“Perform state changes only by applying events. Make them durable by storing the events in a log.”
Looking at the Domain Object pattern example, you can see that all state changes the manager actor performs are coupled to an event that is sent back to the client that requested this change. Because these events contain the full history of how the state of the domain object evolved, you may as well use it for the purpose of making the state changes persistent—this, in turn, makes the state of the domain object persistent. This pattern was described in 2005 by Martin Fowler and picked up by Microsoft Research, and it has shaped the design of the Akka Persistence module.