WhatsApp scaled to 1B users with only 50 engineers

WhatsApp's Engineering culture consists of 3 main principles. WhatsApp consciously keeps the engineering staff small to only about 50 engineers. In terms of servers, WhatsApp prefers to use a smaller number of servers and vertically scale each server to the highest extent possible. Erlang is the programming language of choice for WhatsApp's backend systems. WhatsApp tries to avoid dependencies as much as possible, but they do make use of Mnesia, a distributed database that's part of OTP. Erlang also brings the ability to hotswap code. FreeBSD. FreeBSD is the OS WhatsApp uses for their servers. You can view the full talk on WhatsApp's engineering here.