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| <title>mod_lbmethod_byrequests - Apache HTTP Server</title> |
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| <p class="apache">Apache HTTP Server Version 2.3</p> |
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| <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.3</a> > <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"> en </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"><h3 class="directives">Directives</h3> |
| <p>This module provides no |
| directives.</p> |
| <h3>Topics</h3> |
| <ul id="topics"> |
| <li><img alt="" src="../images/down.gif" /> <a href="#requests">Request Counting Algorithm</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> |
| </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'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> |
| |
| <div class="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></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"> </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> |
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| <p><span>Available Languages: </span><a href="../en/mod/mod_lbmethod_byrequests.html" title="English"> en </a></p> |
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| <p class="apache">Copyright 2011 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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