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Default MongoDB Read Concerns/Write Concerns

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  • Read Concern
  • Write Concern
  • Causally Consistency Guarantees

Read Concern

Read/Write Concern Inheritance

Default Read Concern

The default read concern is as follows:

Operations
Default Read Concern
Reads against primary

"local"

Note

  • This read concern can return data that may be rolled back.

  • This read concern does not guarantee causal consistency.

Reads against secondaries.

"local"

Note

  • This read concern can return data that may be rolled back.

  • This read concern does not guarantee causal consistency.

Specify Read Concern: MongoDB Drivers

Additional Information

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

Write Concern

Read/Write Concern Inheritance

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" }.

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

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.

←  Default MongoDB PortServer Sessions →

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