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376 lines
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376 lines
12 KiB
XML
<?xml version="1.0"?>
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<!DOCTYPE modulesynopsis SYSTEM "../style/modulesynopsis.dtd">
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<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
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<!-- $LastChangedRevision$ -->
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<!--
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<modulesynopsis metafile="mod_proxy_balancer.xml.meta">
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<name>mod_proxy_balancer</name>
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<description><module>mod_proxy</module> extension for load balancing </description>
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<status>Extension</status>
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<sourcefile>proxy_balancer.c</sourcefile>
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<identifier>proxy_balancer_module</identifier>
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<compatibility>Available in version 2.1 and later</compatibility>
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<summary>
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<p>This module <em>requires</em> the service of <module
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>mod_proxy</module>. It provides load balancing support for
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<code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols
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</p>
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<p>Thus, in order to get the ability of load balancing,
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<module>mod_proxy</module> and <module>mod_proxy_balancer</module>
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have to be present in the server.</p>
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<note type="warning"><title>Warning</title>
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<p>Do not enable proxying until you have <a
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href="mod_proxy.html#access">secured your server</a>. Open proxy
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servers are dangerous both to your network and to the Internet at
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large.</p>
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</note>
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</summary>
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<seealso><module>mod_proxy</module></seealso>
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<section id="scheduler">
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<title>Load balancer scheduler algorithm</title>
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<p>At present, there are 2 load balancer scheduler algorithms available
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for use: Request Counting and Weighted Traffic Counting. These are controlled
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via the <code>lbmethod</code> value of the Balancer definition. See
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the <directive module="mod_proxy">Proxy</directive> directive for
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more information.</p>
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</section>
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<section id="example">
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<title>Example of a balancer configuration</title>
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<p>Before we dive into the technical details, here's an example of
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how you might use <module>mod_proxy_balancer</module> to provide
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load balancing between two back-end servers:
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</p>
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<example>
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<Proxy balancer://mycluster><br />
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BalancerMember http://192.168.1.50:80<br />
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BalancerMember http://192.168.1.51:80<br />
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</Proxy><br />
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ProxyPass /test balancer://mycluster/
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</example>
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</section>
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<section id="requests">
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<title>Request Counting Algorithm</title>
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<p>Enabled via <code>lbmethod=byrequests</code>, the idea behind this
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scheduler is that we distribute the requests among the
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various workers to ensure that each gets their configured share
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of the number of requests. It works as follows:</p>
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<p><dfn>lbfactor</dfn> is <em>how much we expect this worker
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to work</em>, or <em>the workers's work quota</em>. This is
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a normalized value representing their "share" of the amount of
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work to be done.</p>
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<p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work
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to fulfill its quota of work</em>.</p>
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<p>The <dfn>worker</dfn> is a member of the load balancer,
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usually a remote host serving one of the supported protocols.</p>
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<p>We distribute each worker's work quota to the worker, and then look
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which of them needs to work most urgently (biggest lbstatus). This
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worker is then selected for work, and its lbstatus reduced by the
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total work quota we distributed to all workers. Thus the sum of all
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lbstatus does not change(*) and we distribute the requests
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as desired.</p>
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<p>If some workers are disabled, the others will
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still be scheduled correctly.</p>
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<example><pre><code>for each worker in workers
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worker lbstatus += worker lbfactor
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total factor += worker lbfactor
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if worker lbstatus > candidate lbstatus
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candidate = worker
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candidate lbstatus -= total factor</code></pre>
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</example>
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<p>If a balancer is configured as follows:</p>
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<table style="data">
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<tr><th>worker</th>
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<th>a</th>
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<th>b</th>
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<th>c</th>
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<th>d</th></tr>
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<tr><th>lbfactor</th>
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<td>25</td>
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<td>25</td>
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<td>25</td>
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<td>25</td></tr>
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<tr><th>lbstatus</th>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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<td>0</td></tr>
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</table>
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<p>And <var>b</var> gets disabled, the following schedule is produced:</p>
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<table style="data">
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<tr><th>worker</th>
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<th>a</th>
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<th>b</th>
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<th>c</th>
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<th>d</th></tr>
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<tr><th>lbstatus</th>
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<td><em>-50</em></td>
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<td>0</td>
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<td>25</td>
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<td>25</td></tr>
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<tr><th>lbstatus</th>
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<td>-25</td>
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<td>0</td>
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<td><em>-25</em></td>
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<td>50</td></tr>
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<tr><th>lbstatus</th>
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<td>0</td>
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<td>0</td>
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<td>0</td>
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<td><em>0</em></td></tr>
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<tr><td colspan="5">(repeat)</td></tr>
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</table>
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<p>That is it schedules: <var>a</var> <var>c</var> <var>d</var>
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<var>a</var> <var>c</var> <var>d</var> <var>a</var> <var>c</var>
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<var>d</var> ... Please note that:</p>
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<table style="data">
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<tr><th>worker</th>
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<th>a</th>
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<th>b</th>
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<th>c</th>
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<th>d</th></tr>
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<tr><th>lbfactor</th>
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<td>25</td>
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<td>25</td>
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<td>25</td>
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<td>25</td></tr>
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</table>
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<p>Has the exact same behavior as:</p>
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<table style="data">
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<tr><th>worker</th>
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<th>a</th>
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<th>b</th>
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<th>c</th>
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<th>d</th></tr>
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<tr><th>lbfactor</th>
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<td>1</td>
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<td>1</td>
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<td>1</td>
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<td>1</td></tr>
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</table>
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<p>This is because all values of <dfn>lbfactor</dfn> are normalized
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with respect to the others. For:</p>
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<table style="data">
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<tr><th>worker</th>
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<th>a</th>
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<th>b</th>
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<th>c</th></tr>
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<tr><th>lbfactor</th>
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<td>1</td>
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<td>4</td>
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<td>1</td></tr>
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</table>
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<p>worker <var>b</var> will, on average, get 4 times the requests
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that <var>a</var> and <var>c</var> will.</p>
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<p>The following asymmetric configuration works as one would expect:</p>
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<table style="data">
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<tr><th>worker</th>
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<th>a</th>
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<th>b</th></tr>
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<tr><th>lbfactor</th>
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<td>70</td>
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<td>30</td></tr>
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<tr><td colspan="2"> </td></tr>
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<tr><th>lbstatus</th>
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<td><em>-30</em></td>
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<td>30</td></tr>
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<tr><th>lbstatus</th>
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<td>40</td>
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<td><em>-40</em></td></tr>
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<tr><th>lbstatus</th>
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<td><em>10</em></td>
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<td>-10</td></tr>
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<tr><th>lbstatus</th>
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<td><em>-20</em></td>
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<td>20</td></tr>
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<tr><th>lbstatus</th>
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<td><em>-50</em></td>
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<td>50</td></tr>
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<tr><th>lbstatus</th>
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<td>20</td>
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<td><em>-20</em></td></tr>
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<tr><th>lbstatus</th>
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<td><em>-10</em></td>
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<td>10</td></tr>
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<tr><th>lbstatus</th>
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<td><em>-40</em></td>
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<td>40</td></tr>
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<tr><th>lbstatus</th>
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<td>30</td>
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<td><em>-30</em></td></tr>
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<tr><th>lbstatus</th>
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<td><em>0</em></td>
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<td>0</td></tr>
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<tr><td colspan="3">(repeat)</td></tr>
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</table>
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<p>That is after 10 schedules, the schedule repeats and 7 <var>a</var>
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are selected with 3 <var>b</var> interspersed.</p>
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</section>
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<section id="traffic">
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<title>Weighted Traffic Counting Algorithm</title>
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<p>Enabled via <code>lbmethod=bytraffic</code>, the idea behind this
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scheduler is very similar to the Request Counting method, with
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the following changes:</p>
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<p><dfn>lbfactor</dfn> is <em>how much traffic, in bytes, we want
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this worker to handle</em>. This is also a normalized value
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representing their "share" of the amount of work to be done,
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but instead of simply counting the number of requests, we take
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into account the amount of traffic this worker has seen.</p>
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<p>If a balancer is configured as follows:</p>
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<table style="data">
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<tr><th>worker</th>
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<th>a</th>
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<th>b</th>
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<th>c</th></tr>
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<tr><th>lbfactor</th>
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<td>1</td>
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<td>2</td>
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<td>1</td></tr>
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</table>
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<p>Then we mean that we want <var>b</var> to process twice the
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amount of bytes than <var>a</var> or <var>c</var> should. It does
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not necessarily mean that <var>b</var> would handle twice as
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many requests, but it would process twice the I/O. Thus, the
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size of the request and response are applied to the weighting
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and selection algorithm.</p>
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</section>
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<section id="environment">
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<title>Exported Environment Variables</title>
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<p>At present there are 6 environment variables exported:</p>
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<dl>
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<!-- ============= BALANCER_SESSION_STICKY =============== -->
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<dt><var><a name="balancer_session_sticky" id="balancer_session_sticky">BALANCER_SESSION_STICKY</a></var></dt>
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<dd>
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<p>This is assigned the <var>stickysession</var> value used in the current
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request. It is the cookie or parameter name used for sticky sessions</p>
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</dd>
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<!-- ============= BALANCER_SESSION_ROUTE ================ -->
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<dt><var><a name="balancer_session_route" id="balancer_session_route">BALANCER_SESSION_ROUTE</a></var></dt>
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<dd>
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<p>This is assigned the <var>route</var> parsed from the current
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request.</p>
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</dd>
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<!-- ============= BALANCER_NAME ========================= -->
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<dt><var><a name="balancer_name" id="balancer_name">BALANCER_NAME</a></var></dt>
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<dd>
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<p>This is assigned the name of the balancer used for the current
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request. The value is something like <code>balancer://foo</code>.</p>
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</dd>
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<!-- ============= BALANCER_WORKER_NAME ================== -->
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<dt><var><a name="balancer_worker_name" id="balancer_worker_name">BALANCER_WORKER_NAME</a></var></dt>
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<dd>
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<p>This is assigned the name of the worker used for the current request.
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The value is something like <code>http://hostA:1234</code>.</p>
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</dd>
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<!-- ============= BALANCER_WORKER_ROUTE ================= -->
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<dt><var><a name="balancer_worker_route" id="balancer_worker_route">BALANCER_WORKER_ROUTE</a></var></dt>
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<dd>
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<p>This is assigned the <var>route</var> of the worker that will be
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used for the current request.</p>
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</dd>
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<!-- ============= BALANCER_ROUTE_CHANGED ================= -->
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<dt><var><a name="balancer_route_changed" id="balancer_route_changed">BALANCER_ROUTE_CHANGED</a></var></dt>
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<dd>
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<p>This is set to 1 if the session route does not match the
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worker route (BALANCER_SESSION_ROUTE != BALANCER_WORKER_ROUTE) or the
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session does not yet have an established route. This can be used to
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determine when/if the client needs to be sent an updated route
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when sticky sessions are used.</p>
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</dd>
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</dl>
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</section>
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<section id="enable">
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<title>Enabling Balancer Manager Support</title>
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<p>This module <em>requires</em> the service of
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<module>mod_status</module>.
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Balancer manager enables dynamic update of balancer
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members. You can use balancer manager to change the balance
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factor or a particular member, or put it in the off line
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mode.
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</p>
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<p>Thus, in order to get the ability of load balancer management,
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<module>mod_status</module> and <module>mod_proxy_balancer</module>
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have to be present in the server.</p>
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<p>To enable load balancer management for browsers from the foo.com
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domain add this code to your <code>httpd.conf</code>
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configuration file</p>
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<example>
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<Location /balancer-manager><br />
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SetHandler balancer-manager<br />
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<br />
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Order Deny,Allow<br />
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Deny from all<br />
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Allow from .foo.com<br />
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</Location>
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</example>
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<p>You can now access load balancer manager by using a Web browser
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to access the page
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<code>http://your.server.name/balancer-manager</code></p>
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</section>
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</modulesynopsis>
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