zerodown 
Package zerodown provides a library that makes it easy to build socket based servers that can be gracefully terminated & restarted (that is, without dropping any connections).
Usage
Demo HTTP Server with graceful termination and restart: https://github.com/douglarek/zerodown/blob/master/zerodowndemo/server.go
-
Install the demo application
go get github.com/douglarek/zerodown/zerodowndemo -
Start it in the first terminal
zerodowndemo -
In a second terminal start a slow HTTP request
curl 'http://localhost:8080/?duration=20s' -
In a third terminal trigger a graceful server restart:
kill -USR2 [zerodowndemo pid] -
Trigger another shorter request that finishes before the earlier request:
curl 'http://localhost:8080/?duration=0s'
If done quickly enough, this shows the second quick request will be served by the new process while the slow first request will be served by the first server. It shows how the active connection was gracefully served before the server was shutdown. It is also showing that at one point both the new as well as the old server was running at the same time.