Event sourcing is a design pattern in which the system state is captured as a sequence of events. Every change to the system state is captured as an event. This sequence of events can be used to reconstruct the state of the entity/entire system at any point in time. Key Concepts of Event Sourcing 1. Event: An event is a record of something that has happened in the system. For example, in an e-commerce system, events could be "Order Placed", "Order Shipped", "Payment Received", etc. 2.