Event Sourcing

Most services store current state by updating rows in place. Event sourcing stores every change as an immutable event and derives current state by replaying those events.

Instead of storing a bank balance directly, the system stores events like money deposited, money withdrawn, and fee charged. The balance is computed from that event log.

This chapter covers append-only logs, replay, snapshots, projections, eventual consistency, and when audit and replay benefits justify the added complexity.