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