Upgrade a Replica Set to 4.2

Important

Before you attempt any upgrade, please familiarize yourself with thecontent of this document.

If you need guidance on upgrading to 4.2, MongoDB offers major versionupgrade services to help ensurea smooth transition without interruption to your MongoDB application.

Upgrade Recommendations and Checklists

When upgrading, consider the following:

Upgrade Version Path

To upgrade an existing MongoDB deployment to 4.2, you must berunning a 4.0-series release.

To upgrade from a version earlier than the 4.0-series, you mustsuccessively upgrade major releases until you have upgraded to4.0-series. For example, if you are running a 3.6-series, you mustupgrade first to 4.0before you can upgrade to 4.2.

Preparedness

Before beginning your upgrade, see the Compatibility Changes in MongoDB 4.2 document toensure that your applications and deployments are compatible withMongoDB 4.2. Resolve the incompatibilities in your deployment beforestarting the upgrade.

Before upgrading MongoDB, always test your application in a stagingenvironment before deploying the upgrade to your productionenvironment.

Downgrade Consideration

Once upgraded to 4.2, if you need to downgrade, we recommend downgrading to the latest patch release of 4.0.

Read Concern Majority (3-Member Primary-Secondary-Arbiter Architecture)

Starting in MongoDB 3.6, MongoDB enables support for"majority" read concern by default.

You can disable read concern "majority" to prevent thestorage cache pressure from immobilizing a three-member replica setwith a primary-secondary-arbiter (PSA) architecture or a shardedcluster with a three-member PSA shards.

Note

Disabling "majority" read concern affects support fortransactions on sharded clusters. Specifically:

• A transaction cannot use read concern "snapshot" ifthe transaction involves a shard that has disabled readconcern “majority”.
• A transaction that writes to multiple shards errors if any of thetransaction’s read or write operations involves a shard that hasdisabled read concern "majority".

However, it does not affect transactionson replica sets. For transactions on replica sets, you can specifyread concern "majority" (or "snapshot"or "local" ) for multi-document transactions even ifread concern "majority" is disabled.

Disabling "majority" read concern disables supportfor Change Streams for MongoDB 4.0 and earlier. For MongoDB4.2+, disabling read concern "majority" has no effect on changestreams availability.

When upgraded to 4.2 with read concern “majority” disabled, you canuse change stream for your deployment.

For more information, see Disable Read Concern Majority.

Change Stream Resume Tokens

MongoDB 4.2 uses the version 1 (i.e. v1) change streamsresume tokens, introduced inversion 4.0.7.

The resume token _data type depends on the MongoDB versions and,in some cases, the feature compatibility version (fcv) at the timeof the change stream’s opening/resumption (i.e. a change in fcvvalue does not affect the resume tokens for already opened changestreams):

When upgrading from MongoDB 4.0.6 or earlier to MongoDB 4.2

During the upgrade process, a client may try to resume changestreams using the new v1 resume token when connected to a memberthat has not been updated (i.e. only accepts v0 token) andfail. In such cases, the client must wait for the 4.2upgrade to complete before resuming change streams.

Prerequisites

All Members Version

All replica set members must be running version 4.0. Toupgrade a replica set from an 3.6-series and earlier, firstupgrade all members of the replica set to the latest4.0-series release, andthen follow the procedure to upgrade from MongoDB 4.0 to4.2.

MMAPv1 to WiredTiger Storage Engine

MongoDB 4.2 removes support for the deprecated MMAPv1 storage engine.

If your 4.0 deployment uses MMAPv1, you must change the 4.0 deploymentto WiredTiger Storage Engine before upgrading to MongoDB 4.2. For details,see Change Replica Set to WiredTiger.

Review Current Configuration

With MongoDB 4.2, the mongod andmongos processes will not start withMMAPv1 Specific Configuration Options. Previous versions of MongoDB runningWiredTiger ignored MMAPv1 configurations options if they were specified.With MongoDB 4.2, you must remove these from your configuration.

Feature Compatibility Version

The 4.0 replica set must havefeatureCompatibilityVersion set to 4.0.

To ensure that all members of the replica set havefeatureCompatibilityVersion set to 4.0, connect to eachreplica set member and check the featureCompatibilityVersion:

db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

All members should return a result that includes"featureCompatibilityVersion" : { "version" : "4.0" }.

To set or update featureCompatibilityVersion, run thefollowing command on the primary. A majority of the data-bearingmembers must be available:

db.adminCommand( { setFeatureCompatibilityVersion: "4.0" } )

For more information, seesetFeatureCompatibilityVersion.

Replica Set Member State

Ensure that no replica set member is in ROLLBACK orRECOVERING state.

Download 4.2 Binaries

Via Package Manager

If you installed MongoDB from the MongoDB apt, yum, dnf, orzypper repositories, you should upgrade to 4.2 using your packagemanager.

Follow the appropriate 4.2 installation instructions for your Linux system. Thiswill involve adding a repository for the new release, then performingthe actual upgrade process.

Manually

If you have not installed MongoDB using a package manager, you canmanually download the MongoDB binaries from the MongoDB DownloadCenter.

See 4.2 installation instructions for more information.

Upgrade Process

You can upgrade from MongoDB 4.0 to 4.2 using a“rolling” upgrade to minimize downtime by upgrading the membersindividually while the other members are available.

Upgrade secondary members of the replica set.

Upgrade the secondarymembers of the replica set one at a time:

• Shut down the mongod instance and replace the 4.0binary with the 4.2 binary.
• Restart the member.

Step down the replica set primary.

Connect a mongo shell to the primary and users.stepDown() to step down the primary and force anelection of a new primary.

Upgrade the primary.

When rs.status()shows that the primary has stepped down and another memberhas assumed PRIMARY state, upgrade the stepped-down primary:

• Shut down the stepped-down primary and replace themongod binary with the 4.2 binary.
• Restart the member.

Enable backwards-incompatible 4.2 features.

At this point, you can run the 4.2 binaries without the4.2 features that are incompatible with 4.0.

To enable these 4.2 features, set the feature compatibilityversion (FCV) to 4.2.

Tip

Enabling these backwards-incompatible features can complicate thedowngrade process since you must remove any persistedbackwards-incompatible features before you downgrade.

It is recommended that after upgrading, you allow your deployment torun without enabling these features for a burn-in period to ensurethe likelihood of downgrade is minimal. When you are confident thatthe likelihood of downgrade is minimal, enable these features.

Tip

Ensure that no initial sync is in progress. RunningsetFeatureCompatibilityVersion command while an initialsync is in progress will cause the initial sync to restart.

On the primary, run the setFeatureCompatibilityVersion command in the admin database:

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

This command must perform writes to an internal systemcollection. If for any reason the command does not completesuccessfully, you can safely retry the command on the primary asthe operation is idempotent.

Post Upgrade

• TLS Options Replace Deprecated SSL Options
• Starting in MongoDB 4.2, MongoDB deprecates the SSL options for themongod, the mongos, and the mongo shell aswell as the corresponding net.ssl Options configurationfile options.

To avoid deprecation messages, use the new TLS options for themongod, the mongos, and the mongo shell.

• For the command-line TLS options, refer to the mongod, mongos, andmongo shell pages.
• For the corresponding mongod and mongos configuration fileoptions, refer to the configuration file page.
• For the connection string tls options, refer to theconnection string page.
  • 4.2-Compatible Drivers Retry Writes by Default
  • The official MongoDB 3.6 and 4.0-compatible drivers required including theretryWrites=true option in the connectionstring to enable retryable writes for that connection.

The official MongoDB 4.2-compatible drivers enable Retryable Writes bydefault. Applications upgrading to the 4.2-compatible drivers that requireretryable writes may omit the retryWrites=trueoption. Applications upgrading to the 4.2-compatible drivers that requiredisabling retryable writes must includeretryWrites=false in the connection string.

Additional Upgrade Procedures

• To upgrade a standalone, see Upgrade a Standalone to 4.2.
• To upgrade a sharded cluster, see Upgrade a Sharded Cluster to 4.2.