分布式事务：参考架构
领域变更和发件箱行共享一个数据库事务；中继以至少一次的方式发布；消费者是幂等的；禁止在没有发件箱的情况下独立写入数据库和代理。
Events carry a schema identifier; additive optional fields are the default evolution; rename is a breaking change; consumers ignore unknown fields; compatibility is verified against a registry in CI.