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