Yes, that's how Unicorn works. But HAProxy isn't designed to inherit another process's current state (see this other comment: https://news.ycombinator.com/item?

Yes, that's how Unicorn works. But HAProxy isn't designed to inherit another process's current state (see this other comment: https://news.ycombinator.com/item?id=9371064 https://news.ycombinator.com/item?id=9371064), and it's probably quite complicated and error-prone. If you're going down that route, it's probably much easier to write the code required to simply re-read the config and apply a diff to the internal data structures.