We use both. This is running production workloads in WA today however Erlang is the "bulk" of our concurrency model, Rust is becoming more heavily used across all the products of Meta. You'll see more logic rewrites with Ractor coming out of Meta open-source in the coming months.

At the rewrites happening for performance reasons?

Anything you’ve published/will publish about this?

We've traditionally had non-pure Erlang services at WA for things like backing storage, interfacing with other infra, etc. This is just sort of the natural extension of that. There might be some services which we eventually rewrite but for now it's more "new" stuff that's being built with this. Some things will get published soon-ish, but I can't say more at the moment.

EDIT: The rewrites I mentioned are not WA specific, more Meta which is rewriting a lot of C++ and Python stuff to Rust constantly :p

Neat. How big is the dev team working on this Erlang & Erlang-adjacent stuff?

Not sure what's public here, so let's just say "big", as in 100's+.

Wait. This package is in use in production at WhatsApp and the only mention of that is many levels deep in a comment thread on HN? That's seriously burying the lede.