Downgrade 5.0 Replica Set to 4.4

1. Cluster Default Read and Write Concerns

MongoDB 5.0 changed the default value for cluster-wide read and write concerns, and downgrading to MongoDB 4.4 might change those defaults back. Consider manually configuring your cluster's default read and write concern before downgrading:

• To manually configure a default value for a cluster's read or write concern, use the setDefaultRWConcern command.
• If your cluster includes an arbiter, and you had previously disabled "Majority" read concern to prevent cache pressure in certain situations, you may want to configure --enableMajorityReadConcern false or replication.enableMajorityReadConcern: false once you downgrade.

2. Document Fields with . or $ Characters

MongoDB 5.0 adds support for including the . or $ characters in document field names. You must delete any documents containing field names that include the . or $ characters before downgrading to MongoDB 4.4.

3. Slim-format Timezone Data Files

MongoDB 5.0 enables support for slim-format timezone data files. If using slim-format timezone data files in your deployment, as provided to MongoDB with the --timeZoneInfo command line option or processManagement.timeZoneInfo configuration file setting, you must downgrade to MongoDB 4.4.7 or later, or else revert your timezone data files to use the previous non-slim-format data files.

4. Downgrade Feature Compatibility Version (fCV)

First, verify the following:

• Ensure that no initial sync is in progress. Running setFeatureCompatibilityVersion command while an initial sync is in progress will cause the initial sync to restart.

• Ensure that no nodes have a newlyAdded field in their replica set configuration. Run the following command on each node in your replica set to verify this:

  use local
  db.system.replset.find( { "members.newlyAdded" : { $exists : true } } );

  The newlyAdded field only appears in a node's replica set configuration document during and shortly after an initial sync.

• Ensure that no replica set member is in ROLLBACK or RECOVERING state.

Next, to downgrade the featureCompatibilityVersion of your replica set:

1. Connect a mongo shell to the primary.
2. Downgrade the featureCompatibilityVersion to "4.4".

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

   The setFeatureCompatibilityVersion command performs writes to an internal system collection and is idempotent. If for any reason the command does not complete successfully, retry the command on the primary.

3. To ensure that all members of the replica set reflect the updated featureCompatibilityVersion, connect to each replica set member and check the featureCompatibilityVersion:

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

   All members should return a result that includes:

   "featureCompatibilityVersion" : { "version" : "4.4" }

   If any member returns a featureCompatibilityVersion of "5.0", wait for the member to reflect version "4.4" before proceeding.

For more information on the returned featureCompatibilityVersion value, see View FeatureCompatibilityVersion.

5. Remove fCV 5.0 Persisted Features

The following steps are necessary only if fCV has ever been set to "5.0".

Remove all persisted 5.0 features that are incompatible with 4.4. These include:

Time-series Collections

Remove all time series collections.

Runtime Audit Filter Management

• Reset the defaults on the primary server in the group with db.admin.runCommand. The primary should be the last configuration server in the group to be updated.

  db.admin.runCommand(
     {
        setAuditConfig: 1,
        filter: {},
        auditAuthorizationSuccess: false
     }
  )

  The configuration document can also be removed after downgrade:

  config.settings.remove({_id: 'audit'});

• Disable Runtime Audit Filter Management on each node by setting auditLog.runtimeConfiguration to false in the node's configuration file.
• Update the audit filters for this instance in the local configuration file.

6. Remove 5.0 Features

Remove all persisted features that use 5.0 features. These include but are not limited to:

• If any view definitions include 5.0 operators, such as $dateAdd or $sampleRate, they must be removed. See New Aggregation Operators for the full list.