Hierarchical IoT pub/sub broker

Design a hierarchical pub/sub broker for an IoT fleet: ~10 million devices publishing to topics like region/building/floor/device/metric, and operator dashboards subscribing with wildcards (region/+/+/+/temperature or region/#). It must support millions of concurrent subscriptions, route a published message to all matching subscribers within ~200ms, and survive a broker node loss without losing in-flight messages for QoS-1 subscribers. Cover topic matching, subscription state, and scale-out.