debian/help/docs/upstream

##
#  File:
#    upstream
#  Description:
#    This file describes how to use upstream blocks.
##

An upstream block allows you to set a list of upstream locations for both
proxy_pass and fastcgi_pass directives.

Examples of upstream blocks:

# PHP listening on the same server
upstream php {
	# ip_hash ensures the same backend is used for client reconnects.
	ip_hash;
	# This assumes we have multiple PHP listeners on different known ports.
	server 127.0.0.1:9000;
	server 127.0.0.1:9001;
	server 127.0.0.1:9002;
	server 127.0.0.1:9003;
	# In addition to listening on ports, we can listen to unix sockets.
	server unix:/tmp/php-cgi.socket;
}

# Multiple backend Apache instances on separate servers
upstream apache {
	# Adding a weight alters the chance the upstream server will be used.
	server apache1.domain.com weight 5;
	server apache2.domain.com;
	server apache3.domain.com;
	# Adding 'down' keeps the upstream from being used. Useful for downtime management.
	server apache4.domain.com down;
	server apache5.domain.com;
}

Using an upstream location:

# Passing PHP to upstream
location ~ \.php$ {
	fastcgi_split_path_info ^(.+\.php)(/.+)$;
	include fastcgi_params;
	fastcgi_pass php;
}

# Passing all requests for /cgi-bin/* to Apache upstreams.
location /cgi-bin {
	proxy_pass apache;
}

For more information see http://wiki.nginx.org/HttpUpstreamModule