Applying the event-first variant of the Event Sourcing pattern

Event sourcing is a key pattern for designing eventually consistent cloud-native systems. Upstream services produce events as their state changes, and downstream services consume these events and produce their own events as needed. This results in a chain of events whereby services collaborate to produce a business process that results in an eventual consistency solution. Each step in this chain must be implemented as an atomic unit of work. Cloud-native systems do not support distributed transactions, because they do not scale horizontally in a cost-effective manner. Therefore, each step must update one, and only one, system. If multiple systems must be updated, then each is updated in a series of steps. In this recipe, we leverage the event-first variant of the Event Sourcing pattern where the atomic unit of work is writing to the event stream. The ultimate persistence of the data is delegated to downstream components.