I doubt WhatsApp chose XMPP first and then ejabberd/Erlang. Even if they had built a custom protocol, the massive-scale concurrency required would have made Erl

I doubt WhatsApp chose XMPP first and then ejabberd/Erlang. Even if they had built a custom protocol, the massive-scale concurrency required would have made Erlang just as good a target to build on. Java could have come close, but Erlang was the language with first class Actor-model and native massive-scale concurrency support. Actually, they do use a custom protocol, one that heavily derives from XMPP. Turns out, they didn't need to build a backend from scratch in their chosen language because a similar one already existed. Without inputs from the WhatsApp engineers on their early choices, my version of things^ is at least just as valid as yours. EDIT: I was wrong. Source: an interview one of the creators of WhatsApp gave to wired: https://www.wired.com/2015/10/whatsapps-co-founder-on-how-the-iconoclastic-app-got-huge/ https://www.wired.com/2015/10/whatsapps-co-founder-on-how-th...