Скачать или смотреть Troubleshooting MongoDB Sharded Cluster Balancing Issues

Encountering issues with balancing in your `MongoDB` sharded cluster? Learn how to troubleshoot and resolve the problem with our detailed guide.
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Troubleshooting MongoDB Sharded Cluster Balancing Issues

If you're working with a sharded cluster in MongoDB and find that the balancer isn't working correctly, it can be frustrating and confusing. One common issue reported by users involves an error message that prevents the balancer from functioning as expected. In this guide, we will explore the problem in detail and provide a comprehensive solution to get your balancer back on track.

Understanding the Problem

After executing the command sh.startBalancer(), users may encounter an error message that looks something like this:

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This error indicates that the balancer is trying to move config.system.sessions, but it cannot create it due to a UUID conflict with an existing collection. Essentially, the balancer is blocked because it perceives that there is already a system.sessions collection with a different UUID, which leads to an operation failure.

How to Fix the Balancing Issue

The good news is that this issue can be resolved by following a few straightforward steps. Here’s what you need to do:

Stop the Balancer
Before making any changes, it's essential to stop the balancer to prevent further complications. You can use the command:

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Create a Temporary User
Create a temporary user with the __system role on all affected shard's primary nodes. This allows you to have the necessary permissions to make changes to the config database.

Log In to Each Shard
Use the temporary user account you created to log in to every affected shard.

Drop the System Sessions Collection
Executing the following command will drop the system.sessions collection from the config database on each shard:

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Restart the Balancer
Once all instances of the system.sessions collection have been removed, you can start the balancer again:

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After completing these steps, MongoDB will automatically recreate the system.sessions collections across shards with the same UUID. This synchronization will allow the balancing process to continue as intended without any further issues.

Conclusion

Dealing with MongoDB sharded cluster balancing issues can be challenging, especially when faced with UUID conflicts. However, by following the steps outlined in this guide, you can effectively troubleshoot and resolve the problem. Remember to stop the balancer first, create a temporary user, drop the conflicting collection, and finally restart the balancer. With these actions, you’ll have your sharded cluster operating smoothly and efficiently again.

If you continue to face similar issues or have further questions, don't hesitate to reach out to the community or consult the official MongoDB documentation for additional resources and support.