setFeatureCompatibilityVersion

setFeatureCompatibilityVersion

Required

The possible values for version are:

confirm

Required

Set to true to confirm the feature compatibility change and allow the operation to proceed.

If you omit the confirm parameter or set confirm to a value other than true, the command fails and returns a warning about modifying the feature compatibility version. The warning states that after you upgrade or downgrade your cluster's fCV, you cannot downgrade the binary version without support assistance.

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.

Downgrade with Backward-Incompatible Data

If you try to downgrade the fCV of a cluster that contains incompatible data in the downloaded version, you receive a CannotDowngrade error. When this error occurs, the cluster remains in the transitionary downgrading state.

To move the cluster out of the downgrading state, either:

• Modify your cluster data to remove backwards-incompatible features, then re-run the setFeatureCompatibilityVersion command with the downgraded version to set the fCV to the downgraded version.

• Run the setFeatureCompatibilityVersion command with the original upgraded version to set the fCV back to the original version.

  Important

  Setting the fCV to the original version stops the downgrade procedure and changes the fCV back to the upgraded version. This procedure does not reset the cluster back to the state before the fCV downgrade began.

  If a failed fCV downgrade's internal metadata is not cleaned up, any subsequent FCV upgrade attempt fails with an error message. You must complete the fCV downgrade before trying to upgrade the fCV.

Downgrade Policy Changes in MongoDB 7.0

Starting in MongoDB 7.0, you cannot downgrade your deployment's fCV to or from a rapid release version of MongoDB.

If you upgrade or downgrade your deployment's fCV, you cannot downgrade your deployment's binary version without assistance from support.

To learn more, see Downgrade 7.0 to 6.0.

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

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.

Cluster-to-Cluster Sync and User Write Blocking

Starting in MongoDB 6.0, if you need to downgrade the feature compatibility version, ensure you disable cluster-to-cluster replication and user write blocking.

1. If you enabled cluster-to-cluster replication, disable it.

2. If you enabled user write blocking, disable it:

   db.runCommand( { setUserWriteBlockMode: 1, global: false } )

3. Wait for the previous command to complete.

4. Downgrade the feature compatibility version using setFeatureCompatibilityVersion.

For more information on MongoDB Cluster-to-Cluster Sync, see the documentation.

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.

Get FeatureCompatibilityVersion

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

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

The output resembles:

{
  featureCompatibilityVersion: { version: '5.0' },
  ok: 1,
  '$clusterTime': {
    clusterTime: Timestamp({ t: 1660318752, i: 5 }),
    signature: {
      hash: Binary(Buffer.from("ce0cff3621e9b089fa6d8e9a1e1efc1a1ff15dab", "hex"), 0),
      keyId: Long("7129893797260951557")
    }
  },
  operationTime: Timestamp({ t: 1660318752, i: 5 })
}

Set Feature Compatibility Version on MongoDB 7.0 Deployments

db.adminCommand(
   {
      setFeatureCompatibilityVersion: "7.0",
      confirm: true
   }
)

db.adminCommand(
   {
      setFeatureCompatibilityVersion: "6.0",
      confirm: true
   }
)

Set Feature Compatibility Version on MongoDB 6.0 Deployments

db.adminCommand( { setFeatureCompatibilityVersion: "6.0" } )

db.adminCommand( { setFeatureCompatibilityVersion: "5.0" } )

Set Feature Compatibility Version on MongoDB 5.0 Deployments

db.adminCommand( { setFeatureCompatibilityVersion: "5.0" } )

db.adminCommand( { setFeatureCompatibilityVersion: "4.4" } )

Set Feature Compatibility Version on MongoDB 4.4 Deployments

db.adminCommand( { setFeatureCompatibilityVersion: "4.4" } )

db.adminCommand( { setFeatureCompatibilityVersion: "4.2" } )

Set Write Concern Timeout

The following example sets the optional write concern wtimeout field to 5000 (5 seconds).

db.adminCommand( {
   setFeatureCompatibilityVersion: "5.0",
   writeConcern: { wtimeout: 5000 }
} )