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There’s some active HN users who developed WhatsApp. I’m sure they have a wealth of knowledge related to scaling since WhatsApp was initially based on ejabberd

• nivertech 4y ago
  There are several talks by WhatsApp people who describe how they're scaled their systems. They run on FreeBSD and used Dtrace to quickly find bottlenecks in the BEAM VM. BEAM has a very good builtin instrumentation, but if you're only looking on them, you're missing a lot. Also they committed upstream many of their improvements. Not only WhatsApp, but also the initial implementation of APNs (Apple Push Notification System) was based on ejabberd. They even used the same port numbers. I did an ejabberd scaling training in SF in 2011, and there were several Apple guys. It doesn't mean that they're used ejabberd for sure, but it's a hint.