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