centrifugo

Centrifugo is a standalone server that handles all the WebSocket connections so your backend doesn't have to. Your app publishes messages to Centrifugo via HTTP API, and Centrifugo pushes them to connected clients. Language-agnostic: works with any backend. Apache 2.0. Supports WebSocket, SSE (Server-Sent Events), HTTP streaming, and GRPC. Built-in presence tracking (who's online), message history, and channel permissions. Handles millions of concurrent connections. Written in Go.

The open source version is fully featured for most use cases. Centrifugo PRO adds features like push notifications (iOS/APNs, Android/FCM), analytics, SSO, real-time CEL-based channel permissions, and priority support. PRO pricing is custom. Contact them for quotes.

Self-hosting: single binary, minimal config. Connect it to Redis or Nats for scaling across multiple nodes. Ops: moderate at scale (Redis cluster management), trivial for a single node.

Solo and small teams: the open source version handles everything. Medium teams: still free unless you need push notifications or advanced analytics. The catch: adding a separate real-time server is another piece of infrastructure. If your framework already has decent WebSocket support (Phoenix, Rails Action Cable), you might not need it. Centrifugo shines when your backend is a REST API that wasn't built for real-time.