Decoupling a Live App with Domain Events (Part 2)

In Part 1 of this series, we introduced RabbitMQ and built our proof-of-concept: the EventBusService, RabbitMQProvider, a DLX retry pattern, and our first emitter (CommentService.createComment()). By the end of Phase 0, we had one event running reliably in production behind a feature flag. For Phase 1, we applied this pattern to all Tier 1 services. 14 Zod schemas, 25 queue handlers across 3 consumer classes were built, and emit sites were added to CommentService, RecordService, and OccurrenceCrudHelper. We also added 79 new tests. Here is how we executed Phase 1, the design choices we made, and a tricky bug that changed how we guard event bus access. The Dual-Write Strategy Our top priority for Phase 1 was safety. Since IHA has active users, we couldn't risk breaking the app. Switching entirely from direct service calls to event emissions wasn't safe yet. Instead, we used a dual-write strategy. Services now emit events and run their existing inline side effects. If the event broker fa