rack-nackmode

https://github.com/rapportive-oss/rack-nackmode

Middleware that communicates impending shutdown to a load balancer via NACKing (negative acking) health checks. Provided you have at least two load-balanced instances, this allows you to shut down or restart an instance without dropping any requests. Your app needs to inform the middleware when it wants to shut down, and the middleware will call back when it's safe to do so.