Downgrade 3.4 Sharded Cluster to 3.2

1. Downgrade Feature Compatibility Version

1. Connect a mongo shell to the mongos instance.

2. Downgrade the featureCompatibilityVersion to "3.2".

   db.adminCommand({setFeatureCompatibilityVersion: "3.2"})

   This command must perform writes to an internal system collection. If for any reason the command does not complete successfully, you can safely retry the command on the mongos instance as the operation is idempotent.

2. Remove Views

If you have defined any views, drop the views before downgrading MongoDB 3.4 to 3.2.

1. Connect a mongo shell to the mongos instance.

2. To find views, you can run the following in the mongo shell:

   db.adminCommand("listDatabases").databases.forEach(function(d){
      let mdb = db.getSiblingDB(d.name);
      mdb.getCollectionInfos({type: "view"}).forEach(function(c){
         print(mdb[c.name]);
      });
   });

   In each database that contains views, drop the system.views collection to drop all views in that database.

   If running with access control, you must have privileges to drop the system.views collection for the database. See Create a Role to Drop system.views Collection across Databases.

3. Remove Collation Option from Collections and Indexes

If you have defined any non-"simple" collation for a collection or an index, remove the collection or index before downgrading MongoDB 3.4 to 3.2.

1. Connect a mongo shell to the mongos instance.

2. To find collections with collation specifications, you can run the following in the mongo shell:

   db.adminCommand("listDatabases").databases.forEach(function(d){
      let mdb = db.getSiblingDB(d.name);
      mdb.getCollectionInfos( { "options.collation": { $exists: true } } ).forEach(function(c){
         print(mdb[c.name]);
      });
   });

   You can migrate the content of the collection to a new collection without the collation specification (one way is via the aggregation pipeline stage $out).

3. To find indexes with collation specification, you can run the following in the mongo shell:

   db.adminCommand("listDatabases").databases.forEach(function(d){
      let mdb = db.getSiblingDB(d.name);
      mdb.getCollectionInfos().forEach(function(c){
         let currentCollection = mdb.getCollection(c.name);
         currentCollection.getIndexes().forEach(function(i){
            if (i.collation){
               printjson(i);
            }
         });
      });
   });

   Drop the indexes with a collation specification. After the downgrade, recreate the dropped indexes.

4. Convert Data of Type Decimal

1. Connect a mongo shell to the mongos instance.

2. Convert any data of decimal type. In versions of MongoDB earlier than 3.4, operations against documents that contain decimal type may fail. For some possible conversion options, see Model Monetary Data.

   To detect the presence of decimal, you can run db.collection.validate(true) against the collections which may contain decimal data.

   db.collection.validate(true) reports on decimal data only when featureCompatibilityVersion is "3.2".

5. Downgrade Index Versions

If you have v: 2 indexes (i.e. the default version for indexes created in MongoDB 3.4 if featureCompatibilityVersion: "3.4"), reindex the collection to recreate all indexes on the collection as v: 1 before downgrading MongoDB.

You must perform this operation on both the shards and the config servers:

1. Connect a mongo shell to the mongos instance.

2. To find indexes with v: 2, you can run the following in the mongo shell:

   db.adminCommand("listDatabases").databases.forEach(function(d){
      let mdb = db.getSiblingDB(d.name);
      mdb.getCollectionInfos().forEach(function(c){
         let currentCollection = mdb.getCollection(c.name);
         currentCollection.getIndexes().forEach(function(i){
            if (i.v === 2){
               printjson(i);
            }
         });
      });
   });

3. If a shard is a replica set, repeat this procedure on each member of the shard as the reindex operation does not propagate to the secondaries.

   Tip

   If connecting a mongo shell to a secondary member, use db.getMongo() to allow reads from secondaries.

4. Repeat the process on each member of the config server replica set.

Considerations

While the downgrade is in progress, you cannot make changes to the collection metadata. For example, during the downgrade, do not do any of the following:

• sh.enableSharding()

• sh.shardCollection()

• sh.addShard()

• db.createCollection()

• db.collection.drop()

• db.dropDatabase()

• any operation that creates a database

• any other operation that modifies the cluster meta-data in any way. See Sharding Reference for a complete list of sharding commands. Note, however, that not all commands on the Sharding Reference page modify the cluster meta-data.

Downgrade a Sharded Cluster