Default MongoDB Read Concerns/Write Concerns

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Read Concern

Write Concern
Causally Consistency Guarantees

Read Concern

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Default Read Concern

The default read concern is as follows:

Operations	Default Read Concern
Reads against primary	`"local"` This read concern can return data that may be rolled back. This read concern does not guarantee causal consistency.
Reads against secondaries.	`"local"` This read concern can return data that may be rolled back. This read concern does not guarantee causal consistency.

Specify Read Concern: MongoDB Drivers

The following information applies to operations that are run outside transactions. For read concern information related to operations that are run inside transactions, click the Operations in Transactions tab.

Using the MongoDB drivers, you can override the default read concern and set read concern for operations at the following levels:

Level	Description
Client level	Applies to operations unless a finer-grained read concern for an operation is set at the database/collection/operation level.
Database level	Applies to operations on the database's collections (i.e. overrides the client read concern) unless a read concern has been set at the collection level or the operation level. Does not apply to operations inside transactions.
Collection level	Applies for read operations on the collection (i.e. overrides the database/client read concern) unless a read concern has been set at the operation level. Does not apply to operations inside transactions.
Operation level	Applies for the specific read operation (i.e. overrides the database/client/collection read concern). The ability to set read concern at the operation depends on the driver. Refer to your driver's documentation. Does not apply to operations inside transactions.

The following information applies to operations that are run inside transactions. For read concern information related to operations that are run outside transactions, click the Operations outside Transactions tab.

Using the MongoDB drivers,you can override the default read concern and set read concern for transactions at the following levels:

Level	Description
Client level	Applies to transactions unless a finer-grained read concern is set at the session/transaction level. All operations within a transaction use the transaction read concern: Any read concern set at the operation, collection, or database level is ignored inside the transaction.
Session level	Applies to transactions started in the session (i.e. overrides the client read concern) unless a finer-grained read concern level is set at a specific transaction level. All operations within a transaction use the transaction read concern: Any read concern set at the operation, collection, or database level is ignored inside the transaction. See Transactions and Read Concern for more information.
Transaction level	Applies to the specific transaction (i.e. overrides the client/session read concern). All operations within a transaction use the transaction read concern: Any read concern set at the operation, collection, or database level is ignored inside the transaction. See Transactions and Read Concern for more information.

Additional Information

For more information on the available read concerns, see Read Concern.

Write Concern

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Default Write Concern

Starting in MongoDB 5.0, the implicit default write concern is w: majority. However, special considerations are made for deployments containing arbiters:

The voting majority of a replica set is 1 plus half the number of voting members, rounded down. If the number of data-bearing voting members is not greater than the voting majority, the default write concern is { w: 1 }.
In all other scenarios, the default write concern is { w: "majority" }.
For a sharded cluster, the default write concern is always retrieved from the config server. Since the config server must have zero arbiters, the implicit default write concern for a sharded cluster is always "majority". Even if a shard is in a Primary-Secondary-Arbiter topology, it still has a default write concern of "majority". Starting in MongoDB 5.1, this configuration is disallowed if the cluster-wide write concern is not set.

Specifically, MongoDB uses the following formula to determine the default write concern:

if [ (#arbiters > 0) AND (#non-arbiters <= majority(#voting-nodes)) ]
    defaultWriteConcern = { w: 1 }
else
    defaultWriteConcern = { w: "majority" }

For example, consider the following deployments and their respective default write concerns:

Non-Arbiters	Arbiters	Voting Nodes	Majority of Voting Nodes	Implicit Default Write Concern
2	1	3	2	`{ w: 1 }`
4	1	5	3	`{ w: "majority" }`

In the first example:
- There are 2 non-arbiters and 1 arbiter for a total of 3 voting nodes.
- The majority of voting nodes (1 plus half of 3, rounded down) is 2.
- The number of non-arbiters (2) is equal to the majority of voting nodes (2), resulting in an implicit write concern of { w: 1 }.
In the second example:
- There are 4 non-arbiters and 1 arbiter for a total of 5 voting nodes.
- The majority of voting nodes (1 plus half of 5, rounded down) is 3.
- The number of non-arbiters (4) is greater than the majority of voting nodes (3), resulting in an implicit write concern of { w: "majority" }.

Specify Write Concern: MongoDB Drivers

Using the MongoDB drivers, you can override the default write concern and set write concern for operations at the following levels:

Level	Description
Client level	Applies to operations unless a finer-grained write concern for an operation is set at the operation/database/collection.
Database level	Applies to write operations on the database's collections (i.e. overrides the client write concern) unless a write concern has been set at the collection level or the operation level. Does not apply to operations inside transactions.
Collection level	Applies to write operations on the collection (i.e. overrides the database and client write concern) unless a write concern has been set at the operation level. Does not apply to operations inside transactions.
Operation level	Applies to the specific write operation. The ability to set write concern at the operation depends on the driver. Refer to your driver's documentation. Does not apply to operations inside transactions.

Using the MongoDB drivers, you can override the default write concern and set write concern for for transactions at the following levels:

Level	Description
Client level	Applies to transactions unless a finer-grained write concern for transactions are set at the session/transaction level. Transaction write concern applies to the commit operation and the operations inside the transaction. All operations within a transaction use the transaction write concern: Any write concern set at the operation, collection, or database level is ignored inside the transaction.
Session level	Applies for transactions started in the session unless the write concern level is set at a specific transaction level. Transaction write concern applies to the commit operation and the operations inside the transaction. All operations within a transaction use the transaction write concern: Any write concern set at the operation, collection, or database level is ignored inside the transaction.
Transaction level	Applies to the specific transaction. Transaction write concern applies to the commit operation and the operations inside the transaction. All operations within a transaction use the transaction write concern: Any write concern set at the operation, collection, or database level is ignored inside the transaction.

See Transactions and Write Concern for more information.

Additional Information

For more information on the available write concerns, see Write Concern.

Causally Consistency Guarantees

With causally consistent client sessions, the client sessions only guarantee causal consistency if:

the associated read operations use "majority" read concern, and
the associated write operations use "majority" write concern.

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