nginx vs Varnish: What are the differences?
nginx: A high performance free open source web server powering busiest sites on the Internet. nginx [engine x] is an HTTP and reverse proxy server, as well as a mail proxy server, written by Igor Sysoev. According to Netcraft nginx served or proxied 30.46% of the top million busiest sites in Jan 2018; Varnish: High-performance HTTP accelerator. Varnish Cache is a web application accelerator also known as a caching HTTP reverse proxy. You install it in front of any server that speaks HTTP and configure it to cache the contents. Varnish Cache is really, really fast. It typically speeds up delivery with a factor of 300 - 1000x, depending on your architecture.
nginx belongs to "Web Servers" category of the tech stack, while Varnish can be primarily classified under "Web Cache".
"High-performance http server", "Performance" and "Easy to configure" are the key factors why developers consider nginx; whereas "High-performance", "Very Fast" and "Very Stable" are the primary reasons why Varnish is favored.
nginx and Varnish are both open source tools. nginx with 9.11K GitHub stars and 3.44K forks on GitHub appears to be more popular than Varnish with 908 GitHub stars and 216 GitHub forks.
According to the StackShare community, nginx has a broader approval, being mentioned in 8676 company stacks & 2560 developers stacks; compared to Varnish, which is listed in 1006 company stacks and 140 developer stacks.