We run a fleet of Nest services on ECS and PM2, with Kafka for domain integration and Firebase at the API gateway. That stack only works if we agree on a few defaults: who owns data, when to use events versus HTTP, how contracts are defined, and where auth stops. This post is our engineering playbook for those defaults—not a claim that every service already matches them. Some rules describe where we want to be (for example strict one-database-per-service everywhere); others we apply today with known gaps (consumer backoff patterns still being defined).

The post has three parts: ten foundation principles , patterns for crossing domain boundaries (with Order and User as the running example), and a payment debit/credit case study on why implicit event listeners are painful and what explicit commands buy us.

1. Each microservice owns its data

Rule: one domain, one database.…