Who Did This? Identity Across Async Boundaries

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. Authentication is handled at the edge. The gateway is an OAuth2 Resource Server; it validates the token once and propagates the caller's identity downstream as headers: // api-gateway — IdentityPropagationFilter (@Order(2), after security) IdentityContext identity = identityContextExtractor . extract ( jwt ); // Always set all three headers (empty when absent) to mask any spoofed values. enrichedRequest . putHeader ( IdentityHeaders . USER_NAME , nullToEmpty ( identity . username ())); enrichedRequest . putHeader ( IdentityHeaders . USER_EMAIL , nullToEmpty ( identity . email ())); enrichedRequest . putHeader ( IdentityHeaders . USER_ROLES , identity . rolesAsString ( DELIM )); One detail here matters more than it looks. The headers are always overwritten , even when a claim is absent. If a client tries to inject X-User-Name: admin on the inbound request, the gateway stomps it with the validated value (or empty). Downstream trust in those headers is only safe because the perimeter guarantees they cannot be forged. Miss that, and you've built an impersonation API. So far, so good. The synchronous hop carries identity. The problem starts one line later. The hidden f