EMQ X Broker is an open sourced MQTT message broker, which implements both MQTT V3.1 and V3.1.1 protocol specifications and supports MQTT-SN, CoAP, WebSocket, STOMP and SockJS as well. It is a distributed, massively scalable, highly extensible broker which a single node alone can support millions concurrent connections and much higher when clustering. It has been deployed by 5000+ WW users and served 50 million IoT and Mobile connections.

Meanwhile EMQ has the enterprise version which extends the functions and enhances the performance. Please visit https://www.emqx.io/products/enterprise for more information if you're interested in EMQ X Enterprise.