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.

  Queries with the "available" read concern don't require a check to ensure that the correct shard received the query.

  In the event of network partitions, these queries route to the shard considered appropriate before the network partition occurred. They do not wait for consistency guarantees from servers that may be unavailable. If the shard was undergoing chunk migration, queries with "available" can return orphaned documents.

  Warning

  mongos does not guarantee that queries with read concern "available" route to the correct shards. The shard that receives the query returns data, but this shard may not be the authoritative owner of the requested data.

  This can cause queries to return incorrect or unexpected results.

• 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 Write ₀ to a three member replica set:

Note

For simplification, the example assumes:

• All writes prior to Write ₀ have been successfully replicated to all members.

• Write _prev is the previous write before Write ₀.

• No other writes have occured after Write ₀.

Time	Event	Most Recent Write	Most Recent w: "majority" write
t 0	Primary applies Write 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	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	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	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	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 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