Long-running transactions are a common but overlooked cause of rising CDC source latency in AWS DMS migrations. When a transaction stays open, DMS must buffer all subsequent changes until it commits, causing latency to climb from seconds to hours. This post explains how DMS processes transactions, how to diagnose latency spikes, and provides ready-to-use bash monitoring scripts for Oracle, PostgreSQL, MySQL, and SQL Server. The scripts detect transactions exceeding a configurable threshold (default 15 minutes), send alerts via Amazon SNS, enforce TLS/SSL connections, retrieve credentials from AWS Secrets Manager, and filter false positives. A step-by-step Oracle demo shows the full workflow from DMS task setup to alert receipt. Scheduling via crontab enables continuous proactive monitoring.
