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<modulesynopsis metafile="mod_proxy_balancer.xml.meta">

<name>mod_proxy_balancer</name>

<description><module>mod_proxy</module> extension for

load balancing </description>

<status>Extension</status>

<sourcefile>proxy_balancer.c</sourcefile>

<identifier>proxy_balancer_module</identifier>

<compatibility>Available in version 2.1 and later</compatibility>

<summary>

<p>This module <em>requires</em> the service of <module

>mod_proxy</module>. It provides load balancing support for

<code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols

</p>

<p>Thus, in order to get the ability of load balancing,

<module>mod_proxy</module> and <module>mod_proxy_balancer</module>

have to be present in the server.</p>

<note type="warning"><title>Warning</title>

<p>Do not enable proxying until you have <a

href="mod_proxy.html#access">secured your server</a>. Open proxy

servers are dangerous both to your network and to the Internet at

large.</p>

</note>

</summary>

<seealso><module>mod_proxy</module></seealso>

<section id="scheduler">

<title>Load balancer scheduler algorithm</title>

<p>The idea behind this scheduler is the following:</p>

<p><dfn>lbfactor</dfn> is <em>how much we expect this worker

to work</em>, or <em>the workers's work quota</em>.</p>

<p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work

to fulfill its quota of work</em>.</p>

<p>The <dfn>worker</dfn> is a member of the load balancer,

usually a remote host serving one of the supported protocols.</p>

<p>We distribute each worker's work quota to the worker, and then look

which of them needs to work most urgently (biggest lbstatus). This

worker is then selected for work, and its lbstatus reduced by the

total work quota we distributed to all workers. Thus the sum of all

lbstatus does not change.(*)</p>

<p>If some workers are disabled, the others will

still be scheduled correctly.</p>

<example><pre><code>for each worker in workers

worker lbstatus += worker lbfactor

total factor += worker lbfactor

if worker lbstatus > candidate lbstatus

candidate = worker

candidate lbstatus -= total factor</code></pre>

</example>

<p>If a balancer is configured as follows:</p>

<table style="data">

<tr><th>worker</th>

<th>a</th>

<th>b</th>

<th>c</th>

<th>d</th></tr>

<tr><th>lbfactor</th>

<td>25</td>

<td>25</td>

<td>25</td>

<td>25</td></tr>

<tr><th>lbstatus</th>

<td>0</td>

<td>0</td>

<td>0</td>

<td>0</td></tr>

</table>

<p>And <var>b</var> gets disabled, the following schedule is produced:</p>

<table style="data">

<tr><th>worker</th>

<th>a</th>

<th>b</th>

<th>c</th>

<th>d</th></tr>

<tr><th>lbstatus</th>

<td><em>-50</em></td>

<td>0</td>

<td>25</td>

<td>25</td></tr>

<tr><th>lbstatus</th>

<td>-25</td>

<td>0</td>

<td><em>-25</em></td>

<td>50</td></tr>

<tr><th>lbstatus</th>

<td>0</td>

<td>0</td>

<td>0</td>

<td><em>0</em></td></tr>

<tr><td colspan="5">(repeat)</td></tr>

</table>

<p>That is it schedules: <var>a</var> <var>c</var> <var>d</var>

<var>a</var> <var>c</var> <var>d</var> <var>a</var> <var>c</var>

<var>d</var> ...</p>

<p>The following asymmetric configuration works as one would expect:</p>

<table style="data">

<tr><th>worker</th>

<th>a</th>

<th>b</th></tr>

<tr><th>lbfactor</th>

<td>70</td>

<td>30</td></tr>

<tr><td colspan="2">&nbsp;</td></tr>

<tr><th>lbstatus</th>

<td><em>-30</em></td>

<td>30</td></tr>

<tr><th>lbstatus</th>

<td>40</td>

<td><em>-40</em></td></tr>

<tr><th>lbstatus</th>

<td><em>10</em></td>

<td>-10</td></tr>

<tr><th>lbstatus</th>

<td><em>-20</em></td>

<td>20</td></tr>

<tr><th>lbstatus</th>

<td><em>-50</em></td>

<td>50</td></tr>

<tr><th>lbstatus</th>

<td>20</td>

<td><em>-20</em></td></tr>

<tr><th>lbstatus</th>

<td><em>-10</em></td>

<td>10</td></tr>

<tr><th>lbstatus</th>

<td><em>-40</em></td>

<td>40</td></tr>

<tr><th>lbstatus</th>

<td>30</td>

<td><em>-30</em></td></tr>

<tr><th>lbstatus</th>

<td><em>0</em></td>

<td>0</td></tr>

<tr><td colspan="3">(repeat)</td></tr>

</table>

<p>That is after 10 schedules, the schedule repeats and 7 <var>a</var>

are selected with 3 <var>b</var> interspersed.</p>

</section>

<section id="enable">

<title>Enabling Balancer Manager Support</title>

<p>This module <em>requires</em> the service of

<module>mod_status</module>.

Balancer manager enables dynamic update of balancer

members. You can use balancer manager to change the balance

factor or a particular member, or put it in the off line

mode.

</p>

<p>Thus, in order to get the ability of load balancer management,

<module>mod_status</module> and <module>mod_proxy_balancer</module>

have to be present in the server.</p>

<p>To enable load balancer management for browsers from the foo.com

domain add this code to your <code>httpd.conf</code>

configuration file</p>

<example>

&lt;Location /balancer-manager&gt;<br />

SetHandler balancer-manager<br />

<br />

Order Deny,Allow<br />

Deny from all<br />

Allow from .foo.com<br />

&lt;/Location&gt;

</example>

<p>You can now access load balancer manager by using a Web browser

to access the page

<code>http://your.server.name/balancer-manager</code></p>

</section>

</modulesynopsis>