Question: How do you upgrade a MongoDB replica set?

Answer

Upgrading a MongoDB replica set involves several steps to ensure that the process is smooth and does not cause downtime. Below are the steps to upgrade a MongoDB replica set:

1. Review the Release Notes: Before starting, review the release notes and the compatibility changes for the version you are planning to upgrade to. This helps in identifying any deprecated features or configuration changes required.

2. Back Up Your Data: Always back up your data before starting the upgrade process. This ensures that you have a recovery point in case something goes wrong during the upgrade.

3. Upgrade Secondary Nodes First: Start the upgrade with the secondary nodes of the replica set. To do this, you can follow these general steps for each secondary node:

   • Step Down as Secondary (if necessary): If you are upgrading the primary node, first step it down to make another node the primary. You can use the rs.stepDown() command for this.
   • Stop the MongoDB service on the secondary node.
   • Upgrade the MongoDB binaries to the new version.
   • Restart the MongoDB service.
   • Verify the upgrade by connecting to the MongoDB shell and checking the version using db.version().

4. Monitor the Replica Set: After upgrading each secondary, monitor the replica set status using rs.status() to ensure that the node rejoins the replica set and catches up with any changes it missed during the upgrade.

5. Upgrade the Primary Node: After all secondary nodes have been upgraded and are fully synchronized,

   • Step down the primary node using rs.stepDown() to force an election and transition it to a secondary role.
   • Follow the same upgrade process as for the secondary nodes.
   • The replica set will automatically elect a new primary among the upgraded secondaries.

6. Verify the Upgrade: Once all members have been upgraded, connect to the MongoDB shell and run rs.status() to ensure all members are online and the replica set is functioning as expected. Also, verify the application's functionality to ensure compatibility with the upgraded MongoDB version.

7. Upgrade MongoDB Drivers if Necessary: Depending on the version upgrade, you may also need to upgrade your MongoDB drivers in your application to ensure compatibility.

Example:

The specific commands and operations might vary based on your operating system and MongoDB version, but the overall process remains consistent. For example, stopping the MongoDB service on a Linux system typically involves a command like sudo systemctl stop mongod, and starting it again with sudo systemctl start mongod.

# Example command to step down the primary
mongo --eval 'rs.stepDown()'

# Sample commands to stop and start MongoDB on a Linux system
sudo systemctl stop mongod
sudo systemctl start mongod

# Check MongoDB version
mongo --eval 'db.version()'

Conclusion

Upgrading a MongoDB replica set requires careful planning and execution. By following these steps, you can minimize downtime and ensure a smooth transition to the new MongoDB version.