Master database migration questions using expand-contract patterns, dual writing, schema versioning, and zero downtime execution.
Practice This QuestionAdd new columns/tables in a backward-compatible manner without removing legacy structures.
Update application code to write to both old and new schemas; backfill historical data in batches.
Point application read paths to the new schema and verify telemetry health.
Remove legacy write paths and drop obsolete columns in a subsequent deployment.
“I execute zero-downtime schema migrations using the Expand-Contract (or Parallel Run) pattern. For example, when splitting a single `users` table column `name` into `first_name` and `last_name` across a database serving 20,000 QPS: Step 1 (Expand): We added the nullable `first_name` and `last_name` columns. Step 2 (Dual Write): We deployed application code that wrote to both old and new columns simultaneously while backfilling 10 million historical records in background batches. Step 3 (Read Switch): We flipped application reads to the new columns after verifying zero data mismatch. Step 4 (Contract): After 7 days of monitoring, we removed legacy column writes and dropped the old column.”
“I use migration tools like Flyway or Liquibase with non-blocking schema changes—such as adding indexed columns with `ALGORITHM=INPLACE, LOCK=NONE` in MySQL—to ensure queries are never locked during deploy.”
“I write additive migration scripts that don't delete active columns and test them thoroughly in staging against realistic database dumps.”
| Word | ❌ Common Error | ✅ Correct | Tip |
|---|---|---|---|
| migration | mig-ray-shun | my-GRAY-shuhn | Long 'i' sound at start ('my'). |
| backward-compatible | back-word-com-pat-ible | BAK-werd kuhm-PAT-uh-buhl | Stress on 'PAT'. |
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