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What is WAL (write-ahead logging), and how does it support durability and replication?
Databases & Caching · Intermediate level
Answer
WAL, or write-ahead logging, records changes durably before data pages are considered committed. It enables crash recovery, replication, and point-in-time recovery because the database can replay committed changes.
Technical explanation
WAL/binlog volume affects storage, backup, replica lag, and restore behavior.
Near-zero downtime migration requires initial load, CDC, validation, short write drain, cutover, and rollback planning.
DMS helps move data, but schema conversion, application compatibility, and data validation remain engineering responsibilities.
Hands-on example
Migration runbook:
1. Create target database and schema.
2. Start full-load plus CDC using DMS or logical replication.
3. Monitor lag and task errors.
4. Compare row counts and critical aggregates.
5. Stop writes briefly.
6. Wait for lag to reach zero.
7. Switch endpoint/secret.
8. Monitor app errors and DB load.
9. Keep source read-only until rollback window closes.
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