How would you implement the Event Sourcing pattern?

The Event Sourcing pattern involves storing all changes to the system's state as a sequence of events, rather than just the current state. To implement:

1. All user or system actions are recorded as events (e.g., creation, modification, deletion).
2. These events are stored in an immutable log (event store).
3. The current state is reconstructed by sequentially applying all events to the initial state.

This allows:

• Tracking the history of changes.
• Reproducing the state at any point in time.
• Implementing auditing and rollback.

Example:

• A user created an order — event OrderCreated.
• Changed the address — event AddressChanged.
• The order was paid — event OrderPaid.

To implement, you need to:

• Design the structure of events.
• Organize reliable storage of events.
• Implement a mechanism for reconstructing state from events.

It is also important to handle event versioning and migration when the model changes.