Building a Mission-Critical Integration Blind, Part I: XML, Queues, and Guesswork

No access to the external broker. No spec—just 300 pages of government PDFs. We built an async service (PHP/Symfony/PostgreSQL/RabbitMQ) that processes millions of legally significant requests. Key lessons: let Claude summarize docs without biasing it; use Doctrine transport for transactional guarantees; enums over lookup tables; audit with PostgreSQL triggers; build a Docker mock (STOMP) when you can't reach real systems. A missing XML namespace caused 2 hours of byte‑by‑byte debugging. XAdES GOST signing is hard because of canonicalization edge cases, not crypto. Shipped two weeks early, then added two new data sources in one day each.