nginx (pronounced "engine x") is an open-source HTTP web server, reverse proxy, content cache, load balancer, TCP/UDP proxy server and mail proxy server. A reverse proxy receives requests from the internet and forwards them to application servers behind it. nginx was originally written by Igor Sysoev; F5, Inc. sells enterprise distributions, commercial support and training.
nginx runs as one master process and several worker processes. The master process reads the configuration and manages the workers; the workers handle requests with an event-based model, and their number can follow the number of CPU cores. All behavior is set in a text file, nginx.conf, with directives grouped in blocks such as http, server and location. A typical setup serves static files, such as images and HTML pages, from local folders and passes other requests to an application through a proxy or FastCGI. A reload command applies a new configuration while old workers finish their current requests.
nginx is free and open source under the 2-clause BSD License and is installed on the operator's own servers.