Conclusion

This chapter covered one change of habit. Instead of storing the current state and overwriting it, you store every change as an event. An event is a small immutable record of one fact, named in the past tense. The current state is not stored at all. It is computed by replaying the events in order.

That is the whole pattern. Everything else in the chapter followed from it.

What the Pattern Gives You

The event history is the source of truth, and state is derived from it. That single change gives you four things.

• A complete history