Upgrade a Sharded Cluster to 3.4¶

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Package Updates required on Ubuntu 16.04 for IBM POWER Systems

Due to a lock elision bug present in older versions of the glibc package on Ubuntu 16.04 for POWER, you must upgrade the glibc package to at least glibc 2.23-0ubuntu5 before running MongoDB. Systems with older versions of the glibc package will experience database server crashes and misbehavior due to random memory corruption, and are unsuitable for production deployments of MongoDB

Important

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

If you need guidance on upgrading to 3.4, MongoDB offers major version upgrade services 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 3.4, you must be running a 3.2-series release.

To upgrade from a version earlier than the 3.2-series, you must successively upgrade major releases until you have upgraded to 3.2-series. For example, if you are running a 3.0-series, you must upgrade first to 3.2 before you can upgrade to 3.4.

Preparedness¶

Before beginning your upgrade, see the Compatibility Changes in MongoDB 3.4 document to ensure that your applications and deployments are compatible with MongoDB 3.4. 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 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.

Avoid reconfiguring replica sets that contain members of different MongoDB versions as validation rules may differ across MongoDB versions.

`mongos` and Earlier Versions of `mongod` Instances¶

Version 3.4 mongos instances cannot connect to earlier versions of mongod instances.

The 3.2 and earlier mongo shell is not compatible with 3.4 clusters.

Prerequisites¶

Version 3.2 or Greater

To upgrade a sharded cluster to 3.4, all members of the cluster must be at least version 3.2. The upgrade process checks all components of the cluster and will produce warnings if any component is running version earlier than 3.2.
Config Server as Replica Set (CSRS)

Starting in 3.4, the use of the deprecated mirrored mongod instances as config servers (SCCC) is no longer supported. Before you can upgrade your sharded clusters to 3.4, you must convert your config servers from SCCC to a replica set (CSRS).

To convert your config servers from SCCC to CSRS, see Upgrade Config Servers to Replica Set.
Avoid Metadata Changes during the Upgrade
Optional but Recommended. During the upgrade, if possible, avoid making changes to the collection metadata; for example, avoid performing any of the following operations:
- sh.enableSharding()
- sh.shardCollection()
- sh.addShard()
- db.createCollection()
- db.collection.drop()
- db.dropDatabase()
- any operation that creates a database
- any other operation that modifies the cluster metadata in any way.
See the Sharding Reference for a complete list of sharding commands. Not all commands on the Sharding Reference page modify the cluster metadata.
Disable the balancer
Back up the config Database

Optional but Recommended. As a precaution, take a backup of the config database before upgrading the sharded cluster.

Download 3.4 Binaries¶

Use Package Manager¶

If you installed MongoDB from the MongoDB apt, yum, dnf, or zypper repositories, you should upgrade to 3.4 using your package manager.

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

Download 3.4 Binaries Manually¶

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

See Install MongoDB for more information.

Upgrade Process¶

1

Disable the Balancer.¶

Disable the balancer as described in Disable the Balancer.

2

Upgrade the config servers.¶

If the config servers are replica sets:

Upgrade the secondary members of the replica set one at a time:
1. Shut down the secondary mongod instance and replace the 3.2 binary with the 3.4 binary.
2. Start the 3.4 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 upgrading the next secondary member. To check the member’s state, issue rs.status() in the mongo shell.
  
  Repeat for each secondary member.
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 3.4 binary.
3. Start the 3.4 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

Upgrade the shards.¶

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

Upgrade the secondary members of the replica set one at a time:
1. Shut down the mongod instance and replace the 3.2 binary with the 3.4 binary.
2. Start the 3.4 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 upgrading the next secondary member. To check the member’s state, you can issue rs.status() in the mongo shell.
  
  Repeat for each secondary member.
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()
```
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 3.4 binary.
2. Start the 3.4 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 specify sharding.clusterRole: shardsvr and net.port and start the 3.4 binary:
```
sharding:
   clusterRole: shardsvr
net:
   port: <port>
storage:
   dbpath: <path>
```
  Include any other configuration as appropriate for your deployment.

4

Upgrade the `mongos` instances.¶

Replace each mongos instance with the 3.4 binary and restart. Include any other configuration as appropriate for your deployment.

Note

In 3.4, mongos no longer supports --chunkSize and --noAutoSplit runtime options (or the corresponding sharding.chunkSize and sharding.autoSplit settings). If your 3.2 mongos configuration includes these settings, remove the settings when running the 3.4 mongos.

mongos --configdb csReplSet/<rsconfigsver1:port1>,<rsconfigsver2:port2>,<rsconfigsver3:port3>

5

Re-enable the balancer.¶

Using a 3.4 mongo shell, re-enable the balancer as described in Enable the Balancer.

The 3.2 and earlier mongo shell is not compatible with 3.4 clusters.

6

Enable backwards-incompatible 3.4 features.¶

At this point, you can run the 3.4 binaries without the 3.4 features that are incompatible with 3.2.

To enable these 3.4 features, set the feature compatibility version to 3.4.

Warning

Enabling these backwards-incompatible features can complicate the downgrade process. For details, see Remove 3.4 Incompatible Features.

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.

On a mongos instance, run the setFeatureCompatibilityVersion command in the admin database:

db.adminCommand( { setFeatureCompatibilityVersion: "3.4" } )

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 as the operation is idempotent.

Additional Upgrade Procedures¶

To upgrade a standalone, see Upgrade a Standalone to 3.4.
To upgrade a replica set, see Upgrade a Replica Set to 3.4.

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