If application migration is the main course of a cloud project, database migration is the dessert that nobody looks forward to. Databases hold the most critical business data, are the most tightly coupled component in most architectures, and have the least tolerance for error. A botched application migration is recoverable. A botched database migration can be catastrophic.
The first principle of database migration is to choose your target before starting. The choice between lift-and-shift (migrating the same database engine to a cloud VM), managed service (moving to a cloud-managed version of the same engine — RDS, Cloud SQL, Azure Database), and re-platforming (moving to a cloud-native alternative — Aurora, Spanner, Cosmos DB) has profound implications for effort, cost, and capability. Each step up the spectrum requires more engineering work and risk management but delivers more long-term benefit.
Schema assessment is the most underestimated pre-migration task. Every database has accumulated technical debt: deprecated columns still referenced in forgotten stored procedures, implicit data type conversions that work in one database engine and fail in another, undocumented constraints enforced at the application layer. A thorough schema assessment using automated tools (AWS Schema Conversion Tool, Flyway) reveals the work ahead before the migration clock starts.
Change Data Capture (CDC) is the recommended migration technique for databases that cannot afford extended downtime. CDC tools like Debezium stream every change from the source database to the target in real time, allowing you to build and validate the target database while the source continues operating. Cutover becomes a matter of draining the CDC queue and redirecting traffic — a window measured in minutes rather than hours.
Post-migration validation is as important as the migration itself. Run parallel query execution against both databases, compare results, and monitor application performance for at least two weeks after cutover before decommissioning the source.