setFeatureCompatibilityVersion

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Definition

Compatibility
Syntax
Behavior
Examples

Definition

setFeatureCompatibilityVersion: Enables or disables the features that persist data incompatible with earlier versions of MongoDB. You can only issue the setFeatureCompatibilityVersion against the admin database.

Warning

Enabling backwards-incompatible features can complicate the downgrade process since you must remove any persisted backwards-incompatible features before you downgrade.

It is recommended that after upgrading, you allow your deployment to run without enabling backwards-incompatible features for a burn-in period to ensure the likelihood of downgrade is minimal. When you are confident that the likelihood of downgrade is minimal, enable these features.

Compatibility

This command is available in deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Important

This command is not supported in M0, M2, M5, and M10+ clusters. For more information, see Unsupported Commands.

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

The command takes the following form:

db.adminCommand( {
   setFeatureCompatibilityVersion: <version>,
   writeConcern: { wtimeout: <timeout> }
} )

The values for the version are:

Version	Description
`"5.0"`	Available on MongoDB 5.0 Deployments Enables the 5.0 features that persist data incompatible with MongoDB 4.4.
`"4.4"`	Available on MongoDB 4.4 and 5.0 Deployments Enables the 4.4 features that persist data incompatible with MongoDB 4.2.
`"4.2"`	Available on MongoDB 4.4 Deployments Disables the 4.4 features that persist data incompatible with MongoDB 4.2.

writeConcern

Optional

The writeConcern specifies the write concern wtimeout value in milliseconds:

The time period that the primary waits for acknowledgment from the majority of the replica set members. If the acknowledgment is not received in the time period, the operation fails.
Default is 60000 milliseconds. Use a longer time period if the secondary members of the replica set have a delay that exceeds the wtimeout default.

Note

For a standalone, run the command on the standalone mongod instance.
For a replica set, run the command on the primary. A majority of the data-bearing members must be available.
For a sharded cluster, run the command on a mongos instance.

Behavior

Conflicts with Background Operations

Certain background operations may prevent execution of setFeatureCompatibilityVersion. Use currentOp to identify any ongoing operations.

Sync Failures

If you trigger a setFeatureCompatibilityVersion change during an initial sync, the sync may fail with an OplogOperationUnsupported error message when replaying entries on the oplog application phase. The sync following this attempt succeeds because the operation phase no longer replays the operation.

Default Values

Deployments	`featureCompatibilityVersion`
For new 5.0 deployments	`"5.0"`
For 5.0 deployments upgraded from 4.4	`"4.4"` until you `setFeatureCompatibilityVersion` to `"5.0"`.

Idempotency

This command must perform writes to an internal system collection. If for any reason the command does not complete successfully, you can safely retry the command as the operation is idempotent.

Feature Compatibility in Arbiters

Arbiters do not replicate the admin.system.version collection. Because of this, arbiters always have a feature compatibility version equal to the downgrade version of the binary, regardless of the fCV value of the replica set.

For example, an arbiter in a MongoDB 5.0 cluster, has an fCV value of 4.4.

Examples

View FeatureCompatibilityVersion

To view the featureCompatibilityVersion for a mongod instance, run the following command on a mongod instance:

Note

The operation is undefined on the mongos instances. For a sharded cluster that has access control enabled, to run the command against a member of the shard replica set, you must connect to the member as a shard local user.

db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

The output from this command will resemble one of the following, depending on the current state of the mongod:

If the deployment has the default featureCompatibilityVersion, or if the setFeatureCompatibilityVersion command has run successfully against the deployment, the featureCompatibilityVersion has the form:
```
"featureCompatibilityVersion" : {
   "version" : <version>
}
```
If the mongod is in a partially upgraded or downgraded state, the featureCompatibilityVersion has the following form:
```
"featureCompatibilityVersion" : {
   "version" : <version> ,
   "targetVersion" : <target version>
}
```
For instance, if a sharded cluster has a shard replica set that is read only when you run setFeatureCompatibilityVersion command against the mongos, the command will fail, and the featureCompatibilityVersion of the config servers will include the targetVersion field.
Or if a replica set becomes read only while setFeatureCompatibilityVersion is running, the command will fail, and the featureCompatibilityVersion of the replica set will include the targetVersion field as well.