sozu

A reverse proxy that can reload its configuration without dropping a single connection. If you're running web services and need a load balancer in front of them, Sozu does what nginx and HAProxy do, but with true zero-downtime config changes: no reload signal, no worker restart, no connection reset.

Written in Rust. Memory-safe, fast, lightweight. Handles HTTP/1.1, HTTP/2, WebSocket, and TLS. The config changes happen through a command socket: you send a message, the proxy updates instantly. No file edits, no restarts. Small but the project has been around since 2016, originally built by Clever Cloud (a French PaaS provider) for their own infrastructure.

Completely free under AGPL-3.0. No paid tier.

The catch: the ecosystem is tiny compared to nginx or Traefik. Documentation is sparse. Community support is limited; you're mostly on your own. If hot config reloading is your primary requirement, Envoy and HAProxy also handle this well with much larger communities. Sozu is interesting technology but unless you specifically need Rust-native proxy with zero-downtime reloads and don't mind being an early adopter, nginx or Traefik are safer choices.