Docs Menu

Default MongoDB Read Concerns/Write Concerns

On this page

  • Read Concern
  • Write Concern
  • Causally Consistency Guarantees
Read/Write Concern Inheritance

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.

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

Read/Write Concern Inheritance

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" }.

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

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.
Give Feedback
© 2021 MongoDB, Inc.

About

  • Careers
  • Legal Notices
  • Privacy Notices
  • Security Information
  • Trust Center
© 2021 MongoDB, Inc.