Read Concern "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).

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. However, a query with "available" read concern may return orphan documents if the shard is undergoing chunk migrations since the "available" read concern, unlike "local" read concern, does not contact the shard’s primary nor the config servers for updated metadata.

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.

Example

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.
• Write_prev 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: Write 0 Secondary 1: Write prev Secondary 2: Write prev	Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev
t1	Secondary1 applies write0	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write prev	Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev
t2	Secondary2 applies write0	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev
t3	Primary is aware of successful replication to Secondary1 and sends acknowledgement to client	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write 0 Secondary 1: Write prev Secondary 2: Write prev
t4	Primary is aware of successful replication to Secondary2	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write 0 Secondary 1: Write prev Secondary 2: Write prev
t5	Secondary1 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: “majority” write	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0	Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write prev
t6	Secondary2 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: “majority” write	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.

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