Storing Events

Once an event has been produced, it needs to be stored permanently. This is the role of the event store.

An event store is not just a database – it's a system designed to keep an immutable, ordered history of all events.

In Our Library

Let's say a member borrows a book. The system produces a BookBorrowed event. That event is saved to the event store – typically as part of a stream.

Each stream groups all events that belong to a specific entity. In this case, the stream might be tied to a copy or a loan.

The event store guarantees:

• Append-only: events are only added, never changed or removed
• Ordering: events stay in the order they were written
• Persistence: events are stored reliably and durably

The Event Log Is the Source of Truth

With Event Sourcing, there is no separate "current state" stored in a database. Instead, the system rebuilds the state by replaying the stored events.

That means the event store is not just an audit log – it's the foundation of how your application works.

Benefits of Storing Events This Way

• You can always trace what happened and when
• You can rebuild projections or read models at any time
• You can debug or explain business behavior using historical data
• You can create new views of the data later – without changing the write logic

In short: event storage gives you flexibility, traceability, and reliability.

Next up: Publishing Events – learn how stored events are made visible to the rest of the system.