Transactions and Read Preference

Operations in a transaction use the transaction-level read preference.

Using the drivers, you can set the transaction-level read preference at the transaction start:

• If the transaction-level read preference is unset, the transaction uses the session-level read preference.
• If transaction-level and the session-level read preference are unset, the transaction uses the client-level read preference. By default, the client-level read preference is primary.

Multi-document transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.

Operations in a transaction use the transaction-level read concern. That is, any read concern set at the collection and database level is ignored inside the transaction.

You can set the transaction-level read concern at the transaction start.

• If the transaction-level read concern is unset, the transaction-level read concern defaults to the session-level read concern.
• If transaction-level and the session-level read concern are unset, the transaction-level read concern defaults to the client-level read concern. By default, client-level read concern is "local" for reads against the primary. See also:
  • Transactions and Read Preference
  • Default MongoDB Read Concerns/Write Concerns

Transactions support the following read concern levels:

"local"

• Read concern "local" returns the most recent data available from the node but can be rolled back.
• For transactions on sharded cluster, "local" read concern cannot guarantee that the data is from the same snapshot view across the shards. If snapshot isolation is required, use "snapshot" read concern.

"majority"

• Read concern "majority" returns data that has been acknowledged by a majority of the replica set members (i.e. data cannot be rolled back) if the transaction commits with write concern “majority”.
• If the transaction does not use write concern “majority” for the commit, the "majority" read concern provides no guarantees that read operations read majority-committed data.
• For transactions on sharded cluster, "majority" read concern cannot guarantee that the data is from the same snapshot view across the shards. If snapshot isolation is required, use "snapshot" read concern.

"snapshot"