Commit 8b23eb99 authored by Andre Malo's avatar Andre Malo
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@106723 13f79535-47bb-0310-9956-ffa450edef68
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docs/manual/mod/mod_proxy_balancer.xml

+94 −130
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@@ -51,149 +51,113 @@ load balancing </description> 
<section id="scheduler">

    <title>Load balancer scheduler algorithm</title>

    <p>The idea behind this scheduler is the following:</p>

    <p><strong>lbfactor</strong> is <em>how much we expect this worker



    <p><dfn>lbfactor</dfn> is <em>how much we expect this worker

    to work</em>, or <em>the workers's work quota</em>.</p>

    <p><strong>lbstatus</strong> is <em>how urgent this worker has to work



    <p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work

    to fulfill its quota of work</em>.</p>

    <p>The <strong>worker</strong> is a member of the load balancer,

    usually a remote host serving one of the supported protocols</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.(*)</p>



    <p>If some workers are disabled, the others will

    still be scheduled correctly.</p>

    <p>If a balancer is configured as follows:</p>



    <!-- Very ugly, but i can not make the table right aligned -->    

<code>

  <table>

    <tr>

        <td width="80">worker</td>

        <td width="30" style="text-align: right;">a</td>

        <td width="30" style="text-align: right;">b</td>

        <td width="30" style="text-align: right;">c</td>

        <td width="30" style="text-align: right;">d</td>

    </tr>

    <tr>

        <td>lbfactor</td>

        <td style="text-align: right;">25</td>

        <td style="text-align: right;">25</td>

        <td style="text-align: right;">25</td>

        <td style="text-align: right;">25</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">0</td>

        <td style="text-align: right;">0</td>

        <td style="text-align: right;">0</td>

        <td style="text-align: right;">0</td>

    </tr>

    <p>If a balancer is configured as follows:</p>

    

    <table style="data">

    <tr><th>worker</th>

        <th>a</th>

        <th>b</th>

        <th>c</th>

        <th>d</th></tr>

    <tr><th>lbfactor</th>

        <td>25</td>

        <td>25</td>

        <td>25</td>

        <td>25</td></tr>

    <tr><th>lbstatus</th>

        <td>0</td>

        <td>0</td>

        <td>0</td>

        <td>0</td></tr>

    </table>

</code>    

    <p>And b gets disabled, the following schedule is produced:</p>

<code>

  <table>

    <tr>

        <td width="80">lbstatus</td>

        <td width="30" style="text-align: right;">-50</td>

        <td width="30" style="text-align: right;">0</td>

        <td width="30" style="text-align: right;">25</td>

        <td width="30" style="text-align: right;">25</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">-25</td>

        <td style="text-align: right;">0</td>

        <td style="text-align: right;">-25</td>

        <td style="text-align: right;">50</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">0</td>

        <td style="text-align: right;">0</td>

        <td style="text-align: right;">0</td>

        <td style="text-align: right;">0</td>

    </tr>

    <tr>

        <td colspan="5">(repeat)</td>

    </tr>



    <p>And <var>b</var> gets disabled, the following schedule is produced:</p>



    <table style="data">

    <tr><th>worker</th>

        <th>a</th>

        <th>b</th>

        <th>c</th>

        <th>d</th></tr>

    <tr><th>lbstatus</th>

        <td>-50</td>

        <td>0</td>

        <td>25</td>

        <td>25</td></tr>

    <tr><th>lbstatus</th>

        <td>-25</td>

        <td>0</td>

        <td>-25</td>

        <td>50</td></tr>

    <tr><th>lbstatus</th>

        <td>0</td>

        <td>0</td>

        <td>0</td>

        <td>0</td></tr>

    <tr><td colspan="5">(repeat)</td></tr>

    </table>

</code>



    <p>That is it schedules: a c d a c d a c d ...</p>

    <p>The following asymmetric configuration works as one would expect:</p>

<code>

  <table>

    <tr>

        <td width="80">worker</td>

        <td width="30" style="text-align: right;">a</td>

        <td width="30" style="text-align: right;">b</td>

    </tr>

    <tr>

        <td>lbfactor</td>

        <td style="text-align: right;">70</td>

        <td style="text-align: right;">30</td>

    </tr>

    <tr>

        <td colspan="3">&nbsp;</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">-30</td>

        <td style="text-align: right;">30</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">40</td>

        <td style="text-align: right;">-40</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">10</td>

        <td style="text-align: right;">-10</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">-20</td>

        <td style="text-align: right;">20</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">-50</td>

        <td style="text-align: right;">50</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">20</td>

        <td style="text-align: right;">-20</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">-10</td>

        <td style="text-align: right;">10</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">-40</td>

        <td style="text-align: right;">40</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">30</td>

        <td style="text-align: right;">-30</td>

    </tr>

    <tr>

        <td>lbstatus</td>

        <td style="text-align: right;">0</td>

        <td style="text-align: right;">0</td>

    </tr>

    <tr>

        <td colspan="3">(repeat)</td>

    </tr>



    <table style="data">

    <tr><th>worker</th>

        <th>a</th>

        <th>b</th></tr>

    <tr><th>lbfactor</th>

        <td>70</td>

        <td>30</td></tr>

    <tr><th>lbstatus</th>

        <td>-30</td>

        <td>30</td></tr>

    <tr><th>lbstatus</th>

        <td>40</td>

        <td>-40</td></tr>

    <tr><th>lbstatus</th>

        <td>10</td>

        <td>-10</td></tr>

    <tr><th>lbstatus</th>

        <td>-20</td>

        <td>20</td></tr>

    <tr><th>lbstatus</th>

        <td>-50</td>

        <td>50</td></tr>

    <tr><th>lbstatus</th>

        <td>20</td>

        <td>-20</td></tr>

    <tr><th>lbstatus</th>

        <td>-10</td>

        <td>10</td></tr>

    <tr><th>lbstatus</th>

        <td>-40</td>

        <td>40</td></tr>

    <tr><th>lbstatus</th>

        <td>30</td>

        <td>-30</td></tr>

    <tr><th>lbstatus</th>

        <td>0</td>

        <td>0</td></tr>

    <tr><td colspan="3">(repeat)</td></tr>

    </table>

</code>



    <p>That is after 10 schedules, the schedule repeats and 7 a are selected

    with 3 b interspersed.</p>