What is Database Migration?

Database migration is the process of modifying database schemas and transforming existing data to new structures. Migrations enable applications to evolve database structure over time whilst preserving data integrity and enabling rollback if issues arise.

Migration Scenarios

Adding Columns

Adding new columns to tables:

ALTER TABLE users ADD COLUMN phone_number VARCHAR(20);

Straightforward migrations adding columns with default values for existing rows.

Removing Columns

Removing unused columns:

ALTER TABLE users DROP COLUMN obsolete_field;

Care required if removing columns breaks existing code.

Renaming Columns

Renaming columns for clarity:

ALTER TABLE users RENAME COLUMN user_email TO email;

Requires code changes to use new column names.

Adding Indices

Creating indices for performance:

CREATE INDEX idx_user_email ON users(email);

Improves query performance at cost of increased disk space and write overhead.

Data Transformations

Transforming existing data:

UPDATE users SET email = LOWER(email);
UPDATE orders SET status = 'completed' WHERE status = 'done';

Complex transformations may require custom scripts.

Migration Challenges

Zero-Downtime Migrations

Production migrations must not cause service disruption:

  • Deploy code supporting both old and new schema
  • Migrate data during low-traffic periods
  • Monitor closely for issues
  • Enable rapid rollback if problems occur

Backward Compatibility

Code must work with both old and new schema during migration:

// Handle both old and new column names
const email = row.new_email || row.old_email;

Data Consistency

Transforming large datasets may take significant time:

  • Perform migrations during maintenance windows
  • Large migrations should not lock tables
  • Monitor progress and rollback if necessary

Rollback Capability

Migrations must be reversible enabling rollback if problems occur:

// Up migration
ALTER TABLE users ADD COLUMN phone_number VARCHAR(20);
// Down migration
ALTER TABLE users DROP COLUMN phone_number;

Irreversible operations (dropping data) complicate rollback.

Migration Best Practices

Version Control

Migrations should be versioned and tracked:

  • Each migration has unique identifier (timestamp or sequence)
  • Migrations are stored in repository
  • Migration history enables rollback to any version

Automate Migrations

Manual migrations are error-prone:

  • Migration tools automate execution
  • Versioning prevents duplicate execution
  • Automated rollback reverses migrations

Test Migrations

Migrations should be tested:

  • Test against production data copies
  • Verify performance impact
  • Test rollback procedures
  • Validate data integrity after migration

Minimal Locking

Large migrations should not lock tables:

  • Use tools enabling lock-free migrations
  • Perform data transformation in background
  • Minimise time tables are locked

Monitoring

Production migrations require close monitoring:

  • Monitor query performance
  • Watch error rates
  • Track migration progress
  • Enable rapid rollback if issues occur

Migration Tools

Liquibase

Language-independent migration management supporting multiple databases.

Flyway

Lightweight database migration tool emphasising simplicity.

Alembic

Python-based database migration tool for SQLAlchemy.

Custom Scripts

Custom scripts provide ultimate flexibility but less automation.

PixelForce Migration Practices

PixelForce manages database migrations carefully. Migrations are versioned, automated, and tested. Zero-downtime migrations minimise service disruption. Comprehensive monitoring enables rapid issue detection.

Migration Strategies

Big Bang Migration

All changes happen simultaneously. Simple but risky for large migrations.

Staged Migration

Migration occurs in phases:

  • Phase 1: Deploy supporting code
  • Phase 2: Create new schema in parallel
  • Phase 3: Migrate data gradually
  • Phase 4: Switch to new schema
  • Phase 5: Remove old schema

Staged migrations reduce risk by isolating changes.

Shadow Traffic

Route subset of traffic to new schema before full migration:

  • New code processes requests against both old and new schema
  • Compare results verifying consistency
  • Full migration only after verification
  • Catches compatibility issues before widespread impact

Database migrations are complex operations requiring careful planning and execution. Well-managed migrations enable safe schema evolution.