Comment by ipsum2 2 days ago
Yep. Zeus is a fork of Zookeeper, Hack is a fork of PHP, etc. It's usually needed to make it work with the internal environment.
The few things that don't have forks are usually the open source projects like React or PyTorch, but even those have some custom features added to make it work with FB internals.
Am I completely off-base/confused thinking that the GFE originally started life (like back under csilver) as a fork of boa[0]?
[0]: http://www.boa.org/
Few companies experienced the explosive growth fb did, though many will claim to have done so. Hack made the existing codebase of php scale to insane levels while reaching escape velocity for the overall company to even attempt to transition away or shrink the php codebase, as i recall (i was an SRE, not a dev)
zeus likewise.
This is also how things work at Google.
Google also maintains a monorepo with "forks" of all software that they use. History diverges, but is occasionally synchronized for things like security updates etc.