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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 load balancing </td></tr>
  28 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
  29 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>proxy_balancer_module</td></tr>
  30 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>proxy_balancer.c</td></tr>
  31 <tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.1 and later</td></tr></table>
  32 <h3>Summary</h3>
  33
  34     <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
  35     <code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols
  36     </p>
  37
  38     <p>Thus, in order to get the ability of load balancing,
  39     <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>
  40     have to be present in the server.</p>
  41
  42     <div class="warning"><h3>Warning</h3>
  43       <p>Do not enable proxying until you have <a href="mod_proxy.html#access">secured your server</a>. Open proxy
  44       servers are dangerous both to your network and to the Internet at
  45       large.</p>
  46     </div>
  47 </div>
  48 <div id="quickview"><h3 class="directives">Directives</h3>
  49 <p>This module provides no
  50             directives.</p>
  51 <h3>Topics</h3>
  52 <ul id="topics">
  53 <li><img alt="" src="../images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li>
  54 <li><img alt="" src="../images/down.gif" /> <a href="#requests">Request Counting Algorithm</a></li>
  55 <li><img alt="" src="../images/down.gif" /> <a href="#traffic">Weighted Traffic Counting Algorithm</a></li>
  56 <li><img alt="" src="../images/down.gif" /> <a href="#enable">Enabling Balancer Manager Support</a></li>
  57 </ul><h3>See also</h3>
  58 <ul class="seealso">
  59 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
  60 </ul></div>
  61 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
  62 <div class="section">
  63 <h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2>
  64
  65     <p>At present, there are 2 load balancer scheduler algorithms available
  66     for use: Request Counting and Weighted Traffic Counting. These are controlled
  67     via the <code>lbmethod</code> value of the Balancer definition. See
  68     the <code class="directive"><a href="../mod/mod_proxy.html#proxy">Proxy</a></code> directive for
  69     more information.</p>
  70
  71 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
  72 <div class="section">
  73 <h2><a name="requests" id="requests">Request Counting Algorithm</a></h2>
  74
  75     <p>Enabled via <code>lbmethod=byrequests</code>, the idea behind this
  76     scheduler is that we distribute the requests among the
  77     various workers to ensure that each gets their configured share
  78     of the number of requests. It works as follows:</p>
  79
  80     <p><dfn>lbfactor</dfn> is <em>how much we expect this worker
  81     to work</em>, or <em>the workers's work quota</em>. This is
  82     a normalized value representing their "share" of the amount of
  83     work to be done.</p>
  84
  85     <p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work
  86     to fulfill its quota of work</em>.</p>
  87
  88     <p>The <dfn>worker</dfn> is a member of the load balancer,
  89     usually a remote host serving one of the supported protocols.</p>
  90
  91     <p>We distribute each worker's work quota to the worker, and then look
  92     which of them needs to work most urgently (biggest lbstatus).  This
  93     worker is then selected for work, and its lbstatus reduced by the
  94     total work quota we distributed to all workers.  Thus the sum of all
  95     lbstatus does not change(*) and we distribute the requests
  96     as desired.</p>
  97
  98     <p>If some workers are disabled, the others will
  99     still be scheduled correctly.</p>
 100
 101     <div class="example"><pre><code>for each worker in workers
 102     worker lbstatus += worker lbfactor
 103     total factor    += worker lbfactor
 104     if worker lbstatus &gt; candidate lbstatus
 105         candidate = worker
 106
 107 candidate lbstatus -= total factor</code></pre></div>
 108
 109     <p>If a balancer is configured as follows:</p>
 110
 111     <table><tr><th>worker</th>
 112         <th class="data">a</th>
 113         <th class="data">b</th>
 114         <th class="data">c</th>
 115         <th class="data">d</th></tr>
 116 <tr><th>lbfactor</th>
 117         <td class="data">25</td>
 118         <td class="data">25</td>
 119         <td class="data">25</td>
 120         <td class="data">25</td></tr>
 121 <tr><th>lbstatus</th>
 122         <td class="data">0</td>
 123         <td class="data">0</td>
 124         <td class="data">0</td>
 125         <td class="data">0</td></tr>
 126 </table>
 127
 128     <p>And <var>b</var> gets disabled, the following schedule is produced:</p>
 129
 130     <table><tr><th>worker</th>
 131         <th class="data">a</th>
 132         <th class="data">b</th>
 133         <th class="data">c</th>
 134         <th class="data">d</th></tr>
 135 <tr><th>lbstatus</th>
 136         <td class="data"><em>-50</em></td>
 137         <td class="data">0</td>
 138         <td class="data">25</td>
 139         <td class="data">25</td></tr>
 140 <tr><th>lbstatus</th>
 141         <td class="data">-25</td>
 142         <td class="data">0</td>
 143         <td class="data"><em>-25</em></td>
 144         <td class="data">50</td></tr>
 145 <tr><th>lbstatus</th>
 146         <td class="data">0</td>
 147         <td class="data">0</td>
 148         <td class="data">0</td>
 149         <td class="data"><em>0</em></td></tr>
 150 <tr><td class="data" colspan="5">(repeat)</td></tr>
 151 </table>
 152
 153     <p>That is it schedules: <var>a</var> <var>c</var> <var>d</var>
 154     <var>a</var> <var>c</var> <var>d</var> <var>a</var> <var>c</var>
 155     <var>d</var> ... Please note that:</p>
 156
 157     <table><tr><th>worker</th>
 158         <th class="data">a</th>
 159         <th class="data">b</th>
 160         <th class="data">c</th>
 161         <th class="data">d</th></tr>
 162 <tr><th>lbfactor</th>
 163         <td class="data">25</td>
 164         <td class="data">25</td>
 165         <td class="data">25</td>
 166         <td class="data">25</td></tr>
 167 </table>
 168
 169     <p>Has the exact same behavior as:</p>
 170
 171     <table><tr><th>worker</th>
 172         <th class="data">a</th>
 173         <th class="data">b</th>
 174         <th class="data">c</th>
 175         <th class="data">d</th></tr>
 176 <tr><th>lbfactor</th>
 177         <td class="data">1</td>
 178         <td class="data">1</td>
 179         <td class="data">1</td>
 180         <td class="data">1</td></tr>
 181 </table>
 182
 183     <p>This is because all values of <dfn>lbfactor</dfn> are normalized
 184     with respect to the others. For:</p>
 185
 186     <table><tr><th>worker</th>
 187         <th class="data">a</th>
 188         <th class="data">b</th>
 189         <th class="data">c</th></tr>
 190 <tr><th>lbfactor</th>
 191         <td class="data">1</td>
 192         <td class="data">4</td>
 193         <td class="data">1</td></tr>
 194 </table>
 195
 196     <p>worker <var>b</var> will, on average, get 4 times the requests
 197     that <var>a</var> and <var>c</var> will.</p>
 198
 199     <p>The following asymmetric configuration works as one would expect:</p>
 200
 201     <table><tr><th>worker</th>
 202         <th class="data">a</th>
 203         <th class="data">b</th></tr>
 204 <tr><th>lbfactor</th>
 205         <td class="data">70</td>
 206         <td class="data">30</td></tr>
 207 <tr><td class="data" colspan="2">&nbsp;</td></tr>
 208 <tr><th>lbstatus</th>
 209         <td class="data"><em>-30</em></td>
 210         <td class="data">30</td></tr>
 211 <tr><th>lbstatus</th>
 212         <td class="data">40</td>
 213         <td class="data"><em>-40</em></td></tr>
 214 <tr><th>lbstatus</th>
 215         <td class="data"><em>10</em></td>
 216         <td class="data">-10</td></tr>
 217 <tr><th>lbstatus</th>
 218         <td class="data"><em>-20</em></td>
 219         <td class="data">20</td></tr>
 220 <tr><th>lbstatus</th>
 221         <td class="data"><em>-50</em></td>
 222         <td class="data">50</td></tr>
 223 <tr><th>lbstatus</th>
 224         <td class="data">20</td>
 225         <td class="data"><em>-20</em></td></tr>
 226 <tr><th>lbstatus</th>
 227         <td class="data"><em>-10</em></td>
 228         <td class="data">10</td></tr>
 229 <tr><th>lbstatus</th>
 230         <td class="data"><em>-40</em></td>
 231         <td class="data">40</td></tr>
 232 <tr><th>lbstatus</th>
 233         <td class="data">30</td>
 234         <td class="data"><em>-30</em></td></tr>
 235 <tr><th>lbstatus</th>
 236         <td class="data"><em>0</em></td>
 237         <td class="data">0</td></tr>
 238 <tr><td class="data" colspan="3">(repeat)</td></tr>
 239 </table>
 240
 241     <p>That is after 10 schedules, the schedule repeats and 7 <var>a</var>
 242     are selected with 3 <var>b</var> interspersed.</p>
 243 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 244 <div class="section">
 245 <h2><a name="traffic" id="traffic">Weighted Traffic Counting Algorithm</a></h2>
 246
 247     <p>Enabled via <code>lbmethod=bytraffic</code>, the idea behind this
 248     scheduler is very similar to the Request Counting method, with
 249     the following changes:</p>
 250
 251     <p><dfn>lbfactor</dfn> is <em>how much traffic, in bytes, we want
 252     this worker to handle</em>. This is also a normalized value
 253     representing their "share" of the amount of work to be done,
 254     but instead of simply counting the number of requests, we take
 255     into account the amount of traffic this worker has seen.</p>
 256
 257     <p>If a balancer is configured as follows:</p>
 258
 259     <table><tr><th>worker</th>
 260         <th class="data">a</th>
 261         <th class="data">b</th>
 262         <th class="data">c</th></tr>
 263 <tr><th>lbfactor</th>
 264         <td class="data">1</td>
 265         <td class="data">2</td>
 266         <td class="data">1</td></tr>
 267 </table>
 268
 269     <p>Then we mean that we want <var>b</var> to process twice the
 270     amount of bytes than <var>a</var> or <var>c</var> should. It does
 271     not necessarily mean that <var>b</var> would handle twice as
 272     many requests, but it would process twice the I/O. Thus, the
 273     size of the request and response are applied to the weighting
 274     and selection algorithm.</p>
 275
 276 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 277 <div class="section">
 278 <h2><a name="enable" id="enable">Enabling Balancer Manager Support</a></h2>
 279
 280     <p>This module <em>requires</em> the service of
 281     <code class="module"><a href="../mod/mod_status.html">mod_status</a></code>.
 282     Balancer manager enables dynamic update of balancer
 283     members. You can use balancer manager to change the balance
 284     factor or a particular member, or put it in the off line
 285     mode.
 286     </p>
 287
 288     <p>Thus, in order to get the ability of load balancer management,
 289     <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>
 290     have to be present in the server.</p>
 291
 292     <p>To enable load balancer management for browsers from the foo.com
 293     domain add this code to your <code>httpd.conf</code>
 294     configuration file</p>
 295 <div class="example"><p><code>
 296     &lt;Location /balancer-manager&gt;<br />
 297     SetHandler balancer-manager<br />
 298 <br />
 299     Order Deny,Allow<br />
 300     Deny from all<br />
 301     Allow from .foo.com<br />
 302     &lt;/Location&gt;
 303 </code></p></div>
 304
 305     <p>You can now access load balancer manager by using a Web browser
 306     to access the page
 307     <code>http://your.server.name/balancer-manager</code></p>
 308 </div>
 309 </div>
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 311 <p><span>Available Languages: </span><a href="../en/mod/mod_proxy_balancer.html" title="English">&nbsp;en&nbsp;</a> |
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 314 <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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