Implementing Domain-Driven Design

by Vernon Vaughn

There are varying definitions of Event Sourcing, so some clarification is fitting. We are discussing the use where every operational command executed on any given Aggregate instance in the domain model will publish at least one Domain Event that describes the execution outcome. Each of the events is saved to an Event Store (8) in the order in which it occurred. When each Aggregate is retrieved from its Repository, the instance is reconstituted by playing back the Events in the order in which they previously occurred.10 In other words, first the very earliest Event is played back, and the Aggregate applies the Event to itself, modifying its state. Next, the second-oldest Event is played back in the same manner. This continues until all Events, from the oldest to the most recent, are completely played back and applied. At that point the Aggregate exists in the state it had upon the most recent execution of some command behavior.