Upgrade a Replica Set to 4.0

Important

MongoDB 4.0 may lose data during unclean shutdowns on macOS 10.12.x and 10.13.x.

For details, see WT-4018.

Important

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

If you need guidance on upgrading to 4.0, 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 4.0, you must berunning a 3.6-series release.

To upgrade from a version earlier than the 3.6-series, you mustsuccessively upgrade major releases until you have upgraded to3.6-series. For example, if you are running a 3.4-series, you mustupgrade first to 3.6before you can upgrade to 4.0.

Preparedness

Before beginning your upgrade, see the Compatibility Changes in MongoDB 4.0 document toensure that your applications and deployments are compatible withMongoDB 4.0. 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 4.0, if you need to downgrade, we recommend downgrading to the latest patch release of 3.6.

Initial Syncs

Before starting the upgrade, ensure that no initial sync is inprogress. Performing the upgrade while an initial sync is inprogress will cause the initial sync to restart.

Default Bind to Localhost

The following procedure includes the command-line option—bind_ip or the configuration optionnet.bindIp when restarting the replica set.

Starting in MongoDB 3.6, the options must be specified when the replicaset members are run on different hosts or if remote clients connect tothe deployment. Omit if all members are run on the same host and allclients are local to the host.

Warning

Before binding to a non-localhost (e.g. publicly accessible)IP address, ensure you have secured your cluster from unauthorizedaccess. For a complete list of security recommendations, seeSecurity Checklist. At minimum, considerenabling authentication andhardening network infrastructure.

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

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

Starting in 3.6.1 and MongoDB 4.0.3, you can disable read concern"majority" to prevent the storage cache pressure fromimmobilizing a deployment with a three-member replica set with aprimary-secondary-arbiter (PSA) architecture or a sharded cluster witha three-member PSA shards.

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.

For more information, see Disable Read Concern Majority.

Change Streams Resume Tokens

MongoDB 4.0 introduces new hex-encoded string change streamresume tokens:

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):

MongoDB VersionFeature Compatibility VersionResume Token _data Type
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

When upgrading from MongoDB 3.6 to MongoDB 4.0.7 or greater

When upgrading from MongoDB 3.6 to MongoDB 4.0.7 or later, a clientmay try to resume change streams using the new v1 resume tokenwhen connected to a member that has not been updated (i.e. onlyaccepts BinData resume tokens) and fail. In such cases, the clientmust wait for the upgrade to complete before resuming change streams.

After upgrading, if you later decide to downgrade to MongoDB 3.6, toresume a change stream, clients can use a pre-upgrade resume token(if available) on the 3.6 deployment. Otherwise, clients will needto start a new change stream.

Prerequisites

All Members Version

All replica set members must be running version 3.6. Toupgrade a replica set from an 3.4-series and earlier, firstupgrade all members of the replica set to the latest3.6-series release, andthen follow the procedure to upgrade from MongoDB 3.6 to4.0.

Remove Support for MONGODB-CR

Starting in version 4.0, MongoDB removes support for the deprecatedMongoDB Challenge-Response (MONGODB-CR) authentication mechanism.

If your deployment has user credentials stored in MONGODB-CRschema, you must upgrade to Salted Challenge ResponseAuthentication Mechanism (SCRAM)before youupgrade to version 4.0. For information on upgrading to SCRAM, seeUpgrade to SCRAM.

See also

Compatibility Changes in MongoDB 4.0

Remove pv0 for Replica Sets

Starting in version 4.0, MongoDB removes the deprecated replica setprotocol version 0 pv0.

Before upgrading to MongoDB 4.0, you must upgrade to pv1.

To upgrade to pv1, connect a mongo shell to thereplica set primary and perform the following sequence of operations:

  1. cfg = rs.conf();
  2. cfg.protocolVersion=1;
  3. rs.reconfig(cfg);

To reduce the likelihood of w:1 rollbacks,you can also reconfigure the replica set to a highersettings.catchUpTimeoutMillis setting.

For more information on pv1, seeReplica Set Protocol Version.

Remove Master-Slave Replication

MongoDB 4.0 removes support for the deprecated master-slavereplication. Before you can upgrade to MongoDB 4.0, if your deploymentuses master-slave replication, you must upgrade to a replica set.

To convert your master-slave replication, seeConvert a Master-Slave Deployment to a Replica Set.

Remove Support for $isolated

MongoDB drops support for the $isolated operator. If you have anexisting partial index that includes the $isolated operator or aview that includes a $isolated operator, recreate the index orview without the operator in the definition before upgrading.

Feature Compatibility Version

The 3.6 replica set must havefeatureCompatibilityVersion set to 3.6.

To ensure that all members of the replica set havefeatureCompatibilityVersion set to 3.6, connect to eachreplica set member and check the featureCompatibilityVersion:

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

All members should return a result that includes"featureCompatibilityVersion" : { "version" : "3.6" }.

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

  1. db.adminCommand( { setFeatureCompatibilityVersion: "3.6" } )

For more information, seesetFeatureCompatibilityVersion.

Replica Set Member State

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

Download 4.0 Binaries

Via Package Manager

If you installed MongoDB from the MongoDB apt, yum, dnf, orzypper repositories, you should upgrade to 4.0 using your packagemanager.

Follow the appropriate 4.0 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 4.0 installation instructions for more information.

Upgrade Process

You can upgrade from MongoDB 3.6 to 4.0 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 3.6binary with the 4.0 binary.

  • Restart the member.

Note

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 4.0 binary.

  • Restart the member.

Note

Enable backwards-incompatible 4.0 features.

At this point, you can run the 4.0 binaries without the4.0 features that are incompatible with 3.6.

To enable these 4.0 features, set the feature compatibilityversion (FCV) to 4.0.

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:

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

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.

Additional Upgrade Procedures