2.4.x-mod_md/docs/manual/mod/mod_lbmethod_byrequests.xml

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<modulesynopsis metafile="mod_lbmethod_byrequests.xml.meta">

<name>mod_lbmethod_byrequests</name>
<description>Request Counting load balancer scheduler algorithm for <module
>mod_proxy_balancer</module></description>
<status>Extension</status>
<sourcefile>mod_lbmethod_byrequests.c</sourcefile>
<identifier>lbmethod_byrequests_module</identifier>
<compatibility>Split off from <module>mod_proxy_balancer</module> in 2.3</compatibility>

<summary>
<p>This module does not provide any configuration directives of its own.
It requires the services of <module>mod_proxy_balancer</module>, and
provides the <code>byrequests</code> load balancing method..</p>
</summary>
<seealso><module>mod_proxy</module></seealso>
<seealso><module>mod_proxy_balancer</module></seealso>

<section id="requests">
    <title>Request Counting Algorithm</title>
    <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' 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>

    <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>
    </example>

    <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>

    <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><em>-50</em></td>
        <td>0</td>
        <td>25</td>
        <td>25</td></tr>
    <tr><th>lbstatus</th>
        <td>-25</td>
        <td>0</td>
        <td><em>-25</em></td>
        <td>50</td></tr>
    <tr><th>lbstatus</th>
        <td>0</td>
        <td>0</td>
        <td>0</td>
        <td><em>0</em></td></tr>
    <tr><td 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 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>
    </table>

    <p>Has the exact same behavior as:</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>1</td>
        <td>1</td>
        <td>1</td>
        <td>1</td></tr>
    </table>

    <p>This is because all values of <dfn>lbfactor</dfn> are normalized
    with respect to the others. For:</p>

    <table style="data">
    <tr><th>worker</th>
        <th>a</th>
        <th>b</th>
        <th>c</th></tr>
    <tr><th>lbfactor</th>
        <td>1</td>
        <td>4</td>
        <td>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 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><td colspan="2">&nbsp;</td></tr>
    <tr><th>lbstatus</th>
        <td><em>-30</em></td>
        <td>30</td></tr>
    <tr><th>lbstatus</th>
        <td>40</td>
        <td><em>-40</em></td></tr>
    <tr><th>lbstatus</th>
        <td><em>10</em></td>
        <td>-10</td></tr>
    <tr><th>lbstatus</th>
        <td><em>-20</em></td>
        <td>20</td></tr>
    <tr><th>lbstatus</th>
        <td><em>-50</em></td>
        <td>50</td></tr>
    <tr><th>lbstatus</th>
        <td>20</td>
        <td><em>-20</em></td></tr>
    <tr><th>lbstatus</th>
        <td><em>-10</em></td>
        <td>10</td></tr>
    <tr><th>lbstatus</th>
        <td><em>-40</em></td>
        <td>40</td></tr>
    <tr><th>lbstatus</th>
        <td>30</td>
        <td><em>-30</em></td></tr>
    <tr><th>lbstatus</th>
        <td><em>0</em></td>
        <td>0</td></tr>
    <tr><td 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>
</section>

</modulesynopsis>