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16 <p class="apache">Apache HTTP Server Version 2.5</p>
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21 <div id="page-content">
22 <div id="preamble"><h1>Apache Module mod_lbmethod_byrequests</h1>
24 <p><span>Available Languages: </span><a href="../en/mod/mod_lbmethod_byrequests.html" title="English"> en </a></p>
26 <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>
27 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
28 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>lbmethod_byrequests_module</td></tr>
29 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_lbmethod_byrequests.c</td></tr>
30 <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>
33 <p>This module does not provide any configuration directives of its own.
34 It requires the services of <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>, and
35 provides the <code>byrequests</code> load balancing method..</p>
37 <div id="quickview"><h3 class="directives">Directives</h3>
38 <p>This module provides no
42 <li><img alt="" src="../images/down.gif" /> <a href="#requests">Request Counting Algorithm</a></li>
43 </ul><h3>See also</h3>
45 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
46 <li><code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code></li>
48 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
50 <h2><a name="requests" id="requests">Request Counting Algorithm</a></h2>
52 <p>Enabled via <code>lbmethod=byrequests</code>, the idea behind this
53 scheduler is that we distribute the requests among the
54 various workers to ensure that each gets their configured share
55 of the number of requests. It works as follows:</p>
57 <p><dfn>lbfactor</dfn> is <em>how much we expect this worker
58 to work</em>, or <em>the workers's work quota</em>. This is
59 a normalized value representing their "share" of the amount of
62 <p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work
63 to fulfill its quota of work</em>.</p>
65 <p>The <dfn>worker</dfn> is a member of the load balancer,
66 usually a remote host serving one of the supported protocols.</p>
68 <p>We distribute each worker's work quota to the worker, and then look
69 which of them needs to work most urgently (biggest lbstatus). This
70 worker is then selected for work, and its lbstatus reduced by the
71 total work quota we distributed to all workers. Thus the sum of all
72 lbstatus does not change(*) and we distribute the requests
75 <p>If some workers are disabled, the others will
76 still be scheduled correctly.</p>
78 <div class="example"><pre><code>for each worker in workers
79 worker lbstatus += worker lbfactor
80 total factor += worker lbfactor
81 if worker lbstatus > candidate lbstatus
84 candidate lbstatus -= total factor</code></pre></div>
86 <p>If a balancer is configured as follows:</p>
88 <table><tr><th>worker</th>
89 <th class="data">a</th>
90 <th class="data">b</th>
91 <th class="data">c</th>
92 <th class="data">d</th></tr>
94 <td class="data">25</td>
95 <td class="data">25</td>
96 <td class="data">25</td>
97 <td class="data">25</td></tr>
99 <td class="data">0</td>
100 <td class="data">0</td>
101 <td class="data">0</td>
102 <td class="data">0</td></tr>
105 <p>And <var>b</var> gets disabled, the following schedule is produced:</p>
107 <table><tr><th>worker</th>
108 <th class="data">a</th>
109 <th class="data">b</th>
110 <th class="data">c</th>
111 <th class="data">d</th></tr>
112 <tr><th>lbstatus</th>
113 <td class="data"><em>-50</em></td>
114 <td class="data">0</td>
115 <td class="data">25</td>
116 <td class="data">25</td></tr>
117 <tr><th>lbstatus</th>
118 <td class="data">-25</td>
119 <td class="data">0</td>
120 <td class="data"><em>-25</em></td>
121 <td class="data">50</td></tr>
122 <tr><th>lbstatus</th>
123 <td class="data">0</td>
124 <td class="data">0</td>
125 <td class="data">0</td>
126 <td class="data"><em>0</em></td></tr>
127 <tr><td class="data" colspan="5">(repeat)</td></tr>
130 <p>That is it schedules: <var>a</var> <var>c</var> <var>d</var>
131 <var>a</var> <var>c</var> <var>d</var> <var>a</var> <var>c</var>
132 <var>d</var> ... Please note that:</p>
134 <table><tr><th>worker</th>
135 <th class="data">a</th>
136 <th class="data">b</th>
137 <th class="data">c</th>
138 <th class="data">d</th></tr>
139 <tr><th>lbfactor</th>
140 <td class="data">25</td>
141 <td class="data">25</td>
142 <td class="data">25</td>
143 <td class="data">25</td></tr>
146 <p>Has the exact same behavior as:</p>
148 <table><tr><th>worker</th>
149 <th class="data">a</th>
150 <th class="data">b</th>
151 <th class="data">c</th>
152 <th class="data">d</th></tr>
153 <tr><th>lbfactor</th>
154 <td class="data">1</td>
155 <td class="data">1</td>
156 <td class="data">1</td>
157 <td class="data">1</td></tr>
160 <p>This is because all values of <dfn>lbfactor</dfn> are normalized
161 with respect to the others. For:</p>
163 <table><tr><th>worker</th>
164 <th class="data">a</th>
165 <th class="data">b</th>
166 <th class="data">c</th></tr>
167 <tr><th>lbfactor</th>
168 <td class="data">1</td>
169 <td class="data">4</td>
170 <td class="data">1</td></tr>
173 <p>worker <var>b</var> will, on average, get 4 times the requests
174 that <var>a</var> and <var>c</var> will.</p>
176 <p>The following asymmetric configuration works as one would expect:</p>
178 <table><tr><th>worker</th>
179 <th class="data">a</th>
180 <th class="data">b</th></tr>
181 <tr><th>lbfactor</th>
182 <td class="data">70</td>
183 <td class="data">30</td></tr>
184 <tr><td class="data" colspan="2"> </td></tr>
185 <tr><th>lbstatus</th>
186 <td class="data"><em>-30</em></td>
187 <td class="data">30</td></tr>
188 <tr><th>lbstatus</th>
189 <td class="data">40</td>
190 <td class="data"><em>-40</em></td></tr>
191 <tr><th>lbstatus</th>
192 <td class="data"><em>10</em></td>
193 <td class="data">-10</td></tr>
194 <tr><th>lbstatus</th>
195 <td class="data"><em>-20</em></td>
196 <td class="data">20</td></tr>
197 <tr><th>lbstatus</th>
198 <td class="data"><em>-50</em></td>
199 <td class="data">50</td></tr>
200 <tr><th>lbstatus</th>
201 <td class="data">20</td>
202 <td class="data"><em>-20</em></td></tr>
203 <tr><th>lbstatus</th>
204 <td class="data"><em>-10</em></td>
205 <td class="data">10</td></tr>
206 <tr><th>lbstatus</th>
207 <td class="data"><em>-40</em></td>
208 <td class="data">40</td></tr>
209 <tr><th>lbstatus</th>
210 <td class="data">30</td>
211 <td class="data"><em>-30</em></td></tr>
212 <tr><th>lbstatus</th>
213 <td class="data"><em>0</em></td>
214 <td class="data">0</td></tr>
215 <tr><td class="data" colspan="3">(repeat)</td></tr>
218 <p>That is after 10 schedules, the schedule repeats and 7 <var>a</var>
219 are selected with 3 <var>b</var> interspersed.</p>
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225 <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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