Types of upstream servers
An upstream server is a server to which NGINX proxies a connection. This can be on a different physical or virtual machine, but doesn't have to be. The upstream server may be a daemon listening on a UNIX domain socket for connections on the local machine or could be one of many on a different machine listening over TCP. It may be an Apache server, with multiple modules to handle different kinds of requests, or a Rack middleware server, providing an HTTP interface to Ruby applications. NGINX can be configured to proxy to each of them.
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