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<?xml version="1.0"?>
<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<!-- $LastChangedRevision$ -->
<!--
Copyright 2002-2004 The Apache Software Foundation
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
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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>
<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>-50</td>
<td>0</td>
<td>25</td>
<td>25</td></tr>
<tr><th>lbstatus</th>
<td>-25</td>
<td>0</td>
<td>-25</td>
<td>50</td></tr>
<tr><th>lbstatus</th>
<td>0</td>
<td>0</td>
<td>0</td>
<td>0</td></tr>
<tr><td colspan="5">(repeat)</td></tr>
</table>
<p>That is it schedules: a c d a c d a c d ...</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><th>lbstatus</th>
<td>-30</td>
<td>30</td></tr>
<tr><th>lbstatus</th>
<td>40</td>
<td>-40</td></tr>
<tr><th>lbstatus</th>
<td>10</td>
<td>-10</td></tr>
<tr><th>lbstatus</th>
<td>-20</td>
<td>20</td></tr>
<tr><th>lbstatus</th>
<td>-50</td>
<td>50</td></tr>
<tr><th>lbstatus</th>
<td>20</td>
<td>-20</td></tr>
<tr><th>lbstatus</th>
<td>-10</td>
<td>10</td></tr>
<tr><th>lbstatus</th>
<td>-40</td>
<td>40</td></tr>
<tr><th>lbstatus</th>
<td>30</td>
<td>-30</td></tr>
<tr><th>lbstatus</th>
<td>0</td>
<td>0</td></tr>
<tr><td colspan="3">(repeat)</td></tr>
</table>
<p>That is after 10 schedules, the schedule repeats and 7 a are selected
with 3 b 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>
<Location /balancer-manager><br />
SetHandler balancer-manager<br />
<br />
Order Deny,Allow<br />
Deny from all<br />
Allow from .foo.com<br />
</Location>
</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>
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