Downgrade 4.2 Sharded Cluster to 4.0

Before you attempt any downgrade, familiarize yourself with the contentof 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.

If you downgrade, you can only downgrade to a 4.0.12 or later version.You cannot downgrade to a 4.0.11 or earlier version.

Considerations

Starting in MongoDB 4.2, change streams areavailable regardless of the "majority" read concernsupport; that is, read concern majority support can be eitherenabled (default) or disabledto use change streams.

In MongoDB 4.0 and earlier, change streams areavailable only if "majority" read concern support isenabled (default).

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

Create Backup

Optional but Recommended. Create a backup of your database.

Access Control

If your sharded cluster has access control enabled, yourdowngrade user privileges must include additional privileges tomanage indexes on the config database.

db.getSiblingDB("admin").createRole({
     role: "configIndexRole",
     privileges: [
        {
          resource: { db: "config", collection: "" },
          actions: [ "find", "dropIndex", "createIndex", "listIndexes" ]
        }
     ],
     roles: [  ]
});

Add the newly created role to your downgrade user. For example,if you have a user myDowngradeUser in the admin databasethat already has the root role, usedb.grantRolesToUser() to grant the additional role:

db.getSiblingDB("admin").grantRolesToUser( "myDowngradeUser",
   [ { role: "configIndexRole", db: "admin" } ],
   {  w: "majority", wtimeout: 4000 }
);

Prerequisites

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

1. Downgrade Feature Compatibility Version (fCV)

To downgrade the featureCompatibilityVersion of your shardedcluster:

• Connect a mongo shell to the mongos instance.

• Downgrade the featureCompatibilityVersion to "4.0".

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

The setFeatureCompatibilityVersion command performs writesto an internal system collection and is idempotent. If for any reasonthe command does not complete successfully, retry the command on themongos instance.

• To ensure that all members of the sharded cluster reflect theupdated featureCompatibilityVersion, connect to each shard replicaset member and each config server replica set member and check thefeatureCompatibilityVersion:

Tip

For a sharded cluster that has access control enabled, to run thefollowing command against a shard replica set member, you mustconnect to the member as a shard local user.

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 featureCompatibilityVersionvalue, 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. These include:

2a. Index Key Size

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

If you have an index with keys that exceed the Index KeyLimit once fCV is set to "4.0",consider changing the index to a hashed index or to indexing acomputed value. You can also temporarily usefailIndexKeyTooLong set to false before resolvingthe problem. However, with failIndexKeyTooLong set tofalse, queries that use these indexes can return incompleteresults.

2b. Index Name Length

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

If you have an index with a name that exceeds the Index NameLength 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> }
}

See

db.collection.dropIndex() and db.collection.createIndex()

2c. Unique Index Version

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

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

• Script to run on mongos

// 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);
         }
      });
   });
});

• Script to run on shards
• After you have run the script on mongos, you need tocheck individual shards if you have created shard localusers. That is, if you created maintenance usersdirectly on the shards instead of through mongos, runthe script on the primary member of the shard.

// A script to rebuild unique indexes after downgrading fcv 4.2 to 4.0.
// Run this script on shards to drop and recreate unique indexes
// for backwards compatibility with 4.0.
 
let mdb = db.getSiblingDB('admin');
 
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);
      }
   });
});

2d. Remove Wildcard Indexes

For featureCompatibilityVersion (fCV) set to "4.2", MongoDBsupports creating Wildcard Indexes. You must drop allwildcard indexes before downgrading to fCV "4.0".

Use the following script to drop and recreate all wildcard indexes:

// A script to drop wildcard indexes before downgrading fcv 4.2 to 4.0.
// Run this script to drop wildcard 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){
 
         var key = Object.keys(idx.key);
         if (key[0].includes("$**")) {
 
           print("Dropping index: " + idx.name + " from " + idx.ns);
 
           let res = mdb.runCommand({dropIndexes: currentCollection, index: idx.name});
           assert.commandWorked(res);
 
         }
 
      });
   });
});

Important

Downgrading to fCV "4.0" during an in-progress wildcard indexbuild does not automatically drop or kill the index build. Theindex build can complete after downgrading to fcv "4.0",resulting in a valid wildcard index on the collection. Startingthe 4.0 binary against against that data directory will result instartup failures.

Use db.currentOp() to check for any in-progress wildcardindex builds. Once any in-progress wildcard index builds complete,run the script to drop them before downgrading tofCV "4.0".

2e. View Definitions/Collection Validation Definitions that Include 4.2 Operators

Before downgrading the binaries, modify read-only view definitions and collection validation definitionsthat include the 4.2 operators, such as$set, $unset, $replaceWith.

• For the $set stage, use the $addFieldsstage instead.
• For the $replaceWith stage, use the$replaceRoot stage instead.
• For the $unset stage, use the $projectstage instead.

You can modify a view either by:

• dropping the view (db.myview.drop() method) andrecreating the view (db.createView() method) or
• using the collMod command.

You can modify the colleciton validation expressions by:

• using the collMod command.

3. Update tls-Prefixed Configuration

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

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

4. Prepare Downgrade from zstd Compression

zstd Journal Compression

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

For any shard or config server member that uses zstd libraryfor its journal compressor:

• If the member uses zstd for journal compression and zstd Data Compression,
• • If using a configuration file, deletestorage.wiredTiger.engineConfig.journalCompressor touse the default compressor (snappy) or set to another 4.0supported compressor.
  • If using command-line options instead, you will have to update theoptions in the procedure below.
• If the member only uses zstd for journal compression only,

Note

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

• Perform a clean shutdown of the mongod instance:

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

• Update the configuration file to prepare to restart as astandalone:

  • Set storage.journal.enabled to false.
  • Set parameter skipShardingConfigurationChecks to true.
  • Set parameter disableLogicalSessionCacheRefresh totrue in the setParameter section.
  • Comment out the replicationsettings for your deployment.
  • Comment out the sharding.clusterRole setting.
  • Set the net.port to the member’scurrent port, if it is not explicitly set.For example:

storage:
   journal:
      enabled: false
setParameter:
   skipShardingConfigurationChecks: true
   disableLogicalSessionCacheRefresh: true
#replication:
#   replSetName: shardA
#sharding:
#   clusterRole: shardsvr
net:
  port: 27218

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

• Restart the mongod instance:

  • If you are using a configuration file:

mongod -f <path/to/myconfig.conf>

- 

If you are using command-line options instead of aconfiguration file:

  - Include the [<code>--nojournal</code>]($1cde9863b9db90a4.md#cmdoption-mongod-nojournal)option.
  - Set parameter [<code>skipShardingConfigurationChecks</code>]($6058f7932783a3b7.md#param.skipShardingConfigurationChecks) to true.
  - Set parameter <code>disableLogicalSessionCacheRefresh</code> to<code>true</code> in the [<code>--setParameter</code>]($1cde9863b9db90a4.md#cmdoption-mongod-setparameter) option.
  - Remove any [replication command-line options]($1cde9863b9db90a4.md#cli-mongod-replica-set) (such as [<code>--replSet</code>]($1cde9863b9db90a4.md#cmdoption-mongod-replset)).
  - Remove [<code>--shardsvr</code>]($1cde9863b9db90a4.md#cmdoption-mongod-shardsvr)/[<code>--configsvr</code>]($1cde9863b9db90a4.md#cmdoption-mongod-configsvr) option.
  - Explicitly include [<code>--port</code>]($1cde9863b9db90a4.md#cmdoption-mongod-port) set tothe instance’s current port.

mongod --nojournal --setParameter skipShardingConfigurationChecks=true --setParameter disableLogicalSessionCacheRefresh=true --port <samePort> ...

• Perform a clean shutdown of the mongod instance:

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

Confirm that the process is no longer running.

• Update the configuration file to prepare to restart with the new journal compressor:

  • Remove the storage.journal.enabled setting.
  • Remove the skipShardingConfigurationChecks parameter setting.
  • Remove the disableLogicalSessionCacheRefresh parametersetting.
  • Uncomment the replicationsettings for your deployment.
  • Uncomment the sharding.clusterRole setting.
  • Removestorage.wiredTiger.engineConfig.journalCompressor settingto use the default journal compressor or specify a new value.For example:

storage:
   wiredTiger:
      engineConfig:
         journalCompressor: <newValue>
replication:
   replSetName: shardA
sharding:
   clusterRole: shardsvr
net:
  port: 27218

If you use command-line options instead of a configuration file, youwill have to update the command-line options during the restartbelow.

• 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 configurationfile:

  - Remove the [<code>--nojournal</code>]($1cde9863b9db90a4.md#cmdoption-mongod-nojournal) option.
  - Remove the [<code>skipShardingConfigurationChecks</code>]($6058f7932783a3b7.md#param.skipShardingConfigurationChecks) parameter setting.
  - Remove the <code>disableLogicalSessionCacheRefresh</code> parametersetting.
  - Remove the [<code>--wiredTigerJournalCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigerjournalcompressor) command-line option to usethe default journal compressor or update to a new value.
  - Include [<code>--shardsvr</code>]($1cde9863b9db90a4.md#cmdoption-mongod-shardsvr)/[<code>--configsvr</code>]($1cde9863b9db90a4.md#cmdoption-mongod-configsvr) option.
  - Include your [replication command-line options]($1cde9863b9db90a4.md#cli-mongod-replica-set) as well as any additionaloptions for your replica set member.

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

Note

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

zstd Data Compression

Important

If you also usezstd Journal Compression, perform thesesteps after you perform the prerequisite steps for thejournal compressor.

The zstd compression library is available starting inversion 4.2. For any config server member or shard member that has datastored using zstd compression, the downgrade procedure willrequire an initial sync for that member. To prepare:

• Create a new empty data directory forthe mongod instance. This directory will be usedin the downgrade procedure below.

Important

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

• If you use a configuration file, update the file to prepare forthe downgrade procedure:

  • Deletestorage.wiredTiger.collectionConfig.blockCompressorto use the default compressor (snappy) orset to another 4.0 supported compressor.
  • Update storage.dbPath to the newdata directory.

If you use command-line options instead, you will have to updatethe options in the procedure below.

Repeat for any other members that used zstd compression.

zstd Network Compression

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

To prepare for the downgrade:

• For any mongod/mongos instance that uses zstd for network messagecompression and uses a configuration file, update thenet.compression.compressors setting to prepare for therestart during the downgrade procedure.

If you use command-line options instead, you will have to updatethe options in the procedure below.

• For any client that specifies zstd in its URIconnection string, update to remove zstd from thelist.

• For any mongo shell that specifies zstd in its—networkMessageCompressors, update to remove zstd from thelist.

Important

Messages are compressed when both parties enable networkcompression. Otherwise, messages between the parties areuncompressed.

Procedure

Downgrade a Sharded Cluster

Warning

Before proceeding with the downgrade procedure, ensure that allmembers, including delayed replica set members in the shardedcluster, reflect the prerequisite changes. That is, check thefeatureCompatibilityVersion and the removal of incompatiblefeatures for each node before downgrading.

Download the latest 4.0 binaries.

Using either a package manager or a manual download, get the latestrelease in the 4.0 series. If using a package manager, add a newrepository for the 4.0 binaries, then perform the actual downgradeprocess.

Once upgraded to 4.2, if you need to downgrade, we recommend downgrading to the latest patch release of 4.0.

Disable the Balancer.

Connect a mongo shell to a mongos instance inthe sharded cluster, and run sh.stopBalancer() todisable the balancer:

sh.stopBalancer()

Note

If a migration is in progress, the system will complete thein-progress migration before stopping the balancer. You can runsh.isBalancerRunning() to check the balancer’s currentstate.

To verify that the balancer is disabled, runsh.getBalancerState(), which returns false if the balanceris disabled:

sh.getBalancerState()

Starting in MongoDB 4.2, sh.stopBalancer() also disablesauto-splitting for the sharded cluster.

For more information on disabling the balancer, seeDisable the Balancer.

Downgrade the mongos instances.

Downgrade the binaries and restart.

Note

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

• If your mongos command-line options include“tls”-prefixed options, update to“ssl”-prefixed options.
• If the mongos instance included zstd networkmessage compression, remove —networkMessageCompressors to use the defaultsnappy,zlib compressors. Alternatively, specify the list ofcompressors to use.

Downgrade each shard, one at a time.

Downgrade the shards one at a time.

• Downgrade the shard’s secondarymembers one at a time:

  • Shut down the mongod instance.

db.adminCommand( { shutdown: 1 } )

• Replace the 4.2binary 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.

- If your command-line options include [“tls”-prefixedoptions]($1cde9863b9db90a4.md#tls-mongod-options), update to [“ssl”-prefixed]($1cde9863b9db90a4.md#ssl-mongod-options) options.
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> data compression,
  - Update [<code>--dbpath</code>]($1cde9863b9db90a4.md#cmdoption-mongod-dbpath) to the new directory(created during the prerequisites).
  - Remove [<code>--wiredTigerCollectionBlockCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigercollectionblockcompressor) to use the default<code>snappy</code> compressor (or, alternatively, explicitly set to a4.0 supported compressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> journalcompression,
  - Remove [<code>--wiredTigerJournalCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigerjournalcompressor) to use the default <code>snappy</code>compressor (or, alternatively, explicitly set to a 4.0 supportedcompressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance included <code>zstd</code> networkmessage compression,
  - Remove [<code>--networkMessageCompressors</code>]($1cde9863b9db90a4.md#cmdoption-mongod-networkmessagecompressors) to enable message compressionusing the default <code>snappy,zlib</code> compressors. Alternatively,explicitly specify the compressor(s).

• Wait for the member to recover to SECONDARY state beforedowngrading the next secondary member. To checkthe member’s state, connect a mongo shell to the shardand run rs.status() method.

Repeat to downgrade for each secondary member.

• Downgrade the shard arbiter. if any.

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

• Shut down the mongod. SeeStop mongod Processes for additional ways to safelyterminate mongod processes.

db.adminCommand( { shutdown: 1 } )

• Delete the arbiter data directory. Thestorage.dbPath configuration setting or—dbpath command line option specify thedata directory of the arbiter mongod.

rm -rf /path/to/mongodb/datafiles

• 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.

- If your command-line options include [“tls”-prefixedoptions]($1cde9863b9db90a4.md#tls-mongod-options), update to [“ssl”-prefixed]($1cde9863b9db90a4.md#ssl-mongod-options) options.
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> data compression,
  - Update [<code>--dbpath</code>]($1cde9863b9db90a4.md#cmdoption-mongod-dbpath) to the new directory(created during the prerequisites).
  - Remove [<code>--wiredTigerCollectionBlockCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigercollectionblockcompressor) to use the default<code>snappy</code> compressor (or, alternatively, explicitly set to a4.0 supported compressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> journalcompression,
  - Remove [<code>--wiredTigerJournalCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigerjournalcompressor) to use the default <code>snappy</code>compressor (or, alternatively, explicitly set to a 4.0 supportedcompressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance included <code>zstd</code> networkmessage compression,
  - Remove [<code>--networkMessageCompressors</code>]($1cde9863b9db90a4.md#cmdoption-mongod-networkmessagecompressors) to enable message compressionusing the default <code>snappy,zlib</code> compressors. Alternatively,explicitly specify the compressor(s).

• Wait for the member to recover to ARBITER state. To checkthe member’s state, connect a mongo shell to themember and run rs.status() method.

• Downgrade the shard’s primary.

  • Step down the shard’s primary. Connect amongo shell to the primary and users.stepDown() to step down the primary and force anelection of a new primary:

rs.stepDown()

• When rs.status()shows that the primary has stepped down and another memberhas assumed PRIMARY state, downgrade the stepped-down primary:

• Shut down the stepped-down primary.

db.adminCommand( { shutdown: 1 } )

• Replace the 4.2binary 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.

- If your command-line options include [“tls”-prefixedoptions]($1cde9863b9db90a4.md#tls-mongod-options), update to [“ssl”-prefixed]($1cde9863b9db90a4.md#ssl-mongod-options) options.
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> data compression,
  - Update [<code>--dbpath</code>]($1cde9863b9db90a4.md#cmdoption-mongod-dbpath) to the new directory(created during the prerequisites).
  - Remove [<code>--wiredTigerCollectionBlockCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigercollectionblockcompressor) to use the default<code>snappy</code> compressor (or, alternatively, explicitly set to a4.0 supported compressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> journalcompression,
  - Remove [<code>--wiredTigerJournalCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigerjournalcompressor) to use the default <code>snappy</code>compressor (or, alternatively, explicitly set to a 4.0 supportedcompressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance included <code>zstd</code> networkmessage compression,
  - Remove [<code>--networkMessageCompressors</code>]($1cde9863b9db90a4.md#cmdoption-mongod-networkmessagecompressors) to enable message compressionusing the default <code>snappy,zlib</code> compressors. Alternatively,explicitly specify the compressor(s).

Repeat for the remaining shards.

Downgrade the config servers.

• Downgrade the secondarymembers of the config servers replica set (CSRS) one at a time:

  • Shut down the mongod instance.

db.adminCommand( { shutdown: 1 } )

• Replace the 4.2binary 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.

- If your command-line options include [“tls”-prefixedoptions]($1cde9863b9db90a4.md#tls-mongod-options), update to [“ssl”-prefixed]($1cde9863b9db90a4.md#ssl-mongod-options) options.
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> data compression,
  - Update [<code>--dbpath</code>]($1cde9863b9db90a4.md#cmdoption-mongod-dbpath) to the new directory(created during the prerequisites).
  - Remove [<code>--wiredTigerCollectionBlockCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigercollectionblockcompressor) to use the default<code>snappy</code> compressor (or, alternatively, explicitly set to a4.0 supported compressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> journalcompression,
  - Remove [<code>--wiredTigerJournalCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigerjournalcompressor) to use the default <code>snappy</code>compressor (or, alternatively, explicitly set to a 4.0 supportedcompressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance included <code>zstd</code> networkmessage compression,
  - Remove [<code>--networkMessageCompressors</code>]($1cde9863b9db90a4.md#cmdoption-mongod-networkmessagecompressors) to enable message compressionusing the default <code>snappy,zlib</code> compressors. Alternatively,explicitly specify the compressor(s).

• Wait for the member to recover to SECONDARY state beforedowngrading the next secondary member. To checkthe member’s state, connect a mongo shell to the shardand run rs.status() method.

Repeat to downgrade for each secondary member.

• Step down the config server primary.

  • Connect a mongo shell to the primary and users.stepDown() to step down the primary and force anelection of a new primary:

rs.stepDown()

• When rs.status()shows that the primary has stepped down and another memberhas assumed PRIMARY state, downgrade the stepped-down primary:

• Shut down the stepped-down primary.

db.adminCommand( { shutdown: 1 } )

• Replace the 4.2binary 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.

- If your command-line options include [“tls”-prefixedoptions]($1cde9863b9db90a4.md#tls-mongod-options), update to [“ssl”-prefixed]($1cde9863b9db90a4.md#ssl-mongod-options) options.
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> data compression,
  - Update [<code>--dbpath</code>]($1cde9863b9db90a4.md#cmdoption-mongod-dbpath) to the new directory(created during the prerequisites).
  - Remove [<code>--wiredTigerCollectionBlockCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigercollectionblockcompressor) to use the default<code>snappy</code> compressor (or, alternatively, explicitly set to a4.0 supported compressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance used <code>zstd</code> journalcompression,
  - Remove [<code>--wiredTigerJournalCompressor</code>]($1cde9863b9db90a4.md#cmdoption-mongod-wiredtigerjournalcompressor) to use the default <code>snappy</code>compressor (or, alternatively, explicitly set to a 4.0 supportedcompressor).
- If the [<code>mongod</code>]($1cde9863b9db90a4.md#bin.mongod) instance included <code>zstd</code> networkmessage compression,
  - Remove [<code>--networkMessageCompressors</code>]($1cde9863b9db90a4.md#cmdoption-mongod-networkmessagecompressors) to enable message compressionusing the default <code>snappy,zlib</code> compressors. Alternatively,explicitly specify the compressor(s).

Re-enable the balancer.

Once the downgrade of sharded cluster components iscomplete, connect to the mongos and restart the balancer.

sh.startBalancer();

To verify that the balancer is enabled, runsh.getBalancerState():

sh.getBalancerState()

If the balancer is enabled, the method returns true.

Re-enable autosplit.

When stopping the balancer as part of the downgrade process, thesh.stopBalancer() method also disabled auto-splitting.

Once downgraded to MongoDB 4.0, sh.startBalancer() doesnot re-enable auto-splitting. If you wish to re-enableauto-splitting, run sh.enableAutoSplit():

sh.enableAutoSplit()