TBMQ is an open-source MQTT message broker that is capable of handling over 4 million concurrent client connections. It supports high throughput, with a minimum of 3 million messages per second throughput per single cluster node, and low latency delivery. In cluster mode, it can support over 100 million connected clients. TBMQ is designed to handle two main scenarios in MQTT-based solutions: fan-in and fan-out patterns. It prioritizes fault tolerance, high availability, and distributed processing for scalability. The broker supports MQTT v3.x and v5.0 protocols, and has been running in production for over a year before being open-sourced. Documentation and installation options are available on the thingsboard.io website. TBMQ also supports various MQTT 5 features, with others currently in active development.
https://github.com/thingsboard/tbmq