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   8 <title>mod_lbmethod_byrequests - Apache HTTP Server</title>
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  16 <p class="apache">Apache HTTP Server Version 2.5</p>
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  21 <div id="page-content">
  22 <div id="preamble"><h1>Apache Module mod_lbmethod_byrequests</h1>
  23 <div class="toplang">
  24 <p><span>Available Languages: </span><a href="../en/mod/mod_lbmethod_byrequests.html" title="English">&nbsp;en&nbsp;</a></p>
  25 </div>
  26 <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>
  27 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
  28 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>lbmethod_byrequests_module</td></tr>
  29 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_lbmethod_byrequests.c</td></tr>
  30 <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>
  31 <h3>Summary</h3>
  32
  33 <p>This module does not provide any configuration directives of its own.
  34 It requires the services of <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>, and
  35 provides the <code>byrequests</code> load balancing method..</p>
  36 </div>
  37 <div id="quickview"><h3 class="directives">Directives</h3>
  38 <p>This module provides no
  39             directives.</p>
  40 <h3>Topics</h3>
  41 <ul id="topics">
  42 <li><img alt="" src="../images/down.gif" /> <a href="#requests">Request Counting Algorithm</a></li>
  43 </ul><h3>See also</h3>
  44 <ul class="seealso">
  45 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
  46 <li><code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code></li>
  47 </ul></div>
  48 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
  49 <div class="section">
  50 <h2><a name="requests" id="requests">Request Counting Algorithm</a></h2>
  51
  52     <p>Enabled via <code>lbmethod=byrequests</code>, the idea behind this
  53     scheduler is that we distribute the requests among the
  54     various workers to ensure that each gets their configured share
  55     of the number of requests. It works as follows:</p>
  56
  57     <p><dfn>lbfactor</dfn> is <em>how much we expect this worker
  58     to work</em>, or <em>the workers's work quota</em>. This is
  59     a normalized value representing their "share" of the amount of
  60     work to be done.</p>
  61
  62     <p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work
  63     to fulfill its quota of work</em>.</p>
  64
  65     <p>The <dfn>worker</dfn> is a member of the load balancer,
  66     usually a remote host serving one of the supported protocols.</p>
  67
  68     <p>We distribute each worker's work quota to the worker, and then look
  69     which of them needs to work most urgently (biggest lbstatus).  This
  70     worker is then selected for work, and its lbstatus reduced by the
  71     total work quota we distributed to all workers.  Thus the sum of all
  72     lbstatus does not change(*) and we distribute the requests
  73     as desired.</p>
  74
  75     <p>If some workers are disabled, the others will
  76     still be scheduled correctly.</p>
  77
  78     <div class="example"><pre><code>for each worker in workers
  79     worker lbstatus += worker lbfactor
  80     total factor    += worker lbfactor
  81     if worker lbstatus &gt; candidate lbstatus
  82         candidate = worker
  83
  84 candidate lbstatus -= total factor</code></pre></div>
  85
  86     <p>If a balancer is configured as follows:</p>
  87
  88     <table><tr><th>worker</th>
  89         <th class="data">a</th>
  90         <th class="data">b</th>
  91         <th class="data">c</th>
  92         <th class="data">d</th></tr>
  93 <tr><th>lbfactor</th>
  94         <td class="data">25</td>
  95         <td class="data">25</td>
  96         <td class="data">25</td>
  97         <td class="data">25</td></tr>
  98 <tr><th>lbstatus</th>
  99         <td class="data">0</td>
 100         <td class="data">0</td>
 101         <td class="data">0</td>
 102         <td class="data">0</td></tr>
 103 </table>
 104
 105     <p>And <var>b</var> gets disabled, the following schedule is produced:</p>
 106
 107     <table><tr><th>worker</th>
 108         <th class="data">a</th>
 109         <th class="data">b</th>
 110         <th class="data">c</th>
 111         <th class="data">d</th></tr>
 112 <tr><th>lbstatus</th>
 113         <td class="data"><em>-50</em></td>
 114         <td class="data">0</td>
 115         <td class="data">25</td>
 116         <td class="data">25</td></tr>
 117 <tr><th>lbstatus</th>
 118         <td class="data">-25</td>
 119         <td class="data">0</td>
 120         <td class="data"><em>-25</em></td>
 121         <td class="data">50</td></tr>
 122 <tr><th>lbstatus</th>
 123         <td class="data">0</td>
 124         <td class="data">0</td>
 125         <td class="data">0</td>
 126         <td class="data"><em>0</em></td></tr>
 127 <tr><td class="data" colspan="5">(repeat)</td></tr>
 128 </table>
 129
 130     <p>That is it schedules: <var>a</var> <var>c</var> <var>d</var>
 131     <var>a</var> <var>c</var> <var>d</var> <var>a</var> <var>c</var>
 132     <var>d</var> ... Please note that:</p>
 133
 134     <table><tr><th>worker</th>
 135         <th class="data">a</th>
 136         <th class="data">b</th>
 137         <th class="data">c</th>
 138         <th class="data">d</th></tr>
 139 <tr><th>lbfactor</th>
 140         <td class="data">25</td>
 141         <td class="data">25</td>
 142         <td class="data">25</td>
 143         <td class="data">25</td></tr>
 144 </table>
 145
 146     <p>Has the exact same behavior as:</p>
 147
 148     <table><tr><th>worker</th>
 149         <th class="data">a</th>
 150         <th class="data">b</th>
 151         <th class="data">c</th>
 152         <th class="data">d</th></tr>
 153 <tr><th>lbfactor</th>
 154         <td class="data">1</td>
 155         <td class="data">1</td>
 156         <td class="data">1</td>
 157         <td class="data">1</td></tr>
 158 </table>
 159
 160     <p>This is because all values of <dfn>lbfactor</dfn> are normalized
 161     with respect to the others. For:</p>
 162
 163     <table><tr><th>worker</th>
 164         <th class="data">a</th>
 165         <th class="data">b</th>
 166         <th class="data">c</th></tr>
 167 <tr><th>lbfactor</th>
 168         <td class="data">1</td>
 169         <td class="data">4</td>
 170         <td class="data">1</td></tr>
 171 </table>
 172
 173     <p>worker <var>b</var> will, on average, get 4 times the requests
 174     that <var>a</var> and <var>c</var> will.</p>
 175
 176     <p>The following asymmetric configuration works as one would expect:</p>
 177
 178     <table><tr><th>worker</th>
 179         <th class="data">a</th>
 180         <th class="data">b</th></tr>
 181 <tr><th>lbfactor</th>
 182         <td class="data">70</td>
 183         <td class="data">30</td></tr>
 184 <tr><td class="data" colspan="2">&nbsp;</td></tr>
 185 <tr><th>lbstatus</th>
 186         <td class="data"><em>-30</em></td>
 187         <td class="data">30</td></tr>
 188 <tr><th>lbstatus</th>
 189         <td class="data">40</td>
 190         <td class="data"><em>-40</em></td></tr>
 191 <tr><th>lbstatus</th>
 192         <td class="data"><em>10</em></td>
 193         <td class="data">-10</td></tr>
 194 <tr><th>lbstatus</th>
 195         <td class="data"><em>-20</em></td>
 196         <td class="data">20</td></tr>
 197 <tr><th>lbstatus</th>
 198         <td class="data"><em>-50</em></td>
 199         <td class="data">50</td></tr>
 200 <tr><th>lbstatus</th>
 201         <td class="data">20</td>
 202         <td class="data"><em>-20</em></td></tr>
 203 <tr><th>lbstatus</th>
 204         <td class="data"><em>-10</em></td>
 205         <td class="data">10</td></tr>
 206 <tr><th>lbstatus</th>
 207         <td class="data"><em>-40</em></td>
 208         <td class="data">40</td></tr>
 209 <tr><th>lbstatus</th>
 210         <td class="data">30</td>
 211         <td class="data"><em>-30</em></td></tr>
 212 <tr><th>lbstatus</th>
 213         <td class="data"><em>0</em></td>
 214         <td class="data">0</td></tr>
 215 <tr><td class="data" colspan="3">(repeat)</td></tr>
 216 </table>
 217
 218     <p>That is after 10 schedules, the schedule repeats and 7 <var>a</var>
 219     are selected with 3 <var>b</var> interspersed.</p>
 220 </div>
 221 </div>
 222 <div class="bottomlang">
 223 <p><span>Available Languages: </span><a href="../en/mod/mod_lbmethod_byrequests.html" title="English">&nbsp;en&nbsp;</a></p>
 224 </div><div id="footer">
 225 <p class="apache">Copyright 2012 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>
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