centrifugal/centrifugo

Centrifugo is an open-source scalable real-time messaging server. Centrifugo can instantly deliver messages to application online users connected over supported transports (WebSocket, HTTP-streaming, SSE/EventSource, GRPC, SockJS, WebTransport). Centrifugo has the concept of channel subscriptions – so it's a user-facing PUB/SUB server.