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Downgrade 3.4 Sharded Cluster to 3.2

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

Downgrade Path

Once upgraded to 3.4, you cannot downgrade to a 3.2.7 or earlier version. You can only downgrade to a 3.2.8 or later version.

Create Backup

Optional but Recommended. Create a backup of your database.

Prerequisites

Before downgrading the binaries, you must downgrade the feature compatibility version and remove any 3.4 features incompatible with 3.2 or earlier versions as outlined below. These steps are necessary only if featureCompatibilityVersion has ever been set to "3.4".

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.

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, set rs.slaveOk().

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

Procedure

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:

Downgrade a Sharded Cluster

1

Download the latest 3.2 binaries.

Using either a package manager or a manual download, get the latest release in the 3.2 series. If using a package manager, add a new repository for the 3.2 binaries, then perform the actual downgrade process.

Once upgraded to 3.4, you cannot downgrade to a 3.2.7 or earlier version. You can only downgrade to a 3.2.8 or later version.

2

Disable the Balancer.

Turn off the balancer as described in Disable the Balancer.

3

Downgrade the mongos instances.

Downgrade the binaries and restart.

4

Downgrade each shard, one at a time.

Downgrade the shards one at a time. If the shards are replica sets, for each shard:

  1. Downgrade the secondary members of the replica set one at a time:

    1. Shut down the mongod instance and replace the 3.4 binary with the 3.2 binary.

    2. Start the 3.2 binary with the --shardsvr and --port command line options.

      mongod --shardsvr --port <port> --dbpath <path>
      

      Of if using a configuration file, update the file to include sharding.clusterRole: shardsvr and net.port and start:

      sharding:
         clusterRole: shardsvr
      net:
         port: <port>
      storage:
         dbpath: <path>
      

      Include any other configuration as appropriate for your deployment.

    3. Wait for the member to recover to SECONDARY state before downgrading the next secondary member. To check the member’s state, you can issue rs.status() in the mongo shell.

      Repeat for each secondary member.

  2. Step down the replica set primary.

    Connect a mongo shell to the primary and use rs.stepDown() to step down the primary and force an election of a new primary:

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

    1. Shut down the stepped-down primary and replace the mongod binary with the 3.2 binary.

    2. Start the 3.2 binary with the --shardsvr and --port command line options.

      mongod --shardsvr --port <port> --dbpath <path>
      

      Or if using a configuration file, update the file to specify sharding.clusterRole: shardsvr and net.port and start the 3.2 binary:

      sharding:
         clusterRole: shardsvr
      net:
         port: <port>
      storage:
         dbpath: <path>
      

      Include any other configuration as appropriate for your deployment.

5

Downgrade the config servers.

If the config servers are replica sets:

  1. Downgrade the secondary members of the replica set one at a time:

    1. Shut down the secondary mongod instance and replace the 3.4 binary with the 3.2 binary.

    2. Start the 3.2 binary with both the --configsvr and --port options:

      mongod --configsvr --port <port> --dbpath <path>
      

      If using a configuration file, update the file to specify sharding.clusterRole: configsvr and net.port and start the 3.4 binary:

      sharding:
         clusterRole: configsvr
      net:
         port: <port>
      storage:
         dbpath: <path>
      

      Include any other configuration as appropriate for your deployment.

    3. Wait for the member to recover to SECONDARY state before downgrading the next secondary member. To check the member’s state, issue rs.status() in the mongo shell.

      Repeat for each secondary member.

  2. Step down the replica set primary.

    1. Connect a mongo shell to the primary and use rs.stepDown() to step down the primary and force an election of a new primary:

      rs.stepDown()
      
    2. When rs.status() shows that the primary has stepped down and another member has assumed PRIMARY state, shut down the stepped-down primary and replace the mongod binary with the 3.2 binary.

    3. Start the 3.2 binary with both the --configsvr and --port options:

      mongod --configsvr --port <port> --dbpath <path>
      

      If using a configuration file, update the file to specify sharding.clusterRole: configsvr and net.port and start the 3.4 binary:

      sharding:
         clusterRole: configsvr
      net:
         port: <port>
      storage:
         dbpath: <path>
      

      Include any other configuration as appropriate for your deployment.

6

Re-enable the balancer.

Once the downgrade of sharded cluster components is complete, re-enable the balancer.