Event Sourcing

A functional domain model is made of pure functions and immutable types. As Domain Driven Design teaches us, it should be expressed in the language shared by everyone involved in the project.

Let’s consider how functional domain model might work in practice by applying the event sourcing pattern.

Let’s implement the Mastermind game with event sourcing. For now, we’re only focusing on the domain model.

In event sourcing, the state is derived from events that have happened in the past. In a classic approach, state is persisted while events are lost. In an event-sourced system, it’s the events that are persisted while state is derived.

This time, we’re going to refactor our solution towards an object-oriented (OOP) style. The finite state machine we implemented previously enables us to take advantage of polymorphic behaviour while sticking to immutability.