Downgrade 4.4 Replica Set to 4.2

1. Namespace Length

Starting in MongoDB 4.4:

• For featureCompatibilityVersion set to "4.4" or greater, MongoDB raises the limit for unsharded collections and views to 255 bytes, and to 235 bytes for sharded collections. For a collection or a view, the namespace includes the database name, the dot (.) separator, and the collection/view name (e.g. <database>.<collection>),
• For featureCompatibilityVersion set to "4.2" or earlier, the maximum length of unsharded collections and views namespace remains 120 bytes and 100 bytes for sharded collection.

Before downgrading, resolve any collections or views with namespaces that exceed the 120-byte Namespace Length limit for Feature Compatibility Version (fCV) 4.2.

To determine if you have any collections or views with namespaces that exceed the 120-byte limit, connect mongo shell to the primary and run:

db.adminCommand("listDatabases").databases.forEach(function(d){
   let mdb = db.getSiblingDB(d.name);
   mdb.getCollectionInfos( ).forEach(function(c){
      namespace = d.name + "." + c.name
      namespacelenBytes =  encodeURIComponent(namespace).length
      if (namespacelenBytes > 120) {
         print (c.type.toUpperCase() + " namespace exceeds 120 bytes:: " + namespace )
      }
   } )
})

If any collection or view namespace exceeds 120 bytes, then prior to downgrading the fCV:

• Rename the collection using the renameCollection command.
• For views, use db.createView() to recreate the view using a shorter name, then drop the original view.

2. Downgrade Feature Compatibility Version (fCV)

To downgrade the featureCompatibilityVersion of your replica set:

1. Connect a mongo shell to the primary.
2. Downgrade the featureCompatibilityVersion to "4.2".

   db.adminCommand({setFeatureCompatibilityVersion: "4.2"})

   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" : "4.2" }

   If any member returns a featureCompatibilityVersion of "4.4", wait for the member to reflect version "4.2" before proceeding.

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

3. Remove FCV 4.4 Persisted Features

The following steps are necessary only if fCV has ever been set to "4.4".

Remove all persisted 4.4 features that are incompatible with 4.2. These include:

Compound Hashed Indexes

Remove all compound hashed indexes.

Use db.collection.getIndexes() to identify any compound hashed indexes in a collection and use db.collection.dropIndex() to remove those indexes.

4. Remove 4.4 Features

Remove all persisted features that use 4.4 features. These include but are not limited to:

• If any view definitions that include 4.4 operators, such as $unionWith or $function. See also New Aggregation Operators.
• Unhide or drop hidden indexes.