The upstream module

Closely paired with the proxy module is the upstream module. The upstream directive starts a new context in which a group of upstream servers is defined. These servers may be given different weights (the higher the weight, the greater the number of connections NGINX will pass to that particular upstream server), may be of different types (TCP versus UNIX domain), and may even be marked as down for maintenance reasons.

The following table summarizes the directives valid within the upstream context:

The upstream module directives	Explanation
ip_hash	This directive ensures the distribution of connecting clients evenly over all servers by hashing the IP address, keying on its class-C network.
keepalive	This directive specifies the number of connections to the upstream servers that are cached per worker process. When used with the HTTP connections, proxy_http_version should be set to 1.1 and proxy_set_header to Connection "".
least_conn	This directive activates...