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ngx_http_upstream_hash_module

This module is not distributed with the Nginx source. Installation instructions are below.

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).

Directives

• hash

• hash_method

• hash_again

hash

syntax hash $variable

context upstream

Enables upstream hashing of $variable.

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

hash_method

syntax hash_method [ crc32 | simple ]

default simple

context 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 below.

hash_again

syntax hash_again number

default 0

context 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.

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.

The "simple" hash algorithm

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

#define ngx_hash(key, c)   ((u_int) key * 31 + c)

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.

Bugs

Send bug reports to Evan Miller.