advanced

Event sourcing

Store state changes as an append-only event log when audit, replay, temporal queries, and explicit evolution justify the modeling cost.

Event sourcing persists state as an append-only sequence of domain events. Current state is derived by replaying or snapshotting. Benefits: audit trail, temporal queries, and natural integration via the same events. Costs: schema evolution, storage, and mental model complexity.

Trade-off: perfect history and replay versus operational burden and learning curve.

On interviews: contrast event sourcing with CRUD plus outbox; explain snapshots and upcasting old events.

Common pitfalls: events as integration DTOs without domain meaning; unbounded event store without compaction policy; replay without versioning strategy.

Checklist:

Model events as domain facts, versioned.
Use snapshots for long aggregates.
Plan upcasters for schema changes.
Separate internal events from public integration events.