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<div id="preamble"><h1>Apache Module mod_proxy_balancer</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_proxy_balancer.html" title="English"> en </a> |
<a href="../ja/mod/mod_proxy_balancer.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p>
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<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> extension for
load balancing </td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module�Identifier:</a></th><td>proxy_balancer_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source�File:</a></th><td>proxy_balancer.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.1 and later</td></tr></table>
<h3>Summary</h3>
<p>This module <em>requires</em> the service of <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>. 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,
<code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> and <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>
have to be present in the server.</p>
<div class="warning"><h3>Warning</h3>
<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>
</div>
</div>
<div id="quickview"><h3 class="directives">Directives</h3>
<p>This module provides no
directives.</p>
<h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#enable">Enabling Balancer Manager Support</a></li>
</ul><h3>See also</h3>
<ul class="seealso">
<li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2>
<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>
<div class="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></div>
<p>If a balancer is configured as follows:</p>
<table><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th>
<th class="data">c</th>
<th class="data">d</th></tr>
<tr><th>lbfactor</th>
<td class="data">25</td>
<td class="data">25</td>
<td class="data">25</td>
<td class="data">25</td></tr>
<tr><th>lbstatus</th>
<td class="data">0</td>
<td class="data">0</td>
<td class="data">0</td>
<td class="data">0</td></tr>
</table>
<p>And <var>b</var> gets disabled, the following schedule is produced:</p>
<table><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th>
<th class="data">c</th>
<th class="data">d</th></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-50</em></td>
<td class="data">0</td>
<td class="data">25</td>
<td class="data">25</td></tr>
<tr><th>lbstatus</th>
<td class="data">-25</td>
<td class="data">0</td>
<td class="data"><em>-25</em></td>
<td class="data">50</td></tr>
<tr><th>lbstatus</th>
<td class="data">0</td>
<td class="data">0</td>
<td class="data">0</td>
<td class="data"><em>0</em></td></tr>
<tr><td class="data" 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><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th></tr>
<tr><th>lbfactor</th>
<td class="data">70</td>
<td class="data">30</td></tr>
<tr><td class="data" colspan="2"> </td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-30</em></td>
<td class="data">30</td></tr>
<tr><th>lbstatus</th>
<td class="data">40</td>
<td class="data"><em>-40</em></td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>10</em></td>
<td class="data">-10</td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-20</em></td>
<td class="data">20</td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-50</em></td>
<td class="data">50</td></tr>
<tr><th>lbstatus</th>
<td class="data">20</td>
<td class="data"><em>-20</em></td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-10</em></td>
<td class="data">10</td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-40</em></td>
<td class="data">40</td></tr>
<tr><th>lbstatus</th>
<td class="data">30</td>
<td class="data"><em>-30</em></td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>0</em></td>
<td class="data">0</td></tr>
<tr><td class="data" 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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="enable" id="enable">Enabling Balancer Manager Support</a></h2>
<p>This module <em>requires</em> the service of
<code class="module"><a href="../mod/mod_status.html">mod_status</a></code>.
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,
<code class="module"><a href="../mod/mod_status.html">mod_status</a></code> and <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>
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>
<div class="example"><p><code>
<Location /balancer-manager><br />
SetHandler balancer-manager<br />
<br />
Order Deny,Allow<br />
Deny from all<br />
Allow from .foo.com<br />
</Location>
</code></p></div>
<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>
</div>
</div>
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