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**Event sourcing** is an approach to data storage in which the system stores **not the current state of an object, but the sequence of all events** that led to that state.

### The essence

Instead of the usual "overwrite the data" model, the system records every change as an event:

> "Order created", "Item added", "Order paid".

The current state can be obtained by replaying all the events in order.

### Example

A regular database stores:

```javascript
Order: {id: 1, status: "paid"}
```

Event sourcing stores events:

```javascript
1. OrderCreated
2. ItemAdded
3. PaymentReceived
```

The "paid" state is computed when needed.

### Why this is needed

1. **Full history of changes**: you can precisely reconstruct what happened and when.
2. **Audit and tracing**: suits financial, logistics, and legal systems.
3. **Easy to react to events**: other services can subscribe to them.
4. **Deferred computation**: you can recompute the state when the logic changes.

### Drawbacks

- Harder to implement: requires replaying events and building state projections.
- Data grows faster.
- Not always convenient for simple CRUD applications.

**Summary:**

> **Event sourcing** stores the history of changes, not the result.
> This makes the system transparent, reproducible, and well suited to event-driven architecture,
> but it requires carefully managing the event stream and the storage.

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