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What is the Event Sourcing Pattern? | Designing Event-Driven Microservices

Event Sourcing is a pattern of storing an object's state as a series of events. Each time the object is updated a new event is written to an append-only log. When the object is loaded from the database, the events are replayed in order, reapplying the necessary changes. The benefit of this approach is that it stores a full history of the object. This can be valuable for debugging, auditing, building new models, and a variety of other situations. It is also a technique that can be used to solve the dual-write problem when working with event-driven architectures.

What is the Transactional Outbox Pattern? | Designing Event-Driven Microservices

The transactional outbox pattern leverages database transactions to update a microservice's state and an outbox table. Events in the outbox will be sent to an external messaging platform such as Apache Kafka. This technique is used to overcome the dual-write problem which occurs when you have to write data to two separate systems such as a database and Apache Kafka. The database transactions can be used to ensure atomic writes between the two tables. From there, a separate process can consume the outbox and update the external system as required.

What is the Dual Write Problem? | Designing Event-Driven Microservices

The dual write problem occurs when you try to write to two separate systems and need them to be atomic. If one write fails, and the other succeeds, you can end up with inconsistent state. This is an easy trap to fall into, and it can be difficult to avoid. We'll explore what causes the dual-write problem and explore both valid and invalid solutions to it.