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   8 <title>mod_proxy_balancer - Apache HTTP Server</title>
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  22 <div id="preamble"><h1>Apache Module mod_proxy_balancer</h1>
  23 <div class="toplang">
  24 <p><span>Available Languages: </span><a href="../en/mod/mod_proxy_balancer.html" title="English">&nbsp;en&nbsp;</a> |
  25 <a href="../ja/mod/mod_proxy_balancer.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a></p>
  26 </div>
  27 <table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> extension for
  28 load balancing </td></tr>
  29 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
  30 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>proxy_balancer_module</td></tr>
  31 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>proxy_balancer.c</td></tr>
  32 <tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.1 and later</td></tr></table>
  33 <h3>Summary</h3>
  34
  35     <p>This module <em>requires</em> the service of <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>. It provides load balancing support for
  36     <code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols
  37     </p>
  38
  39     <p>Thus, in order to get the ability of load balancing,
  40     <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> and <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>
  41     have to be present in the server.</p>
  42
  43     <div class="warning"><h3>Warning</h3>
  44       <p>Do not enable proxying until you have <a href="mod_proxy.html#access">secured your server</a>. Open proxy
  45       servers are dangerous both to your network and to the Internet at
  46       large.</p>
  47     </div>
  48 </div>
  49 <div id="quickview"><h3 class="directives">Directives</h3>
  50 <p>This module provides no
  51             directives.</p>
  52 <h3>Topics</h3>
  53 <ul id="topics">
  54 <li><img alt="" src="../images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li>
  55 <li><img alt="" src="../images/down.gif" /> <a href="#enable">Enabling Balancer Manager Support</a></li>
  56 </ul><h3>See also</h3>
  57 <ul class="seealso">
  58 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
  59 </ul></div>
  60 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
  61 <div class="section">
  62 <h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2>
  63
  64     <p>The idea behind this scheduler is the following:</p>
  65
  66     <p><dfn>lbfactor</dfn> is <em>how much we expect this worker
  67     to work</em>, or <em>the workers's work quota</em>.</p>
  68
  69     <p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work
  70     to fulfill its quota of work</em>.</p>
  71
  72     <p>The <dfn>worker</dfn> is a member of the load balancer,
  73     usually a remote host serving one of the supported protocols.</p>
  74
  75     <p>We distribute each worker's work quota to the worker, and then look
  76     which of them needs to work most urgently (biggest lbstatus).  This
  77     worker is then selected for work, and its lbstatus reduced by the
  78     total work quota we distributed to all workers.  Thus the sum of all
  79     lbstatus does not change.(*)</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> ...</p>
 139
 140     <p>The following asymmetric configuration works as one would expect:</p>
 141
 142     <table><tr><th>worker</th>
 143         <th class="data">a</th>
 144         <th class="data">b</th></tr>
 145 <tr><th>lbfactor</th>
 146         <td class="data">70</td>
 147         <td class="data">30</td></tr>
 148 <tr><td class="data" colspan="2">&nbsp;</td></tr>
 149 <tr><th>lbstatus</th>
 150         <td class="data"><em>-30</em></td>
 151         <td class="data">30</td></tr>
 152 <tr><th>lbstatus</th>
 153         <td class="data">40</td>
 154         <td class="data"><em>-40</em></td></tr>
 155 <tr><th>lbstatus</th>
 156         <td class="data"><em>10</em></td>
 157         <td class="data">-10</td></tr>
 158 <tr><th>lbstatus</th>
 159         <td class="data"><em>-20</em></td>
 160         <td class="data">20</td></tr>
 161 <tr><th>lbstatus</th>
 162         <td class="data"><em>-50</em></td>
 163         <td class="data">50</td></tr>
 164 <tr><th>lbstatus</th>
 165         <td class="data">20</td>
 166         <td class="data"><em>-20</em></td></tr>
 167 <tr><th>lbstatus</th>
 168         <td class="data"><em>-10</em></td>
 169         <td class="data">10</td></tr>
 170 <tr><th>lbstatus</th>
 171         <td class="data"><em>-40</em></td>
 172         <td class="data">40</td></tr>
 173 <tr><th>lbstatus</th>
 174         <td class="data">30</td>
 175         <td class="data"><em>-30</em></td></tr>
 176 <tr><th>lbstatus</th>
 177         <td class="data"><em>0</em></td>
 178         <td class="data">0</td></tr>
 179 <tr><td class="data" colspan="3">(repeat)</td></tr>
 180 </table>
 181
 182     <p>That is after 10 schedules, the schedule repeats and 7 <var>a</var>
 183     are selected with 3 <var>b</var> interspersed.</p>
 184 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 185 <div class="section">
 186 <h2><a name="enable" id="enable">Enabling Balancer Manager Support</a></h2>
 187
 188     <p>This module <em>requires</em> the service of
 189     <code class="module"><a href="../mod/mod_status.html">mod_status</a></code>.
 190     Balancer manager enables dynamic update of balancer
 191     members. You can use balancer manager to change the balance
 192     factor or a particular member, or put it in the off line
 193     mode.
 194     </p>
 195
 196     <p>Thus, in order to get the ability of load balancer management,
 197     <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> and <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>
 198     have to be present in the server.</p>
 199
 200     <p>To enable load balancer management for browsers from the foo.com
 201     domain add this code to your <code>httpd.conf</code>
 202     configuration file</p>
 203 <div class="example"><p><code>
 204     &lt;Location /balancer-manager&gt;<br />
 205     SetHandler balancer-manager<br />
 206 <br />
 207     Order Deny,Allow<br />
 208     Deny from all<br />
 209     Allow from .foo.com<br />
 210     &lt;/Location&gt;
 211 </code></p></div>
 212
 213     <p>You can now access load balancer manager by using a Web browser
 214     to access the page
 215     <code>http://your.server.name/balancer-manager</code></p>
 216 </div>
 217 </div>
 218 <div class="bottomlang">
 219 <p><span>Available Languages: </span><a href="../en/mod/mod_proxy_balancer.html" title="English">&nbsp;en&nbsp;</a> |
 220 <a href="../ja/mod/mod_proxy_balancer.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a></p>
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 222 <p class="apache">Copyright 1995-2005 The Apache Software Foundation or its licensors, as applicable.<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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