Read Concern "snapshot"
New in version 4.0.
Read concern "snapshot" is only available for multi-document
transactions.
A snapshot is a complete copy of the data in a mongod instance
at a specific point in time. You can retrieve snapshot metadata for the
whole cluster or replica set, or for a single config server in a
cluster. For more details, see snapshots.
If the transaction is not part of a causally consistent session, upon transaction commit with write concern
"majority", the transaction operations are guaranteed to have read from a snapshot of majority-committed data.If the transaction is part of a causally consistent session, upon transaction commit with write concern
"majority", the transaction operations are guaranteed to have read from a snapshot of majority-committed data that provides causal consistency with the operation immediately preceding the transaction start.
Operations
For a list of all operations that accept read concerns, see Operations That Support Read Concern.
Read Concern and Transactions
Multi-document transactions support read concern
"snapshot" as well as "local", and
"majority".
Note
You set the read concern at the transaction level, not at the individual operation level. To set the read concern for transactions, see Transactions and Read Concern.
For transactions on a sharded cluster, if any operation in the
transaction involves a shard that has disabled read concern
"majority", you cannot use read
concern "snapshot" for the transaction. You can only use
read concern "local" or "majority" for
the transaction. If you use read concern "snapshot", the
transaction errors and aborts. For more information, see
Disabled Read Concern Majority.