprev
Secondary1: Writeprev
Secondary2: Writeprev |
| t2 | Secondary2 applies write0 | Primary: Write0
Secondary1: Write0
Secondary2: Write0 |
Primary: Writeprev
Secondary1: Writeprev
Secondary2: Writeprev |
| t3 | Primary is aware of successful replication to Secondary1 and sends acknowledgement to client | Primary: Write0
Secondary1: Write0
Secondary2: Write0 |
Primary: Write0
Secondary1: Writeprev
Secondary2: Writeprev |
| t4 | Primary is aware of successful replication to Secondary2 | Primary: Write0
Secondary1: Write0
Secondary2: Write0 |
Primary: Write0
Secondary1: Writeprev
Secondary2: Writeprev |
| t5 | Secondary1 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: “majority” write | Primary: Write0
Secondary1: Write0
Secondary2: Write0 |
Primary: Write0
Secondary1: Write0
Secondary2: Writeprev |
| t6 | Secondary2 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: “majority” write | Primary: Write0
Secondary1: Write0
Secondary2: Write0 |
Primary: Write0
Secondary1: Write0
Secondary2: Write0 |
Then, the following tables summarizes the state of the data that a read operation with "available" read concern would see at time T.
| Read Target | Time T | State of Data |
|---|---|---|
| Primary | After t0 | Data reflects Write0. |
| Secondary1 | Before t1 | Data reflects Writeprev |
| Secondary1 | After t1 | Data reflects Write0 |
| Secondary2 | Before t2 | Data reflects Writeprev |
| Secondary2 | After t2 | Data reflects Write0 |