Mnesia, an Erlang distributed database, serves as an embed- ded storage and replication layer for OTP applications re- quiring low read latency and high availability. EMQX is a publish-subscribe message broker supporting the MQTT pro- tocol that uses Mnesia to replicate its internal state across the cluster. We analyze the limitations of Mnesia’s replica- tion protocol scalability in large clusters under high load. To address these limitations, we developed Mria, an extension to the Mnesia database that provides eventual consistency within a cluster and achieves better horizontal scalability. We validated Mria using a variety of testing techniques, in- cluding model checking, chaos engineering, and formal veri- fication. Replacing Mnesia with Mria allowed us to scale the EMQX cluster to 23 nodes, handle 100 million simultaneous client sessions and achieve a higher sustained load.