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setDefaultRWConcern

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  • Definition
  • Syntax
  • Command Fields
  • Behavior
  • Example

Definition

setDefaultRWConcern

The setDefaultRWConcern administrative command sets the global default read or write concern configuration for a replica set or sharded cluster. setDefaultRWConcern must be run against the admin database.

  • For replica sets, issue the setDefaultRWConcern command on the primary mongod.

  • For sharded clusters, issue the setDefaultRWConcern on a mongos.

Syntax

The command has the following syntax:

db.adminCommand(
  {
    setDefaultRWConcern : 1,
    defaultReadConcern: { <read concern> },
    defaultWriteConcern: { <write concern> },
    writeConcern: { <write concern> },
    comment: <any>
  }
)

Command Fields

The command takes the following fields:

Field
Type
Description
int

Set to 1.

object

Document containing the global read concern configuration. Specify a valid read concern object.

Omit this document to leave the current global read concern unmodified. If omitted, setDefaultRWConcern must specify defaultWriteConcern.

object

Document containing the global default write concern configuration.

  • For the write concern w setting, setDefaultRWConcern supports all write concern values except w : 0.

  • For the write concern wtimeout setting, setDefaultRWConcern defaults to 0 if the setting is omitted. Operations block until the requested write concern is met. If specifying a global default wtimeout, ensure the value is large enough to allow write operations to achieve the requested write concern.

  • To unset the currently configured write concern, specify an empty document {}.

Omit this document to leave the current global write concern unmodified. If omitted, setDefaultRWConcern must specify defaultReadConcern.

object

Optional. A document that specifies the write concern to be used by the setDefaultRWConcern command itself.

If omitted, setDefaultRWConcern uses the previously set global default write concern if one was configured.

comment
any

Optional. A user-provided comment to attach to this command. Once set, this comment appears alongside records of this command in the following locations:

A comment can be any valid BSON type (string, integer, object, array, etc).

setDefaultRWConcern returns an object that contains the currently configured global default read and write concern. See getDefaultRWConcern for more complete documentation on the returned fields.

Behavior

Note

Requires featureCompatibilityVersion 4.4+

Each mongod in the replica set or sharded cluster must have featureCompatibilityVersion set to at least 4.4 to use setDefaultRWConcern.

Starting in MongoDB 5.0, once the Cluster Wide Write Concern (CWWC) is set via the setDefaultRWConcern command the write concern cannot be unset.

MongoDB only applies the global default read or write concern to operations which do not explicitly specify a read or write concern.

If MongoDB applies the global default read or write concern to an operation, that operation behaves as if that read or write concern were explicitly specified by the issuing client.

Replica Sets

Issue setDefaultRWConcern against the replica set primary. The primary replicates the new global default settings to the remaining members of the replica set. Secondaries which have not yet replicated the updated global default settings continue using their local 'stale' copy of the defaults.

Issue the setDefaultRWConcern command with a writeConcern of w : "majority" to ensure the command only returns after the changes have propagated to a majority of replica set members.

Sharded Clusters

Issue the setDefaultRWConcern against a mongos in the cluster. The mongos persists the updated settings to the config server replica set (CSRS). Each mongos periodically issues a getDefaultRWConcern against the CSRS to refresh their local copy of the global settings. A mongos uses its local 'stale' copy of the global defaults during the time period between refreshes.

Issue the setDefaultRWConcern command with a writeConcern of w : "majority" to ensure the command only returns after the changes have propagated to a majority of CSRS members.

When an application issues an operation against the mongos without explicitly specifying a read or write concern setting, the mongos applies the corresponding global default setting.

The global default settings do not propagate to the individual shards. You cannot run setDefaultRWConcern against a shard.

Important

setDefaultRWConcern requires featureCompatibilityVersion 4.4+. If you downgrade your deployment's featureCompatibilityVersion from 4.4 to 4.2, all cluster-wide read and write concern defaults are lost, but mongos instances may continue applying the defaults for up to 30 seconds.

Sharding Administrative Commands Override Write Concern Settings

Sharding administrative commands that perform write operations on the config server, such as the enableSharding or addShard commands, have specific behavior with global default write concern settings:

  • The commands use "majority" regardless of the configured global default write concern.

  • The commands use a minimum wtimeout of 60000. The commands only use the global default write concern wtimeout if it is greater than 60000.

Access Control

For replica sets or sharded clusters enforcing Authentication, setDefaultRWConcern requires that the authenticated user have the setDefaultRWConcern privilege action.

The clusterManager built-in role provides the required privileges to run setDefaultRWConcern.

Example

Set Global Default Write Concern

The following operation sets the global write concern to the following:

db.adminCommand({
  "setDefaultRWConcern" : 1,
  "defaultWriteConcern" : {
    "w" : 2
  }
})

The operation returns a document similar to the following:

{
  "defaultWriteConcern" : {
   "w" : 2
  },
  "updateOpTime" : Timestamp(1586290895, 1),
  "updateWallClockTime" : ISODate("2020-04-07T20:21:41.849Z"),
  "localUpdateWallClockTime" : ISODate("2020-04-07T20:21:41.862Z"),
  "ok" : 1,
  "$clusterTime" : { ... }
  "operationTime" : Timestamp(1586290925, 1)
}

Set Global Default Read Concern

The following operation sets the global read concern to "majority":

db.adminCommand({
  "setDefaultRWConcern" : 1,
  "defaultReadConcern" : { "level" : "majority" }
})

The operation returns a document similar to the following:

{
  "defaultReadConcern" : {
    "level" : "majority"
  },
  "updateOpTime" : Timestamp(1586290895, 1),
  "updateWallClockTime" : ISODate("2020-04-07T20:21:41.849Z"),
  "localUpdateWallClockTime" : ISODate("2020-04-07T20:21:41.862Z"),
  "ok" : 1,
  "$clusterTime" : { ... }
  "operationTime" : Timestamp(1586290925, 1)
}

Set Global Default Read and Write Concern

The following operation sets the global default read and write concern to the following:

db.adminCommand({
  "setDefaultRWConcern" : 1,
  "defaultWriteConcern" : {
    "w" : 2
  },
  "defaultReadConcern" : { "level" : "majority" }
})

The operation returns a document similar to the following:

"defaultWriteConcern" : {
  "w" : 2
},
"defaultReadConcern" : {
  "level" : "majority"
}

Unset Global Default Read and Write Concern

You can:

  • Unset the global default read concern.

  • Only unset the global default write concern if you haven't already set it.

For example, assume the global default read concern is set to level: "majority". To unset the global default read concern, use an empty document {}:

db.adminCommand( {
   "setDefaultRWConcern" : 1,
   "defaultReadConcern" : {}
} )

The operation returns a document that indicates the operation was successful:

{
   defaultReadConcern: { level: 'local' },
   defaultWriteConcern: { w: 2, wtimeout: 0 },
   updateOpTime: Timestamp({ t: 1656696934, i: 1 }),
   updateWallClockTime: ISODate("2022-07-01T17:35:40.578Z"),
   defaultWriteConcernSource: 'global',
   defaultReadConcernSource: 'implicit',
   localUpdateWallClockTime: ISODate("2022-07-01T17:35:40.578Z"),
   ok: 1,
   '$clusterTime': {
      ...
   },
   operationTime: Timestamp({ t: 1656632593, i: 1 })
}

You can only unset the global default write concern if you haven't already set it.

To unset the global default write concern, use an empty document {}:

db.adminCommand( {
   "setDefaultRWConcern" : 1,
   "defaultWriteConcern" : {}
} )

If the global default write concern is:

  • Unset, the operation succeeds.

  • Already set, the operation returns the following error.

MongoServerError: The global default write concern cannot be unset
once it is set.

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