Database migrations are one of the riskiest parts of deployment. A bad migration can take down your application or corrupt data. Here's how to migrate safely with zero downtime.
Migration Fundamentals
The Problem
Traditional deployment:
1. Stop application
2. Run migrations
3. Start application
→ Downtime during migrations
Zero-downtime requirement:
1. Run migrations (app still running)
2. Deploy new code
3. No interruption
→ Migrations must be backward compatible
Migration Tools
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Safe Migration Patterns
Expand-Contract Pattern
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Adding Columns Safely
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Renaming Columns
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Adding Indexes
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Dropping Tables/Columns
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Data Migrations
Background Jobs
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Dual Writes
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Rollback Strategies
Reversible Migrations
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Point-in-Time Recovery
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Feature Flags
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Testing Migrations
Local Testing
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Migration Tests
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Production Checklist
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Common Pitfalls
Lock Timeouts
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Large Table Alterations
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Conclusion
Safe database migrations require planning and discipline. Use the expand-contract pattern, test thoroughly, and always have a rollback plan. The extra effort prevents outages and data loss.
Remember: it's better to take three small migrations than one risky big one.