Event sourcing is a way to store data as events in an append-only log. It only keeps the latest version of the entity state. This method stores the state of a database object as a sequence of events. It is essentially a new event each time the object changed state, from the beginning of the object’s existence. An event can be anything that is generated by a user, a mouse click, a key press on a keyboard, and so on. It is a great way to atomically update the state and publish events. Not just can we query these events, but we can also use the event log to reconstruct past states, and as a foundation to automatically adjust the state to cope with retroactive changes.
Events are immutable, they cannot be changed. This well-known rule of event stores is often the first defining characteristic of event stores and event sourcing.
Read on to see how this concept works and how products like Apache Kafka make event sourcing viable.