The ngx_http_upstream_module module allows to define groups of servers that can be referenced from the proxy_pass and fastcgi_pass directives.

upstream backend {
    server backend1.example.com       weight=5;
    server backend2.example.com:8080;
    server unix:/tmp/backend3;

    server backup1.example.com:8080   backup;
    server backup2.example.com:8080   backup;
}

server {
    location / {
        proxy_pass http://backend;
    }
}

Specifies that a group should use a balancing method where requests are distributed between servers based on client IP addresses. A class C network containing the client IP address is used as a hashing key. The method ensures that requests of the same client will always be passed to the same server except when this server is considered down in which case client requests will be passed to another server and most probably it will also be the same server.

It is not possible to specify a weight for servers using the ip_hash balancing method. If one of the servers needs to be temporarily removed, it should be marked with the down parameter in order to preserve the current hashing of client IP addresses.

Example:

upstream backend {
    ip_hash;

    server backend1.example.com;
    server backend2.example.com;
    server backend3.example.com down;
    server backend4.example.com;
}

Activates cache of connections to upstream servers.

The connections parameter sets the maximum number of keepalive connections to upstream servers that are retained in the cache per one worker process. If there are more idle connections to retain to an upstream than are specified by this number, the least recently used ones will be closed.

It should be noted that keepalive directive does not limit the total number of connections that nginx worker process can open to upstream servers. The new connections are always created on demand. The connections parameter should be set low enough to allow upstream servers to process additional new incoming connections as well.

Example configuration of memcached upstream with keepalive connections:

upstream memcached_backend {
    server 127.0.0.1:11211;
    server 10.0.0.2:11211;

    keepalive 32;
}

server {
    ...

    location /memcached/ {
        set $memcached_key $uri;
        memcached_pass memcached_backend;
    }

}

For HTTP, the proxy_http_version directive should be set to “1.1” and the “Connection” header field should be cleared:

upstream http_backend {
    server 127.0.0.1:8080;

    keepalive 16;
}

server {
    ...

    location /http/ {
        proxy_pass http://http_backend;
        proxy_http_version 1.1;
        proxy_set_header Connection "";
        ...
    }
}

Alternatively, HTTP/1.0 persistent connections can be used by passing the “Connection: Keep-Alive” header field to an upstream server, though this is not recommended.

For FastCGI servers, it is required to set fastcgi_keep_conn for keepalive connections to work:

upstream fastcgi_backend {
    server 127.0.0.1:9000;

    keepalive 8;
}

server {
    ...

    location /fastcgi/ {
        fastcgi_pass fastcgi_backend;
        fastcgi_keep_conn on;
        ...
    }
}

When using load balancer modules other than the default round-robin, it is necessary to activate them before the keepalive directive.

SCGI and uwsgi protocols do not define keepalive connections.

Sets a name and other parameters of the server. A name can be a domain name, an address, a port, or a path of the UNIX-domain socket. If a domain name resolves to several addresses, all of them are used.

weight=number

sets a weight of the server, by default 1.

max_fails=number

sets a number of unsuccessful attempts to communicate with the server during a time set by the fail_timeout parameter after which it will be considered down for a period of time also set by the fail_timeout parameter. By default, the number of unsuccessful attempts is set to 1. A value of zero disables accounting of attempts. What is considered to be an unsuccessful attempt is configured by the proxy_next_upstream and fastcgi_next_upstream directives. The http_404 state is not considered an unsuccessful attempt.

fail_timeout=time

sets

• a time during which the specified number of unsuccessful attempts to communicate with the server should happen for the server to be considered down;
• and a period of time the server will be considered down.

By default, timeout is set to 10 seconds.

backup

marks the server as a backup server. It will be passed requests when the primary servers are down.

down

marks the server as permanently down; used along with the ip_hash directive.

Example:

upstream backend {
    server backend1.example.com     weight=5;
    server 127.0.0.1:8080           max_fails=3 fail_timeout=30s;
    server unix:/tmp/backend3;

    server backup1.example.com:8080 backup;
}

Defines a group of servers. Servers can listen on different ports. In addition, servers listening on TCP and UNIX-domain sockets can be used simultaneously.

Example:

upstream backend {
    server backend1.example.com weight=5;
    server 127.0.0.1:8080       max_fails=3 fail_timeout=30s;
    server unix:/tmp/backend3;
}

Requests are distributed between servers in a round-robin fashion, taking into account the weights of servers. In the above example, each 7 requests will be distributed as follows: 5 requests to backend1.example.com and one request to each of second and third servers. If an error occurs when communicating with the server, a request will be passed to the next server, and so on until all of the functioning servers will be tried. If a successful response could not be obtained from any of the servers, the client will be returned the result of contacting the last server.

The ngx_http_upstream_module module supports the following embedded variables:

$upstream_addr

keeps an IP address and port of the server, or a path to the UNIX-domain socket. If several servers were contacted during request processing, their addresses are separated by commas, e.g. “192.168.1.1:80, 192.168.1.2:80, unix:/tmp/sock”. If an internal redirect from one server group to another happened using “X-Accel-Redirect” or error_page then these server groups are separated by colons, e.g. “192.168.1.1:80, 192.168.1.2:80, unix:/tmp/sock : 192.168.10.1:80, 192.168.10.2:80”.

$upstream_response_time

keeps servers response times in seconds with a milliseconds resolution. Several responses are also separated by commas and colons.

$upstream_status

keeps servers response codes. Several responses are also separated by commas and colons.

$upstream_http_...

keep server response header fields. For example, the “Server” response header field is made available through the $upstream_http_server variable. Note that only the last server’s response header fields are saved.