Centrifugo

Centrifugo is an open-source, scalable real-time messaging server designed for building interactive applications with instant updates such as chat apps, live comments, multiplayer games, and real-time dashboards. While it's not a full chat platform out of the box, it can serve as a backend foundation for creating custom Slack alternatives with real-time communication capabilities.

Key features include:

• Real-time Transports: Supports WebSocket, HTTP-streaming, Server-Sent Events (SSE), WebTransport, and GRPC.
• Scalability: Built-in integrations with Redis and Nats for scaling connections across multiple nodes.
• Seamless Integration: Simple publishing API for easy integration with existing applications.
• Flexible Authentication: Provides flexible authentication mechanisms.
• Channel History: Ability to maintain channel history.
• Online Presence: Supports online presence detection.
• SDK Libraries: Official SDK libraries for web and mobile development.
• Centrifugo PRO: Offers additional features for enterprise environments, such as analytics with ClickHouse, real-time tracing, push notification API, and SSO integrations.