Event Sourcing Pattern

Stateless append-only store

Every change in state is captured as an event object

A system of record : the event store as the source of truth

Pub / Sub : Events are published by the store and consumed / processed by subscribers

Time travelling : the history of events can be played at anytime

Application state can be stored using snapshots

Considerations

Eventual consistency of data

Events must be reversible through compensating events

Make sure the event is only played once

Replay mode: Wrapping external system interactions with a gateway to avoid sending out messages multiple times