DocumentDB Instance restarting

The unexpected restarts of your Amazon DocumentDB instance could be due to several reasons, even if they were not manually initiated. Here are some potential causes and steps to investigate further:

1. Maintenance and updates: Amazon DocumentDB may automatically restart instances to apply critical patches or updates to the database engine or underlying infrastructure. This is part of AWS's managed service to ensure your database is running on the latest, most secure version.

2. Performance issues: If your instance experienced high resource utilization or performance bottlenecks, it might have triggered an automatic restart. This could be due to increased workload or resource contention.

3. Underlying infrastructure issues: Sometimes, problems with the underlying AWS infrastructure can cause instances to restart automatically.

4. Cluster parameter group changes: If there were any recent changes to the cluster parameter group associated with your instance, it might have required a restart to apply those changes.

To investigate the root cause further, you can take the following steps:

1. Check CloudWatch Metrics: Look at metrics like CPU utilization, memory usage, and disk I/O for the period leading up to the restarts. This might reveal any performance issues that could have triggered the restarts.

2. Review DocumentDB logs: Examine the database logs for any error messages or warnings that occurred around the time of the restarts.

3. Check for any recent parameter group changes: If there were any modifications to your cluster's parameter group, this could explain the restarts.

4. Look for any AWS notifications: Check if there were any AWS notifications about planned maintenance or issues in your region around the time of the restarts.

5. Enable Enhanced Monitoring: If not already enabled, turn on Enhanced Monitoring for more detailed performance metrics.

6. Set up CloudWatch Alarms: Create alarms to notify you of high resource utilization or other potential issues that could lead to restarts.

7. Consider Multi-AZ deployment: If you're not already using it, implementing a Multi-AZ deployment can help minimize downtime during such events.

Remember to configure your application to handle connection losses gracefully by implementing retry logic. This can help maintain application stability even if the database instance restarts unexpectedly.

If the issue persists or you need more detailed information about these specific restart events, it's advisable to contact AWS Support for further assistance.

Sources
Rebooting an Amazon DocumentDB instance - Amazon DocumentDB
RebootDBInstance - Amazon DocumentDB
Understand why Amazon RDS for Oracle DB instances automatically restart | AWS re:Post