Memory hook: Assess, copy, catch up, switch, verify.
Must remember
- Inventory engines, versions, schema, extensions, collation, stored code, dependencies and operational requirements. Homogeneous moves differ from heterogeneous moves requiring DDL/DML or application conversion.
- Offline export/import accepts downtime; continuous replication/CDC reduces the final outage for supported source-target combinations. Database Migration Service supports specific paths, not every possible engine conversion.
- Establish secure connectivity and permissions, take the initial copy, then catch up changes. Validate row counts, checksums where appropriate, constraints, sequences, queries and performance.
- Cutover usually requires stopping or controlling source writes, confirming acceptable lag, redirecting applications and checking business transactions. Coordinate caches, connection pools, DNS and scheduled jobs.
- Define a go/no-go threshold and fallback before migration. Returning to the source after new target writes requires reconciliation or supported reverse replication; merely changing DNS can lose data.
- Keep source retention, backups, monitoring and rollback responsibility explicit until acceptance. Remove temporary access only after the agreed recovery window.
Choose under exam pressure
| Requirement | Choice and reason |
|---|---|
| A small database can tolerate a maintenance outage | Tested export/import may be simpler than continuous replication. |
| Minimal downtime is essential | A supported initial-copy-plus-CDC migration with rehearsed cutover. |
Traps
- Schema conversion does not prove application behavior is compatible.
- Near-zero downtime is not zero planning or zero risk.
Active recall
1. Why stop source writes at cutover?
To establish a consistent final replication boundary and avoid divergent writers.
2. What must be checked besides row counts?
Data values, constraints, code behavior, performance and application transactions.
3. Why plan reverse replication early?
Support and topology constraints may make it impossible to add safely after cutover.
4. What can cached DNS or pooled connections do?
Keep applications sending traffic to the old database after a configuration change.
5. When may the old source be removed?
After validation and the agreed rollback/retention period, with recovery evidence.