Event Sourcing

Status: draft · Confidence: medium (0.635) · Basis: verified_sources

Quality notes: generic_source_homepage, no_verified_sources, partial_source_verification

## TL;DR

Event Sourcing stores state changes as a sequence of immutable events, rather than storing current state directly. The current state is derived by replaying events. Events are the source of truth — like a bank ledger (transactions), not just the current balance. Benefits: complete audit trail, temporal queries, easy debugging.

## Core Explanation

Event: `OrderPlaced {orderId, items}`, `OrderShipped {orderId, tracking}`, `OrderDelivered {orderId}`. Current state = fold/reduce over events. CQRS (Command Query Responsibility Segregation) pairs naturally with Event Sourcing: Commands write events, Queries read from denormalized projections. Event store: append-only, immutable. Snapshots: periodically save current state to avoid replaying entire history.

## Further Reading

- [Event Sourcing (Martin Fowler)](undefined)

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