The Event Sourcing pattern is a software design pattern for Microservice architecture that involves storing the complete history of an application's state as a sequence of events. Each event represents a change in the application's state, such as the creation of a new record or the modification of an existing one.

Event sourcing persists the state of a business entity such an Order or a Customer as a sequence of state-changing events. Whenever the state of a business entity changes, a new event is appended to the list of events. Since saving an event is a single operation, it is inherently atomic.

Event Sourcing is a different way of structuring the business logic and persisting aggregates. It persists an aggregate as a sequence of events and each event represents a state change of the aggregate. By doing this, it helps to preserve the history of aggregates, which is valuable for auditing and regulatory purposes.

The event sourcing design pattern creates events regarding the changes in the application state. Also, these events are stored as a sequence of events to help the developers track which change was made when. So, with the help of this, you can always adjust the application state to cope up with the past changes.

In the event sourcing pattern, the application code sends a series of events that crucially describes each action that has taken place on the data to the event store. The events are persisted in an event store that acts as the record system of the current data state.