Re:Buck – A build system developed and used by Facebook
by sdwilsh on Tuesday March 14, 2017 @12:02PM
(#13868653)
Although the daemon doesn't automatically build things. Buck does use watchman (https://github.com/facebook/watchman) to watch files though, and it has a way to run commands when files change: https://facebook.github.io/watchman/docs/watchman-make.html
https://buckbuild.com/command/buckd.html
Note the daemon is automatically started when you `buck build`.
(disclosure: my team created Buck when I was at FB)