"Varnish is often used in deployments alongside Apache, as well
as the open source ngnx web server. According to Per Buer,CEO of
Varnish, it really doesn't matter which web server is used with
Varnish.
"Some people look at Varnish as a way to outsource performance,"
Buer said. "For example, Ruby on Rails is not known for
performance, so many of those developers outsource performance to
Varnish."
"Varnish is not the first open source proxy caching server in
the market either. Though Buer notes it has advantages over other
options, including the open source Squid proxy server."