Migration Execution, Cutover, and Operations
This domain tests the runbook discipline that turns a designed migration into a controlled event and a stable operating model. The exam focuses on cutover sequencing, validation, rollback, and day-2 operations.
Core rules
- Write a cutover runbook with owners, timestamps, pre-checks, freeze window, execution steps, validation, and rollback triggers.
- Lower DNS TTLs before cutover; keep them low through the rollback window.
- Freeze changes on source and target during final sync. Capture row counts, checksums, and application smoke tests.
- Cut over in dependency order: database, then middleware, then application, then integrations. Validate each layer before the next.
- Define go/no-go criteria and a hard rollback deadline. A rollback that starts too late is not a rollback.
- After cutover, update monitoring, backups, patching, and DR. Use OCI Full Stack Disaster Recovery or equivalent runbooks for failover testing.
Worked example
A payroll system cutover is planned for a weekend. The runbook says "migrate DB, then app, then test." That is insufficient. A proper runbook lowers DNS TTL to 60 seconds on Thursday, freezes source changes at 20:00, runs final GoldenGate sync and validation, switches the database at 22:00, starts the app tier at 22:30, runs payroll smoke tests at 23:00, and holds a rollback decision at 23:30. If validation fails, DNS points back to source and replication resumes. The reasoning is that each step has an observable pass/fail and a bounded rollback.
Traps
- No rollback trigger or deadline.
- Forgetting DNS TTL and cached clients.
- Skipping source freeze, causing divergence.
- Treating cutover as the end; not updating backups, monitoring, and DR.