Schema Evolution

Overview

Schema evolution is the process of changing data schemas over time while maintaining compatibility with existing systems and data. It's crucial for long-running distributed systems that need to evolve without breaking existing clients or corrupting stored data.

Types of Compatibility

Forward Compatibility

New schema can read data written with old schema. This allows deploying new code before updating all data producers.

Backward Compatibility

Old schema can read data written with new schema. This allows gradual rollout of schema changes without breaking existing consumers.

Full Compatibility

Both forward and backward compatibility. Provides maximum flexibility but is most restrictive on allowed changes.

Safe Schema Changes

Always Safe

• Adding optional fields with default values
• Removing optional fields
• Adding new enum values (at the end)
• Renaming fields (if using field IDs)

Sometimes Safe

• Changing field types (with compatible types)
• Making optional fields required (if default exists)
• Changing default values

Never Safe

• Removing required fields
• Changing field types incompatibly
• Reordering fields (in some formats)
• Renaming fields (without aliases)

Evolution Strategies

Versioned Schemas

• Maintain multiple schema versions simultaneously
• Route data based on schema version
• Gradual migration between versions

Schema Registry

• Centralized schema management
• Compatibility checking before deployment
• Version tracking and governance

Feature Flags

• Toggle new schema features on/off
• A/B testing with different schemas
• Safe rollback capabilities

Best Practices

1. Plan for Evolution: Design schemas with future changes in mind
2. Use Optional Fields: Make new fields optional with sensible defaults
3. Avoid Breaking Changes: Prefer additive changes over modifications
4. Test Compatibility: Validate changes against existing data
5. Document Changes: Maintain clear change logs and migration guides

Schema evolution is essential for maintaining system reliability while enabling continuous improvement and feature development.