Downgrade 4.2 Standalone to 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.

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);
         }
      });
   });
});

2d. Remove user_1_db_1 System Unique Index

In addition, if you have enabled access control, you must also remove the system unique index user_1_db_1 on the admin.system.users collection.

If fCV has ever been set to "4.2", use the following command to drop the user_1_db_1 system unique index:

db.getSiblingDB("admin").getCollection("system.users").dropIndex("user_1_db_1")

The user_1_db_1 index will automatically be rebuilt when starting the server with the 4.0 binary in the procedure below.

2e. Remove Wildcard Indexes

For featureCompatibilityVersion (fCV) set to "4.2", MongoDB supports creating Wildcard Indexes. You must drop all wildcard 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);
         }
      });
   });
});

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

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

• For the $set stage, use the $addFields stage instead.

• For the $replaceWith stage, use the $replaceRoot stage instead.

• For the $unset stage, use the $project stage instead.

You can modify a view either by:

• dropping the view (db.myview.drop() method) and recreating the view (db.createView() method) or

• using the collMod command.

You can modify the colleciton validation expressions by:

• using the collMod command.

zstd Data Compression

The zstd compression library is available starting in version 4.2.

If your standalone has any data using zstd compression:

1. Stop all writes to your instance.

2. Create a mongodump of your database before starting the downgrade; mongodump outputs uncompressed data.

   mongodump --host=<myhost> --port=<port> --out=mystandalone.uncompressed.fcv4.0

   Include any other options, such as --username, --password, and --authenticationDatabase if your standalone enforces access control.

3. 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.

4. 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.

zstd Journal Compression

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

If the mongod instance uses zstd library for its journal compressor:

• If using a configuration file, delete storage.wiredTiger.engineConfig.journalCompressor to use the default compressor (snappy) or set to another 4.0 supported compressor.

• If using command-line options instead, you will have to update the options in the procedure below.

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 the mongod instance 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.