What is Event Sourcing, and how do event stores differ from traditional databases?

Short answer

Instead of storing only the current state, the system records every change as an immutable historical event (Append-Only Event) and rebuilds the current state by replaying the event stream.

Explanation

In financial systems and shipment tracking, knowing that an account balance is $500 is not enough; the full history of deposits and withdrawals matters. Event Sourcing provides a complete, tamper-resistant audit trail. Libraries such as Marten (on PostgreSQL) or EventStoreDB provide native event recording and fast read projections (Read Projections).

Code example

// Immutable stored events
public record AccountCreated(Guid Id, string Owner);
public record MoneyDeposited(Guid Id, decimal Amount);
public record MoneyWithdrawn(Guid Id, decimal Amount);

Common mistakes

• Editing or deleting previous events in an Event Store; the event log is Immutable forever, and corrections should be new events.

Follow-up questions

• What is CQRS, and how does MediatR help separate reads and writes in C# projects?

Sources

• Microsoft Learn — Event Sourcing pattern