You put a lot of work into authentication. A gateway validates the Keycloak JWT, maps realm roles to authorities, checks that the caller is allowed. By the time a request reaches your service, you know exactly who is calling.
Then the request crosses into async land, and all of that evaporates.
This is the story of the point where identity quietly disappears in an event-driven system, why the dead-letter queue is the worst possible place for it to disappear, and how I made the acting user as durable and replay-safe as the event itself.
The flow everyone believes is fine
The platform is a set of Spring Boot services: an API gateway in front, a user-service on MySQL, a notification-service on PostgreSQL, and Kafka carrying events between them. A user is created, an event is published, a notification is sent.
