Event Sourcing Pattern

Event Sourcing is a pattern where all changes to application state are stored as a sequence of events. Instead of storing just the current state, you store every event that led to that state. This enables powerful capabilities like temporal queries, complete audit trails, and the ability to replay events to reconstruct any point-in-time state.

Key Concepts

• Events: Immutable records of something that happened in the system
• Aggregates: Clusters of domain objects treated as a unit for data changes
• Event Store: Append-only storage for all events
• Projections: Read models built from events
• Snapshots: Periodic state captures for performance optimization

The architecture separates concerns between commands (write operations), events (what happened), and projections (read models). This enables independent scaling of read and write operations while maintaining eventual consistency.

Key Tables

• Events: Stores all domain events with metadata
• Aggregates: Tracks current version of each aggregate
• Projections: Denormalized read models for fast queries

Event Structure

{
  "event_id": "uuid",
  "aggregate_id": "order-123",
  "event_type": "OrderCreated",
  "event_data": {
    "customer_id": "cust-456",
    "items": [...],
    "total_amount": 99.99
  },
  "version": 1,
  "created_at": "2024-01-15T10:30:00Z",
  "correlation_id": "req-789"
}

Implementation Steps

1. Define Events: Create event classes for all domain changes
2. Implement Aggregates: Build aggregate classes that handle commands and produce events
3. Create Event Store: Implement append-only storage for events
4. Build Projections: Create read models that consume events
5. Handle Event Handlers: Implement side effects and external integrations

Code Example: Order Aggregate

class OrderAggregate {
  constructor(private events: OrderEvent[] = []) {}
  
  createOrder(command: CreateOrderCommand): OrderCreatedEvent {
    const event = new OrderCreatedEvent(
      command.orderId,
      command.customerId,
      command.items,
      command.totalAmount
    );
    
    this.events.push(event);
    return event;
  }
  
  cancelOrder(command: CancelOrderCommand): OrderCancelledEvent {
    const event = new OrderCancelledEvent(
      command.orderId,
      command.reason
    );
    
    this.events.push(event);
    return event;
  }
  
  getCurrentState(): OrderState {
    return this.events.reduce((state, event) => 
      this.applyEvent(state, event), new OrderState()
    );
  }
}

Good Use Cases

• Audit Requirements: When you need complete traceability of all changes
• Temporal Queries: When you need to query data as it existed at a specific point in time
• Complex Business Logic: When business rules are complex and change frequently
• Analytics: When you need to analyze how data changes over time
• Compliance: When regulatory requirements demand complete audit trails

Consider Alternatives When

• Simple CRUD: When your application is mostly simple create/read/update/delete operations
• Small Team: When your team lacks experience with event sourcing
• Performance Critical: When you need the fastest possible read performance
• Simple Domain: When your business domain is straightforward and unlikely to change