Downgrade 3.4 Replica Set to 3.2

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

Downgrade Path

Once upgraded to 3.4, you cannot downgrade to a 3.2.7 or earlierversion. 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 featurecompatibility version and remove any 3.4 features incompatible with 3.2 or earlier versions as generallyoutlined below. These steps are necessary only iffeatureCompatibilityVersion has ever been set to "3.4".

1. Downgrade Feature Compatibility Version

  • Connect a mongo shell to the primary.

  • Downgrade the featureCompatibilityVersion to "3.2".

  1. 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, youcan safely retry the command on the primary as the operation isidempotent.

2. Remove Views

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

  • Connect a mongo shell to the primary.

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

  1. db.adminCommand("listDatabases").databases.forEach(function(d){
  2.    let mdb = db.getSiblingDB(d.name);
  3.    mdb.getCollectionInfos({type: "view"}).forEach(function(c){
  4.       print(mdb[c.name]);
  5.    });
  6. });

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

If running with access control, you must have privileges to drop thesystem.views collection for the database. SeeCreate 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 anindex, remove the collection or index before downgrading MongoDB 3.4 to3.2.

  • Connect a mongo shell to the primary.

  • To find collections with collation specifications, you can run thefollowing in the mongo shell:

  1. db.adminCommand("listDatabases").databases.forEach(function(d){
  2.    let mdb = db.getSiblingDB(d.name);
  3.    mdb.getCollectionInfos( { "options.collation": { $exists: true } } ).forEach(function(c){
  4.       print(mdb[c.name]);
  5.    });
  6. });

You can migrate the content of the collection to a new collectionwithout the collation specification (one way is via theaggregation pipeline stage $out).

  • To find indexes with collation specification, you can run thefollowing in the mongo shell:
  1. db.adminCommand("listDatabases").databases.forEach(function(d){
  2.    let mdb = db.getSiblingDB(d.name);
  3.    mdb.getCollectionInfos().forEach(function(c){
  4.       let currentCollection = mdb.getCollection(c.name);
  5.       currentCollection.getIndexes().forEach(function(i){
  6.          if (i.collation){
  7.             printjson(i);
  8.          }
  9.       });
  10.    });
  11. });

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

4. Convert Data of Type Decimal

  • Connect a mongo shell to the primary.

  • Convert any data of decimal type. In versionsof MongoDB earlier than 3.4, operations against documents thatcontain decimal type may fail. For somepossible conversion options, seeModel Monetary Data.

To detect the presence of decimal, you can rundb.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 indexescreated in MongoDB 3.4 if featureCompatibilityVersion: "3.4"),reindex the collection to recreateall indexes on the collection as v: 1 before downgrading MongoDB.

  • Connect a mongo shell to the primary.

  • To find indexes with v: 2, you can run the following in themongo shell:

  1. db.adminCommand("listDatabases").databases.forEach(function(d){
  2.    let mdb = db.getSiblingDB(d.name);
  3.    mdb.getCollectionInfos().forEach(function(c){
  4.       let currentCollection = mdb.getCollection(c.name);
  5.       currentCollection.getIndexes().forEach(function(i){
  6.          if (i.v === 2){
  7.             printjson(i);
  8.          }
  9.       });
  10.    });
  11. });

Repeat the process on secondary members of the replica set as thereindex operation does not propagate to the secondaries.

Tip

If connecting a mongo shell to a secondary member, setrs.slaveOk().

Procedure

Download the latest 3.2 binaries.

Using either a package manager or a manual download, get the latestrelease in the 3.2 series. If using a package manager, add a newrepository for the 3.2 binaries, then perform the actual downgradeprocess.

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

Downgrade secondary members of the replica set.

Downgrade each secondary member of the replica set, one at atime:

  • Shut down the mongod. See Stop mongod Processes for instructions on safely terminating mongod processes.
  • Replace the 3.4 binary with the 3.2 binary and restart.
  • Wait for the member to recover to SECONDARY statebefore downgrading the next secondary. To check the member’s state,use the rs.status() method in the mongo shell.

Step down the primary.

Use rs.stepDown() in the mongo shell tostep down the primary and force the normal failover procedure.

  1. rs.stepDown()

rs.stepDown() expedites the failover procedure and ispreferable to shutting down the primary directly.

Replace and restart former primary mongod.

When rs.status() shows that the primary has stepped downand another member has assumed PRIMARY state, shut down theprevious primary and replace the mongod binary withthe 3.2 binary and start the new instance.