HttpUpstreamRequestHashModule

= ngx_http_upstream_hash_module =

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

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_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 from Github or here:

After extracting, you will need to patch the latest Nginx source (0.7.62 as of this writing). Run patch like this:

cd nginx-0.7.62 # or whatever patch -p0 < /path/to/upstream/hash/directory/nginx.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 hash algorithm
As of 0.3, the hash algorithm and the failover algorithm use a CRC-32 computation compatible with PECL Memcache.

Changes

 * . * 0.3.1: Updated patch to work with latest Nginx sources
 * . * 0.3: Feature: PECL Memcache compatibility. Change: Removed "hash_method" directive.

Bugs
Send bug reports to Evan Miller.