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Downgrade 5.0 Replica Set to 4.4

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  • Downgrade Path
  • Create Backup
  • Access Control
  • Prerequisites
  • Procedure

Before you attempt any downgrade, familiarize yourself with the content of this document.

Important

Before you upgrade or downgrade a replica set, ensure all replica set members are running. If you do not, the upgrade or downgrade will not complete until all members are started.

Once upgraded to 5.0, if you need to downgrade, we recommend downgrading to the latest patch release of 4.4.

Optional but Recommended. Create a backup of your database.

If your replica set has access control enabled, your downgrade user privileges must include privileges to list and manage indexes across databases. A user with root role has the required privileges.

To downgrade from 5.0 to 4.4, you must remove incompatible features that are persisted and/or update incompatible configuration settings. These include:

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:

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.

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.

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.

Note

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.

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

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.

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

Warning

Before proceeding with the downgrade procedure, ensure that all replica set members, including delayed replica set members, reflect the prerequisite changes. That is, check the featureCompatibilityVersion and the removal of incompatible features for each node before downgrading.

1

Using either a package manager or a manual download, get the latest release in the 4.4 series. If using a package manager, add a new repository for the 4.4 binaries, then perform the actual downgrade process.

Important

Before you upgrade or downgrade a replica set, ensure all replica set members are running. If you do not, the upgrade or downgrade will not complete until all members are started.

Once upgraded to 5.0, if you need to downgrade, we recommend downgrading to the latest patch release of 4.4.

2

Downgrade each secondary member of the replica set, one at a time:

  1. Run the following command in mongosh to perform a clean shutdown, or refer to Stop mongod Processes for additional ways to safely terminate the mongod process:
    db.adminCommand( { shutdown: 1 } )
  2. Replace the 5.0 binary with the 4.4 binary and restart.
  3. Wait for the member to recover to SECONDARY state before downgrading the next secondary. To check the member's state, use the rs.status() method in mongosh.
  4. Once the member is in SECONDARY stage, downgrade the next secondary.
3

Skip this step if the replica set does not include an arbiter.

Downgrade the arbiter member of the replica set:

  1. Run the following command in mongosh to perform a clean shutdown, or refer to Stop mongod Processes for additional ways to safely terminate the mongod process:
    db.adminCommand( { shutdown: 1 } )
  2. Delete the contents of the arbiter data directory. The storage.dbPath configuration setting or --dbpath command line option specify the data directory of the arbiter mongod.

    rm -rf /path/to/mongodb/datafiles/*
  3. Replace the 5.0 binary with the 4.4 binary and restart.
  4. Wait for the member to recover to ARBITER state. To check the member's state, connect mongosh to the member and run rs.status() method.
4

Use rs.stepDown() in mongosh to step down the primary and force the normal failover procedure.

rs.stepDown()

rs.stepDown() expedites the failover procedure and is preferable to shutting down the primary directly.

5

When rs.status() shows that the primary has stepped down and another member has assumed PRIMARY state:

  1. Run the following command in mongosh to perform a clean shutdown, or refer to Stop mongod Processes for additional ways to safely terminate the mongod process:
    db.adminCommand( { shutdown: 1 } )
  2. Replace the mongod binary with the 4.4 binary and restart.
←  Downgrade 5.0 Standalone to 4.4Downgrade 5.0 Sharded Cluster to 4.4 →
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