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<?xml version="1.0"?>
<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
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<modulesynopsis metafile="mod_lbmethod_byrequests.xml.meta">
<name>mod_lbmethod_byrequests</name>
<description>Request Counting load balancer scheduler algorithm for <module
>mod_proxy_balancer</module></description>
<status>Extension</status>
<sourcefile>mod_lbmethod_byrequests.c</sourcefile>
<identifier>lbmethod_byrequests_module</identifier>
<compatibility>Split off from <module>mod_proxy_balancer</module> in 2.3</compatibility>
<summary>
<!-- FIXME: --> <p>This document is still under development.</p>
</summary>
<seealso><module>mod_proxy</module></seealso>
<seealso><module>mod_proxy_balancer</module></seealso>
<section id="requests">
<title>Request Counting Algorithm</title>
<p>Enabled via <code>lbmethod=byrequests</code>, the idea behind this
scheduler is that we distribute the requests among the
various workers to ensure that each gets their configured share
of the number of requests. It works as follows:</p>
<p><dfn>lbfactor</dfn> is <em>how much we expect this worker
to work</em>, or <em>the workers's work quota</em>. This is
a normalized value representing their "share" of the amount of
work to be done.</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(*) and we distribute the requests
as desired.</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> ... Please note that:</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>
</table>
<p>Has the exact same behavior as:</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>1</td>
<td>1</td>
<td>1</td>
<td>1</td></tr>
</table>
<p>This is because all values of <dfn>lbfactor</dfn> are normalized
with respect to the others. For:</p>
<table style="data">
<tr><th>worker</th>
<th>a</th>
<th>b</th>
<th>c</th></tr>
<tr><th>lbfactor</th>
<td>1</td>
<td>4</td>
<td>1</td></tr>
</table>
<p>worker <var>b</var> will, on average, get 4 times the requests
that <var>a</var> and <var>c</var> will.</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"> </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>
</modulesynopsis>
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