<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head>
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
<!--
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
              This file is generated from xml source: DO NOT EDIT
        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
      -->
<title>mod_lbmethod_byrequests - Apache HTTP Server Version 2.4</title>
<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link rel="stylesheet" type="text/css" href="../style/css/prettify.css" />
<script src="../style/scripts/prettify.min.js" type="text/javascript">
</script>

<link href="../images/favicon.ico" rel="shortcut icon" /></head>
<body>
<div id="page-header">
<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.4</p>
<img alt="" src="../images/feather.png" /></div>
<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
<div id="path">
<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.4</a> &gt; <a href="./">Modules</a></div>
<div id="page-content">
<div id="preamble"><h1>Apache Module mod_lbmethod_byrequests</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_lbmethod_byrequests.html" title="English">&nbsp;en&nbsp;</a> |
<a href="../fr/mod/mod_lbmethod_byrequests.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a></p>
</div>
<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Request Counting load balancer scheduler algorithm for <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code></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>lbmethod_byrequests_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_lbmethod_byrequests.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Split off from <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> in 2.3</td></tr></table>
<h3>Summary</h3>

<p>This module does not provide any configuration directives of its own.
It requires the services of <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>, and
provides the <code>byrequests</code> load balancing method.</p>
</div>
<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#requests">Request Counting Algorithm</a></li>
</ul><h3 class="directives">Directives</h3>
<p>This module provides no
            directives.</p>
<h3>Bugfix checklist</h3><ul class="seealso"><li><a href="https://www.apache.org/dist/httpd/CHANGES_2.4">httpd changelog</a></li><li><a href="https://bz.apache.org/bugzilla/buglist.cgi?bug_status=__open__&amp;list_id=144532&amp;product=Apache%20httpd-2&amp;query_format=specific&amp;order=changeddate%20DESC%2Cpriority%2Cbug_severity&amp;component=mod_lbmethod_byrequests">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&amp;component=mod_lbmethod_byrequests">Report a bug</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>
<li><code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code></li>
<li><a href="#comments_section">Comments</a></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="requests" id="requests">Request Counting Algorithm</a></h2>
    
    <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' 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>

    <div class="example"><pre><code>for each worker in workers
    worker lbstatus += worker lbfactor
    total factor    += worker lbfactor
    if worker lbstatus &gt; 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> ... Please note that:</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>
</table>

    <p>Has the exact same behavior as:</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">1</td>
        <td class="data">1</td>
        <td class="data">1</td>
        <td class="data">1</td></tr>
</table>

    <p>This is because all values of <dfn>lbfactor</dfn> are normalized
    with respect to the others. For:</p>

    <table><tr><th>worker</th>
        <th class="data">a</th>
        <th class="data">b</th>
        <th class="data">c</th></tr>
<tr><th>lbfactor</th>
        <td class="data">1</td>
        <td class="data">4</td>
        <td class="data">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><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">&nbsp;</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>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/mod/mod_lbmethod_byrequests.html" title="English">&nbsp;en&nbsp;</a> |
<a href="../fr/mod/mod_lbmethod_byrequests.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a></p>
</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&amp;A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
<script type="text/javascript"><!--//--><![CDATA[//><!--
var comments_shortname = 'httpd';
var comments_identifier = 'http://httpd.apache.org/docs/2.4/mod/mod_lbmethod_byrequests.html';
(function(w, d) {
    if (w.location.hostname.toLowerCase() == "httpd.apache.org") {
        d.write('<div id="comments_thread"><\/div>');
        var s = d.createElement('script');
        s.type = 'text/javascript';
        s.async = true;
        s.src = 'https://comments.apache.org/show_comments.lua?site=' + comments_shortname + '&page=' + comments_identifier;
        (d.getElementsByTagName('head')[0] || d.getElementsByTagName('body')[0]).appendChild(s);
    }
    else { 
        d.write('<div id="comments_thread">Comments are disabled for this page at the moment.<\/div>');
    }
})(window, document);
//--><!]]></script></div><div id="footer">
<p class="apache">Copyright 2021 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div><script type="text/javascript"><!--//--><![CDATA[//><!--
if (typeof(prettyPrint) !== 'undefined') {
    prettyPrint();
}
//--><!]]></script>
</body></html>