There’s a number of great introductory articles, so this is going to be a very brief introduction. With event sourcing, instead of storing the “current” state of the entities that are used in our system, we store a stream of events that relate to these entities. Each event is a fact, it describes a state change that occurred to the entity (past tense!). As we all know, facts are indisputable and immutable. For example, suppose we had an application that saved a customer’s details. If we took an event sourcing approach, we would store every change made to that customer’s information as a stream, with the current state derived from a composition of the changes, much like a version control system does. Each individual change record in that stream would be an immutable, indisputable fact.
Having a stream of such events, it’s possible to find out what’s the current state of an entity by folding all events relating to that entity; note, however, that it’s not possible the other way round — when storing the current state only, we discard a lot of valuable historical information.
Event sourcing can peacefully co-exist with more traditional ways of storing state. A system typically handles a number of entity types (e.g. users, orders, products, …), and it’s quite possible that event sourcing is beneficial for only some of them. It’s important to remember that it’s not an all-or-nothing choice, but an additional possibility when it comes to choosing how state is managed in our application.
It’s a helpful article and works hand in hand with a CQRS pattern.