Senior LevelSQL & MySQL
How do you execute zero-downtime database migrations during blue-green application deployments?
❓ Interview Prompt: Explain Zero-Downtime database migration pipelines, multi-phase additive schema changes, backwards compatibility, and shadow column backfilling.
Deploying new code with database schema changes without taking system downtime requires multi-step additive migration pipelines:
Zero-Downtime Rule:
Schema modifications MUST be backwards-compatible with both the old app (Blue) and new app (Green) simultaneously:
1
Step 1 (Additive Change): Add new columns/tables as NULLable or with DEFAULT values. Deploy Green code.
2
Step 2 (Data Backfill): Run asynchronous background workers to populate new column data.
3
Step 3 (Cleanup): Drop legacy unused columns in a subsequent release after Green is fully active.
Code Implementation Example
sql
-- Step 1: Backwards-Compatible Additive Schema Migration
ALTER TABLE users
ADD COLUMN is_verified TINYINT(1) DEFAULT 0 NOT NULL;
-- Both Blue and Green application versions execute safely!💡Executing additive, backwards-compatible schema migrations.
🎯 Interview Key Takeaway
Perform zero-downtime database migrations by using additive, backwards-compatible schema changes.
#SQL#Zero Downtime#Schema Migration#Blue-Green#Senior
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