Event sourcing: store state as an append-only log of domain events

Traditional CRUD overwrites state, losing history. Audits, debugging production incidents, and replaying business processes are painful or impossible when you only have the current state.

Instead of storing the current state, store every state-changing event. The current state is derived by replaying events. Use snapshots for performance when the event log grows long.

class OrderAggregate {
  private events: DomainEvent[] = [];

  place(items: Item[]): void {
    this.apply(new OrderPlaced({ items, timestamp: new Date() }));
  }

  private apply(event: DomainEvent): void {
    this.events.push(event);
    this.mutate(event);
  }

  static rehydrate(events: DomainEvent[]): OrderAggregate {
    const agg = new OrderAggregate();
    events.forEach(e => agg.mutate(e));
    return agg;
  }
}

The event log is a complete audit trail. You can replay events to rebuild state, project into new read models, or time-travel debug. The append-only log is also naturally durable and scalable.

Systems requiring full audit trails, temporal queries, or multiple read projections from the same data