Event sourcing stores state as a sequence of events rather than current values. Here's how to implement it effectively.
What is Event Sourcing
Traditional state storage:
- Store current state
- Update in place
- History lost
Event sourcing:
- Store events that caused state changes
- Derive current state from events
- Complete history preserved
Example:
Instead of: { balance: 150 }
Store events:
1. AccountOpened { balance: 100 }
2. MoneyDeposited { amount: 100 }
3. MoneyWithdrawn { amount: 50 }
Current state derived: { balance: 150 }
Event Definition
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Event Store
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Aggregate
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Repository
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Projections
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Snapshots
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Best Practices
Events:
✓ Make events immutable
✓ Include all necessary data
✓ Use past tense naming
✓ Version event schemas
Storage:
✓ Use append-only storage
✓ Implement optimistic concurrency
✓ Create snapshots for performance
✓ Archive old events
Projections:
✓ Build for specific queries
✓ Handle idempotency
✓ Enable rebuilding
✓ Monitor projection lag
Conclusion
Event sourcing provides complete audit history and enables temporal queries. Start with clear event definitions, implement proper aggregate handling, and build projections for read performance. The pattern works well for domains where history and audit trails are important.