Event Sourcing
Practice this topic in a realistic system design interview
Most systems store only the latest state. If a user changes their email, the old email is replaced. If money is withdrawn from an account, the balance column is overwritten with the new number. That is simple and often exactly right.
The problem appears when history matters. Why is this balance what it is? What happened to this account last week? If you only store the latest value, those answers are gone unless you built a separate audit trail.
Event sourcing keeps the history as the main record. Instead of overwriting state, the system stores every change as an event and calculates the current state from those events.
This chapter covers what event sourcing is, how systems reconstruct their current state from a history of events, how snapshots and projections make this efficient, and the benefits and trade-offs of this approach.