[edit] ngx_http_upstream_hash_module

本模块由第三方提供，不包含在 Nginx 的源码发布版中。安装介绍等请看 [#installation 这里] .

The upstream_hash module provides simple upstream load distribution by hashing a configurable variable (e.g., the request URI, incoming HTTP headers, or some combination). Example usage:

upstream backend {
: server server1;
: server server2;
: hash   $request_uri;
}

Here, Nginx will choose server1 or server2 by hashing the request URI ($request_uri).

[edit] 指令

• [#hash hash]
• [#hash_method hash_method]
• [#hash_again hash_again]

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[edit] hash

语法： hash $variable

作用域： upstream

Enables upstream hashing of $variable.

When present, the "server" directives cannot take any arguments ("weight", "max_fails", etc.).

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[edit] hash_method

语法： hash_method [ crc32 | simple ]

默认值： simple

作用域： upstream

The hash algorithm to use. "crc32" takes the last 15 bits of the crc32 value and modulo's by the number of servers. (This behavior is compatible with libmemcache.) "simple" is described [#hash-algorithm below] .

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[edit] hash_again

语法： hash_again number

默认值： 0

作用域： upstream

Number of times to rehash the value and choose a different server if the backend connection fails. Increase this number to provide high availability.

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[edit] Installation

This module is not distributed with the Nginx source. You can download the request_hash module here: nginx_upstream_hash-0.2.tar.gz

After extracting, you will need to patch the latest Nginx source (0.5.21 as of this writing). Run patch like this:

: cd nginx-0.5.21
: patch -p0 < /path/to/upstream/hash/directory/nginx-0.5.21.patch

Then add the following option to your Nginx ./configure command:

: --add-module=path/to/upstream/hash/directory

Then "make" and "make install" as usual.

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[edit] The "simple" hash algorithm

The "simple" hash algorithm is the same as that used for Nginx's internal hash tables. The function is:

u_int ngx_hash_key(u_char *data, size_t len)
{
: u_int  i, key;

: key = 0;

: for (i = 0; i < len; i++) {
: key *= 31; 
: key += data[i] ;
: }

: return key;
}

The hash value is then modulo'd by the number of servers, and thus a server is picked. A server's index is determined by the order in which it appears in the upstream block.