Event Sourcing is an architectural pattern where an entity's current state is stored as an immutable sequence of events. Instead of saving "user balance is 100", you store "user received 50", "user spent 30", "user received 80".
How it works
Each state change is recorded as an immutable event in an event store. To know the current state, you replay all events for that entity in order. This process is called event replay.
Key benefits
- Complete audit trail: every change has an immutable record with who, when, and what changed.
- State reconstruction: rebuild state at any point in time.
- Flexible projections: create new data views by replaying historical events.
- Natural CQRS integration: events are the source of truth for both models.
Challenges
Complexity is the biggest challenge. Event versioning requires careful planning. State reconstruction can be slow with millions of events (solvable with snapshots). Storage grows indefinitely.
Event Store and tools
Specialized databases like EventStoreDB offer optimized event storage. PostgreSQL can also serve as an event store with well-designed tables and indexing. Kafka is popular for distributed event sourcing.
Event Sourcing is ideal for financial systems, auditing, traceability, and applications where change history is a business requirement. At Vynta, we design event-driven architectures for enterprises needing complete traceability.