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This is an upcoming (i.e. in progress) version of the manual.

Downgrade 4.2 Replica Set to 4.0

MongoDB 4.2 is currently in development as part of the 4.1 development series.

Warning

While the 4.1-dev-series are available, these versions of MongoDB are for testing purposes only and not for production use.

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

Downgrade Path

Once upgraded to 4.2, if you need to downgrade, we recommend downgrading to the latest patch release of 4.0 or, at least, 4.0.7 if you are running change streams and want to seamlessly resume change streams.

Considerations

Starting in MongoDB 4.2, change streams are available regardless of the "majority" read concern support; that is, read concern majority support can be either enabled (default) or disabled to use change streams.

In MongoDB 4.0 and earlier, change streams are available only if "majority" read concern support is enabled (default).

Once you downgrade to 4.0-series, change streams will be disabled if you have disabled read concern "majority".

Create Backup

Optional but Recommended. Create a backup of your database.

Access Control

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.

Prerequisites

To downgrade from 4.2 to 4.0, you must remove incompatible features that are persisted and/or update incompatible configuration settings. These include:

1. Downgrade Feature Compatibility Version (fCV)

Tip

To downgrade the featureCompatibilityVersion of your sharded cluster:

  1. Connect a mongo shell to the primary.

  2. Downgrade the featureCompatibilityVersion to "4.0".

    db.adminCommand({setFeatureCompatibilityVersion: "4.0"})
    

    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" : "4.0" }
    

    If any member returns a featureCompatibilityVersion of "4.2", wait for the member to reflect version "4.0" before proceeding.

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

2. Remove FCV 4.2 Persisted Features

The following steps are necessary only if fCV has ever been set to "4.2".

Remove all persisted 4.2 features that are incompatible with 4.0.

2a. Index Key Size

Starting in MongoDB 4.2, for featureCompatibilityVersion (fCV) set to "4.2" or greater, MongoDB removes the Index Key Limit. For fCV set to "4.0", the limit still applies.

If you have an index with keys that exceed the Index Key Limit once fCV is set to "4.0", consider changing the index to a hashed index or to indexing a computed value. You can also temporarily use failIndexKeyTooLong set to false before resolving the problem. However, with failIndexKeyTooLong set to false, queries that use these indexes can return incomplete results.

2b. Index Name Length

Starting in MongoDB 4.2, for featureCompatibilityVersion (fCV) set to "4.2" or greater, MongoDB removes the Index Name Length. For fCV set to "4.0", the limit still applies.

If you have an index with a name that exceeds the Index Name Length once fCV is set to "4.0", drop and recreate the index with a shorter name.

db.collection.dropIndex( <name | index specification> )

db.collection.createIndex(
   { <index specification> },
   { name: <shorter name> }
}

2c. Unique Index Version

With featureCompatibilityVersion (fCV) "4.2", MongoDB uses a new internal format for unique indexes that is incompatible with MongoDB 4.0. The new internal format applies to both existing unique indexes as well as newly created/rebuilt unique indexes.

If fCV has ever been set to "4.2", use the following script to drop and recreate all unique indexes.

Tip

Perform this operation after you have resolved any index key size and index name length issues first.

Script
// A script to rebuild unique indexes after downgrading fcv 4.2 to 4.0.
// Run this script to drop and recreate unique indexes
// for backwards compatibility with 4.0.

db.adminCommand("listDatabases").databases.forEach(function(d){
   let mdb = db.getSiblingDB(d.name);

   mdb.getCollectionInfos( { type: "collection" } ).forEach(function(c){
      let currentCollection = mdb.getCollection(c.name);

      currentCollection.getIndexes().forEach(function(idx){
         if (idx.unique){
            print("Dropping and recreating the following index:" + tojson(idx))

            assert.commandWorked(mdb.runCommand({dropIndexes: c.name, index: idx.name}));

            let res = mdb.runCommand({ createIndexes: c.name, indexes: [idx] });
            if (res.ok !== 1)
               assert.commandWorked(res);
         }
      });
   });
});

3. Update tls-Prefixed Configuration

Starting in MongoDB 4.2, MongoDB adds "tls"-prefixed options as aliases for the "ssl"-prefixed options.

If your deployments or clients use the "tls"-prefixed options, replace with the corresponding "ssl"-prefixed options for the mongod, the mongos, and the mongo shell and drivers.

4. Prepare Downgrade from zstd Compression

zstd Data Compression

Important

If you also use zstd Journal Compression, perform these steps after you perform the prerequisite steps for the journal compressor.

The zstd compression library is available starting in version 4.2. For any member that has data stored using zstd compression, the downgrade procedure will require an initial sync for that member. To prepare:

  1. Create a new empty data directory for the mongod instance. This directory will be used in the downgrade procedure below.

    Important

    Ensure that the user account running mongod has read and write permissions for the new directory.

  2. If you use a configuration file, update the file to prepare for the downgrade procedure:

    1. Delete storage.wiredTiger.collectionConfig.blockCompressor to use the default compressor (snappy) or set to another 4.0 supported compressor.
    2. Update storage.dbPath to the new data directory.
    If you use command-line options instead, you will have to update the options in the procedure below.

Repeat for any other members that used zstd compression.

zstd Journal Compression

The zstd compression library is available for journal data compression starting in version 4.2.

For any member that uses zstd library for its journal compressor:

If the member uses zstd for journal compression and data compression,
If the member uses zstd for journal compression only,

Note

The following procedure involves restarting the replica member as a standalone without the journal.

  1. Perform a clean shutdown of the mongod instance:

    db.getSiblingDB('admin').shutdownServer()
    
  2. Update the configuration file to prepare to restart as a standalone:

    For example:

    storage:
       journal:
          enabled: false
    #replication:
    #   replSetName: replA
    

    If you use command-line options instead of a configuration file, you will have to update the command-line option during the restart.

  3. Restart the mongod instance:

  4. Perform a clean shutdown of the mongod instance:

    db.getSiblingDB('admin').shutdownServer()
    

    Confirm that the process is no longer running.

  5. Update the configuration file to prepare to restart as a replica set member with the new journal compressor:

    For example:

    storage:
       wiredTiger:
          engineConfig:
             journalCompressor: <newValue>
    replication:
       replSetName: replA
    

    If you use command-line options instead of a configuration file, you will have to update the command-line options during the restart below.

  6. Restart the mongod instance as a replica set member:

    • If you are using a configuration file:

      mongod -f <path/to/myconfig.conf>
      
    • If you are using command-line options instead of a configuration file:

      mongod --wiredTigerJournalCompressor <differentCompressor|none> --replSet ...
      

Note

If you encounter an unclean shutdown for a mongod during the downgrade procedure such that you need to use the journal files to recover, recover the instance using the 4.2 mongod and then retry the downgrade of the instance.

zstd Network Compression

The zstd compression library is available for network message compression starting in version 4.2.

To prepare for the downgrade:

  1. For any replica set member that uses zstd for network message compression and uses a configuration file, update the net.compression.compressors setting to prepare for the restart during the downgrade procedure.

    If you use command-line options instead, you will have to update the options in the procedure below.
  2. For any client that specifies zstd in its URI connection string, update to remove zstd from the list.

  3. For any mongo shell that specifies zstd in its --networkMessageCompressors, update to remove zstd from the list.

Important

Messages are compressed when both parties enable network compression. Otherwise, messages between the parties are uncompressed.

5. Update View Definitions that Include $set

Before downgrading the binaries, modify read-only view definitions that include the $set stage to use the $addFields stage instead.

You can modify a view either by:

Procedure

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

Download the latest 4.0 binaries.

Using either a package manager or a manual download, get the latest release in the 4.0 series. If using a package manager, add a new repository for the 4.0 binaries, then perform the actual downgrade process.

Once upgraded to 4.2, if you need to downgrade, we recommend downgrading to the latest patch release of 4.0 or, at least, 4.0.7 if you are running change streams and want to seamlessly resume change streams.

2

Downgrade secondary members of the replica set.

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

  1. Shut down the mongod. See Stop mongod Processes for additional ways to safely terminate mongod processes.

    db.adminCommand( { shutdown: 1 } )
    
  2. Replace the 4.2 binary with the 4.0 binary and restart.

    Note

    If you use command-line options instead of a configuration file, update the command-line options as appropriate during the restart.

  3. Wait for the member to recover to SECONDARY state. To check the member’s state, connect a mongo shell to the member and run rs.status() method.

  4. Once the member is in SECONDARY stage, downgrade the next secondary.

3

Step down the primary.

Use rs.stepDown() in the mongo shell 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.

4

Replace and restart the former primary.

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

  1. Shut down the previous primary.

    db.adminCommand( { shutdown: 1 } )
    
  2. Replace the mongod binary with the 4.0 binary and restart.

    Note

    If you use command-line options instead of a configuration file, update the command-line options as appropriate during the restart.