详细解释：nginx中ChsHttpUpstreamModule模块配置及各个参数含义

该模块为后端服务器提供简单的负载均衡（轮循调度和客户端 IP）。

示例：

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

指令：

ip_hash
语法: ip_hash

默认值: 无

语境: upstream

This directive causes requests to be distributed between upstreams based on the IP-address of the client.
The key for the hash is the class-C network address of the client. This method guarantees that the client request will always be transferred to the same server. But if this server is considered inoperative, then the request of this client will be transferred to another server. This gives a high probability clients will always connect to the same server.

It is not possible to combine ip_hash and weight methods for connection distribution. If one of the servers must be removed for some time, you must mark that server as *down*.

示例：

upstream backend {
  ip_hash;
  server   backend1.example.com;
  server   backend2.example.com;
  server   backend3.example.com  down;
  server   backend4.example.com;
}

server
语法: server name [parameters]

默认值: 无

语境: upstream

Directive assigns the name and the parameters of server. For the name it is possible to use a domain name, an address, port or unix socket. If domain name resolves to several addresses, then all are used.

weight = NUMBER - set weight of the server, if not set weight is equal to one.
max_fails = NUMBER - number of unsuccessful attempts at communicating with the server within the time period (assigned by parameter fail_timeout) after which it is considered inoperative. If not set, the number of attempts is one. A value of 0 turns off this check. What is considered a failure is defined by proxy_next_upstream or fastcgi_next_upstream (except http_404 errors which do not count towards max_fails).
fail_timeout = TIME - the time during which must occur *max_fails* number of unsuccessful attempts at communication with the server that would cause the server to be considered inoperative, and also the time for which the server will be considered inoperative (before another attempt is made). If not set the time is 10 seconds. fail_timeout has nothing to do with upstream response time, use proxy_connect_timeout and proxy_read_timeout for controlling this.
down - marks server as permanently offline, to be used with the directive ip_hash.
backup - (0.6.7 or later) only uses this server if the non-backup servers are all down or busy
示例：

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

upstream
语法: upstream name { ... }

默认值: 无

语境: http

这个指令描述了一个服务器的集合，该集合可被用于 proxy_pass 和 fastcgi_pass 指令中，作为一个单独的实体。

这些服务器可以是监听在不同的端口，另外，并发使用同时监听 TCP 端口和 Unix 套接字的服务器是可能的。

这些服务器能被分配不同的权重。如果没有指定，则都为一。

示例：

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 according to the servers in round-robin manner with respect of the server weight.
For example of every 7 seven requests given above they will be distributed like this: 5 requests on backend1.example.com and one request to the second and the third of server. If with an attempt at the work with the server error occurred, then the request will be transmitted to the following server and then until all workers of server not are tested. If successful answer is not succeeded in obtaining from all servers, then to client will be returned the result of work with the last server.

变量：

自版本 0.5.18 之后，可通过 log_module 模块将变量值写入日志。

示例：

log_format timing '$remote_addr - $remote_user [$time_local]  $request '
  'upstream_response_time $upstream_response_time '
  'msec $msec request_time $request_time';
 
log_format up_head '$remote_addr - $remote_user [$time_local]  $request '
  'upstream_http_content_type $upstream_http_content_type';

$upstream_addr
Address of the upstream server that handled the request

$upstream_cache_status
Appeared in 0.8.3. Possible values:

MISS
EXPIRED - expired, request was passed to backend
UPDATING - expired, stale response was used due to proxy/fastcgi_cache_use_stale updating
STALE - expired, stale response was used due to proxy/fastcgi_cache_use_stale
HIT
$upstream_status
Upstream server status of the answer

$upstream_response_time
Response time in milliseconds. Several answers are divided by commas and colons.

$upstream_http_$HEADER
Arbitrary HTTP protocol headers, for example:

$upstream_http_host

原始俄文文档：http://sysoev.ru/nginx/docs/http/ngx_http_upstream.html

原始英文wiki文档：http://wiki.nginx.org/NginxHttpUpstreamModule