What is NumUndoRowOperations RDS metric?

In Short: As to my understanding, even though this metric is currently missing from the explicit CloudWatch list in the Aurora User Guide, NumUndoRowOperations measures the number of row operations being processed inside the InnoDB Undo Logs (Rollback Segments).

In Aurora MySQL 3.x (MySQL 8.0), this metric tracks two critical database behaviors:

1. Active Transaction Rollbacks: When a heavy DML transaction (INSERT/UPDATE/DELETE) fails or is explicitly rolled back, the InnoDB engine must undo those changes. This metric reflects the volume of rows currently being reverted using the undo logs.
2. MVCC Generation (Multi-Version Concurrency Control): If you have long-running SELECT queries executing while concurrent write transactions are modifying the same data, Aurora has to reconstruct older versions of those rows from the undo logs to guarantee read consistency.

Why you should monitor it:

A sustained spike or high value in NumUndoRowOperations directly correlates with an increasing History List Length (HLL). If you notice performance degradation or high CPU usage along with this metric, it indicates that your database is spending significant cycles processing or purging old row versions to maintain isolation levels.