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26 <div id="page-content">
27 <div id="preamble"><h1>Apache Module mod_lbmethod_byrequests</h1>
29 <p><span>Available Languages: </span><a href="../en/mod/mod_lbmethod_byrequests.html" title="English"> en </a></p>
31 <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>
32 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
33 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>lbmethod_byrequests_module</td></tr>
34 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_lbmethod_byrequests.c</td></tr>
35 <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>
38 <p>This module does not provide any configuration directives of its own.
39 It requires the services of <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>, and
40 provides the <code>byrequests</code> load balancing method..</p>
42 <div id="quickview"><h3>Topics</h3>
44 <li><img alt="" src="../images/down.gif" /> <a href="#requests">Request Counting Algorithm</a></li>
45 </ul><h3 class="directives">Directives</h3>
46 <p>This module provides no
50 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
51 <li><code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code></li>
52 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
53 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
55 <h2><a name="requests" id="requests">Request Counting Algorithm</a></h2>
57 <p>Enabled via <code>lbmethod=byrequests</code>, the idea behind this
58 scheduler is that we distribute the requests among the
59 various workers to ensure that each gets their configured share
60 of the number of requests. It works as follows:</p>
62 <p><dfn>lbfactor</dfn> is <em>how much we expect this worker
63 to work</em>, or <em>the workers' work quota</em>. This is
64 a normalized value representing their "share" of the amount of
67 <p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work
68 to fulfill its quota of work</em>.</p>
70 <p>The <dfn>worker</dfn> is a member of the load balancer,
71 usually a remote host serving one of the supported protocols.</p>
73 <p>We distribute each worker's work quota to the worker, and then look
74 which of them needs to work most urgently (biggest lbstatus). This
75 worker is then selected for work, and its lbstatus reduced by the
76 total work quota we distributed to all workers. Thus the sum of all
77 lbstatus does not change(*) and we distribute the requests
80 <p>If some workers are disabled, the others will
81 still be scheduled correctly.</p>
83 <div class="example"><pre><code>for each worker in workers
84 worker lbstatus += worker lbfactor
85 total factor += worker lbfactor
86 if worker lbstatus > candidate lbstatus
89 candidate lbstatus -= total factor</code></pre></div>
91 <p>If a balancer is configured as follows:</p>
93 <table><tr><th>worker</th>
94 <th class="data">a</th>
95 <th class="data">b</th>
96 <th class="data">c</th>
97 <th class="data">d</th></tr>
99 <td class="data">25</td>
100 <td class="data">25</td>
101 <td class="data">25</td>
102 <td class="data">25</td></tr>
103 <tr><th>lbstatus</th>
104 <td class="data">0</td>
105 <td class="data">0</td>
106 <td class="data">0</td>
107 <td class="data">0</td></tr>
110 <p>And <var>b</var> gets disabled, the following schedule is produced:</p>
112 <table><tr><th>worker</th>
113 <th class="data">a</th>
114 <th class="data">b</th>
115 <th class="data">c</th>
116 <th class="data">d</th></tr>
117 <tr><th>lbstatus</th>
118 <td class="data"><em>-50</em></td>
119 <td class="data">0</td>
120 <td class="data">25</td>
121 <td class="data">25</td></tr>
122 <tr><th>lbstatus</th>
123 <td class="data">-25</td>
124 <td class="data">0</td>
125 <td class="data"><em>-25</em></td>
126 <td class="data">50</td></tr>
127 <tr><th>lbstatus</th>
128 <td class="data">0</td>
129 <td class="data">0</td>
130 <td class="data">0</td>
131 <td class="data"><em>0</em></td></tr>
132 <tr><td class="data" colspan="5">(repeat)</td></tr>
135 <p>That is it schedules: <var>a</var> <var>c</var> <var>d</var>
136 <var>a</var> <var>c</var> <var>d</var> <var>a</var> <var>c</var>
137 <var>d</var> ... Please note that:</p>
139 <table><tr><th>worker</th>
140 <th class="data">a</th>
141 <th class="data">b</th>
142 <th class="data">c</th>
143 <th class="data">d</th></tr>
144 <tr><th>lbfactor</th>
145 <td class="data">25</td>
146 <td class="data">25</td>
147 <td class="data">25</td>
148 <td class="data">25</td></tr>
151 <p>Has the exact same behavior as:</p>
153 <table><tr><th>worker</th>
154 <th class="data">a</th>
155 <th class="data">b</th>
156 <th class="data">c</th>
157 <th class="data">d</th></tr>
158 <tr><th>lbfactor</th>
159 <td class="data">1</td>
160 <td class="data">1</td>
161 <td class="data">1</td>
162 <td class="data">1</td></tr>
165 <p>This is because all values of <dfn>lbfactor</dfn> are normalized
166 with respect to the others. For:</p>
168 <table><tr><th>worker</th>
169 <th class="data">a</th>
170 <th class="data">b</th>
171 <th class="data">c</th></tr>
172 <tr><th>lbfactor</th>
173 <td class="data">1</td>
174 <td class="data">4</td>
175 <td class="data">1</td></tr>
178 <p>worker <var>b</var> will, on average, get 4 times the requests
179 that <var>a</var> and <var>c</var> will.</p>
181 <p>The following asymmetric configuration works as one would expect:</p>
183 <table><tr><th>worker</th>
184 <th class="data">a</th>
185 <th class="data">b</th></tr>
186 <tr><th>lbfactor</th>
187 <td class="data">70</td>
188 <td class="data">30</td></tr>
189 <tr><td class="data" colspan="2"> </td></tr>
190 <tr><th>lbstatus</th>
191 <td class="data"><em>-30</em></td>
192 <td class="data">30</td></tr>
193 <tr><th>lbstatus</th>
194 <td class="data">40</td>
195 <td class="data"><em>-40</em></td></tr>
196 <tr><th>lbstatus</th>
197 <td class="data"><em>10</em></td>
198 <td class="data">-10</td></tr>
199 <tr><th>lbstatus</th>
200 <td class="data"><em>-20</em></td>
201 <td class="data">20</td></tr>
202 <tr><th>lbstatus</th>
203 <td class="data"><em>-50</em></td>
204 <td class="data">50</td></tr>
205 <tr><th>lbstatus</th>
206 <td class="data">20</td>
207 <td class="data"><em>-20</em></td></tr>
208 <tr><th>lbstatus</th>
209 <td class="data"><em>-10</em></td>
210 <td class="data">10</td></tr>
211 <tr><th>lbstatus</th>
212 <td class="data"><em>-40</em></td>
213 <td class="data">40</td></tr>
214 <tr><th>lbstatus</th>
215 <td class="data">30</td>
216 <td class="data"><em>-30</em></td></tr>
217 <tr><th>lbstatus</th>
218 <td class="data"><em>0</em></td>
219 <td class="data">0</td></tr>
220 <tr><td class="data" colspan="3">(repeat)</td></tr>
223 <p>That is after 10 schedules, the schedule repeats and 7 <var>a</var>
224 are selected with 3 <var>b</var> interspersed.</p>
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