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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).

  • 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.

Tip

See also:

Read concern "available" is unavailable for use with causally consistent sessions and transactions.

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.

Timeline of a write operation to a three member replica set.
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 acknowledgment 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.

Timeline of a write operation to a three member replica set.
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
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