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🧩 Event Sourcing

βœ… Overview​

Style obtaining current state by accumulating "events that changed state" instead of state itself, and replaying them.

βœ… Problems Addressed​

  • Complete retention of update history (Audit / Reproduction).
  • Detection of conflict and ensuring consistency with asynchronous processing.
  • Making it easy to grasp meaning of state change (why it became this state) later.

βœ… Basic Philosophy & Rules​

  • All change operations are saved as events.
  • Current State = Replay result of past events.
  • Often uses Event Store (DB dedicated to events).

Conceptual Diagram​

Event Sourcing Architecture diagram

Source: Microsoft, β€œEvent Sourcing pattern – Azure Architecture Center”.
https://learn.microsoft.com/en-us/azure/architecture/patterns/event-sourcing

βœ… Suitable Applications​

  • Areas where history management is essential like Finance / Accounting.
  • Domains compatible with DDD Aggregate.
  • Systems requiring automatic reprocessing / recalculation.

❌ Unsuitable Cases​

  • Simple CRUD where necessity of history retention is thin.
  • Cases where event volume is too large and replay cost becomes a problem.

βœ… History​

  • Influenced event-centric calculation models like Actor Model.
  • Combination with CQRS is common.

βœ… Related Styles​

  • CQRS: Easily used as model on write side.
  • EDA: Naturally integrated with event-driven systems.

βœ… Representative Frameworks​

  • EventStoreDB
    Most widely used Event Store dedicated to Event Sourcing.

  • Kafka / Kinesis / Pub/Sub
    Implementation accumulating events as Append-only log and performing state reconstruction by Replay is possible.

  • Axon Framework (Java)
    Easy to implement integrating Event Sourcing and CQRS.

  • Temporal / Cadence
    Supports long-term event history management and workflow replay.

βœ… Design Patterns Supporting This Style​

  • Memento
    Used for saving / restoring snapshots.

  • Command
    Treats event as operation object generating it.

  • Observer
    Structure where event subscriber receives state change as notification.

  • Iterator
    Used for sequential replay of Event Stream.

  • State
    Aggregate updates internal state applying events.

βœ… Summary​

Event Sourcing is an architecture realizing meaningful state management by treating history as first-class data.