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Release Notes for MongoDB 4.2

Upgrade a Replica Set to 4.2

On this page

  • Upgrade Recommendations and Checklists
  • Read Concern Majority (3-Member Primary-Secondary-Arbiter Architecture)
  • Change Stream Resume Tokens
  • All Members Version
  • MMAPv1 to WiredTiger Storage Engine
  • Increase Open Files ulimit Setting
  • Review Current Configuration
  • Feature Compatibility Version
  • Replica Set Member State
  • Via Package Manager
  • Manually

Important

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

If you need guidance on upgrading to 4.2, MongoDB professional services offer major version upgrade support to help ensure a 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 be running a 4.0-series release.

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

Check Driver Compatibility

Before you upgrade MongoDB, check that you're using a MongoDB 4.2-compatible driver. Consult the driver documentation for your specific driver to verify compatibility with MongoDB 4.2.

Upgraded deployments that run on incompatible drivers might encounter unexpected or undefined behavior.

Preparedness

Before beginning your upgrade, see the Compatibility Changes in MongoDB 4.2 document to ensure that your applications and deployments are compatible with MongoDB 4.2. Resolve the incompatibilities in your deployment before starting the upgrade.

Before upgrading MongoDB, always test your application in a staging environment before deploying the upgrade to your production environment.

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 the storage cache pressure from immobilizing a three-member replica set with a primary-secondary-arbiter (PSA) architecture or a sharded cluster with a three-member PSA shards.

Note

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

  • A transaction cannot use read concern "snapshot" if the transaction involves a shard that has disabled read concern "majority".

  • A transaction that writes to multiple shards errors if any of the transaction's read or write operations involves a shard that has disabled read concern "majority".

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

Disabling "majority" read concern prevents collMod commands which modify an index from rolling back. If such an operation needs to be rolled back, you must resync the affected nodes with the primary node.

Disabling "majority" read concern has no effect on change streams availability.

When upgraded to 4.2 with read concern "majority" disabled, you can use change streams for your deployment.

For more information, see Primary-Secondary-Arbiter Replica Sets.

Change Stream Resume Tokens

MongoDB 4.2 uses the version 1 (i.e. v1) change streams resume tokens, introduced in version 4.0.7.

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

MongoDB Version
Feature Compatibility Version
Resume Token _data Type
MongoDB 4.2 and later
"4.2" or "4.0"
Hex-encoded string (v1)
MongoDB 4.0.7 and later
"4.0" or "3.6"
Hex-encoded string (v1)
MongoDB 4.0.6 and earlier
"4.0"
Hex-encoded string (v0)
MongoDB 4.0.6 and earlier
"3.6"
BinData
MongoDB 3.6
"3.6"
BinData

Important

When upgrading from MongoDB 4.0.6 or earlier to MongoDB 4.2

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

Prerequisites

All Members Version

All replica set members must be running version 4.0. To upgrade a replica set from an 3.6-series and earlier, first upgrade all members of the replica set to the latest 4.0-series release, and then follow the procedure to upgrade from MongoDB 4.0 to 4.2.

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 deployment to WiredTiger Storage Engine before upgrading to MongoDB 4.2. For details, see Change Replica Set to WiredTiger.

Increase Open Files ulimit Setting

Starting in MongoDB 4.2, incoming connections to a mongod or mongos instance require two file descriptors. In previous versions of MongoDB, incoming connections required one file descriptor.

Prior to upgrading from MongoDB 4.0 to 4.2, you may need to increase the value of your open files ulimit setting (-n).

Review Current Configuration

With MongoDB 4.2, the mongod and mongos processes will not start with MMAPv1 Specific Configuration Options. Previous versions of MongoDB running WiredTiger 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 have featureCompatibilityVersion set to 4.0.

To ensure that all members of the replica set have featureCompatibilityVersion set to 4.0, 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" : "4.0" }.

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

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

For more information, see setFeatureCompatibilityVersion.

Replica Set Member State

Ensure that no replica set member is in ROLLBACK or RECOVERING state.

Download 4.2 Binaries

Via Package Manager

If you installed MongoDB from the MongoDB apt, yum, dnf, or zypper repositories, you should upgrade to 4.2 using your package manager.

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

Manually

If you have not installed MongoDB using a package manager, you can manually download the MongoDB binaries from the MongoDB Download Center.

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 members individually while the other members are available.

1

Upgrade secondary members of the replica set.

Upgrade the secondary members of the replica set one at a time:

  1. Shut down the mongod instance and replace the 4.0 binary with the 4.2 binary.

  2. Restart the 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.

3

Upgrade the primary.

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

  1. Shut down the stepped-down primary and replace the mongod binary with the 4.2 binary.

  2. Restart the member.

4

Enable backwards-incompatible 4.2 features.

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

To enable these 4.2 features, set the feature compatibility version (fCV) to 4.2.

Tip

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

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

Tip

Ensure that no initial sync is in progress. Running setFeatureCompatibilityVersion command while an initial sync 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 system collection. If for any reason the command does not complete successfully, you can safely retry the command on the primary as the operation is idempotent.

Post Upgrade

TLS Options Replace Deprecated SSL Options
Starting in MongoDB 4.2, MongoDB deprecates the SSL options for the mongod, the mongos, and the mongo shell as well as the corresponding net.ssl Options configuration file options.To avoid deprecation messages, use the new TLS options for the mongod, the mongos, and the mongo.

  • For the command-line TLS options, refer to the mongod, mongos, and mongo shell pages.

  • For the corresponding mongod and mongos configuration file options, refer to the configuration file page.

  • For the connection string tls options, refer to the connection string page.

4.2+ compatible Drivers Retry Writes by Default
Drivers compatible with MongoDB 4.2 and higher enable Retryable Writes by default. Earlier drivers require the retryWrites=true option. The retryWrites=true option can be omitted in applications that use drivers compatible with MongoDB 4.2 and higher.

To disable retryable writes, applications that use drivers compatible with MongoDB 4.2 and higher must include retryWrites=false in the connection string.

Additional Upgrade Procedures

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Upgrade a Standalone to 4.2