What architecture does Algolia use to provide an high performance search engine?

The Algolia service is powered by a home-made search engine implemented in C++ which is hosted inside a NGINX server as a NGINX module. The search engine has originally been designed for mobile devices: it was actually a mobile SDK (providing an offline search engine SDK to be embedded in an app) before being an online API. Good thing is that on a mobile devices you need to optimize everything at the milliseconds level because you've no RAM/CPU/DISK \o/ Now that the same technology runs on high-end clusters of machines, it's even faster :)

The whole stack is described here and summarized here.