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| <modulesynopsis metafile="mod_proxy_balancer.xml.meta"> |
| |
| <name>mod_proxy_balancer</name> |
| <description><module>mod_proxy</module> extension for load balancing </description> |
| <status>Extension</status> |
| <sourcefile>proxy_balancer.c</sourcefile> |
| <identifier>proxy_balancer_module</identifier> |
| <compatibility>Available in version 2.1 and later</compatibility> |
| |
| <summary> |
| <p>This module <em>requires</em> the service of <module |
| >mod_proxy</module>. It provides load balancing support for |
| <code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols |
| </p> |
| |
| <p>Thus, in order to get the ability of load balancing, |
| <module>mod_proxy</module> and <module>mod_proxy_balancer</module> |
| have to be present in the server.</p> |
| |
| <note type="warning"><title>Warning</title> |
| <p>Do not enable proxying until you have <a |
| href="mod_proxy.html#access">secured your server</a>. Open proxy |
| servers are dangerous both to your network and to the Internet at |
| large.</p> |
| </note> |
| </summary> |
| <seealso><module>mod_proxy</module></seealso> |
| |
| <section id="scheduler"> |
| <title>Load balancer scheduler algorithm</title> |
| <p>At present, there are 2 load balancer scheduler algorithms available |
| for use: Request Counting and Weighted Traffic Counting. These are controlled |
| via the <code>lbmethod</code> value of the Balancer definition. See |
| the <directive module="mod_proxy">Proxy</directive> directive for |
| more information.</p> |
| |
| </section> |
| |
| <section id="requests"> |
| <title>Request Counting Algorithm</title> |
| <p>Enabled via <code>lbmethod=byrequests</code>, the idea behind this |
| scheduler is that we distribute the requests among the |
| various workers to ensure that each gets their configured share |
| of the number of requests. It works as follows:</p> |
| |
| <p><dfn>lbfactor</dfn> is <em>how much we expect this worker |
| to work</em>, or <em>the workers's work quota</em>. This is |
| a normalized value representing their "share" of the amount of |
| work to be done.</p> |
| |
| <p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work |
| to fulfill its quota of work</em>.</p> |
| |
| <p>The <dfn>worker</dfn> is a member of the load balancer, |
| usually a remote host serving one of the supported protocols.</p> |
| |
| <p>We distribute each worker's work quota to the worker, and then look |
| which of them needs to work most urgently (biggest lbstatus). This |
| worker is then selected for work, and its lbstatus reduced by the |
| total work quota we distributed to all workers. Thus the sum of all |
| lbstatus does not change(*) and we distribute the requests |
| as desired.</p> |
| |
| <p>If some workers are disabled, the others will |
| still be scheduled correctly.</p> |
| |
| <example><pre><code>for each worker in workers |
| worker lbstatus += worker lbfactor |
| total factor += worker lbfactor |
| if worker lbstatus > candidate lbstatus |
| candidate = worker |
| |
| candidate lbstatus -= total factor</code></pre> |
| </example> |
| |
| <p>If a balancer is configured as follows:</p> |
| |
| <table style="data"> |
| <tr><th>worker</th> |
| <th>a</th> |
| <th>b</th> |
| <th>c</th> |
| <th>d</th></tr> |
| <tr><th>lbfactor</th> |
| <td>25</td> |
| <td>25</td> |
| <td>25</td> |
| <td>25</td></tr> |
| <tr><th>lbstatus</th> |
| <td>0</td> |
| <td>0</td> |
| <td>0</td> |
| <td>0</td></tr> |
| </table> |
| |
| <p>And <var>b</var> gets disabled, the following schedule is produced:</p> |
| |
| <table style="data"> |
| <tr><th>worker</th> |
| <th>a</th> |
| <th>b</th> |
| <th>c</th> |
| <th>d</th></tr> |
| <tr><th>lbstatus</th> |
| <td><em>-50</em></td> |
| <td>0</td> |
| <td>25</td> |
| <td>25</td></tr> |
| <tr><th>lbstatus</th> |
| <td>-25</td> |
| <td>0</td> |
| <td><em>-25</em></td> |
| <td>50</td></tr> |
| <tr><th>lbstatus</th> |
| <td>0</td> |
| <td>0</td> |
| <td>0</td> |
| <td><em>0</em></td></tr> |
| <tr><td colspan="5">(repeat)</td></tr> |
| </table> |
| |
| <p>That is it schedules: <var>a</var> <var>c</var> <var>d</var> |
| <var>a</var> <var>c</var> <var>d</var> <var>a</var> <var>c</var> |
| <var>d</var> ... Please note that:</p> |
| |
| <table style="data"> |
| <tr><th>worker</th> |
| <th>a</th> |
| <th>b</th> |
| <th>c</th> |
| <th>d</th></tr> |
| <tr><th>lbfactor</th> |
| <td>25</td> |
| <td>25</td> |
| <td>25</td> |
| <td>25</td></tr> |
| </table> |
| |
| <p>Has the exact same behavior as:</p> |
| |
| <table style="data"> |
| <tr><th>worker</th> |
| <th>a</th> |
| <th>b</th> |
| <th>c</th> |
| <th>d</th></tr> |
| <tr><th>lbfactor</th> |
| <td>1</td> |
| <td>1</td> |
| <td>1</td> |
| <td>1</td></tr> |
| </table> |
| |
| <p>This is because all values of <dfn>lbfactor</dfn> are normalized |
| with respect to the others. For:</p> |
| |
| <table style="data"> |
| <tr><th>worker</th> |
| <th>a</th> |
| <th>b</th> |
| <th>c</th></tr> |
| <tr><th>lbfactor</th> |
| <td>1</td> |
| <td>4</td> |
| <td>1</td></tr> |
| </table> |
| |
| <p>worker <var>b</var> will, on average, get 4 times the requests |
| that <var>a</var> and <var>c</var> will.</p> |
| |
| <p>The following asymmetric configuration works as one would expect:</p> |
| |
| <table style="data"> |
| <tr><th>worker</th> |
| <th>a</th> |
| <th>b</th></tr> |
| <tr><th>lbfactor</th> |
| <td>70</td> |
| <td>30</td></tr> |
| <tr><td colspan="2"> </td></tr> |
| <tr><th>lbstatus</th> |
| <td><em>-30</em></td> |
| <td>30</td></tr> |
| <tr><th>lbstatus</th> |
| <td>40</td> |
| <td><em>-40</em></td></tr> |
| <tr><th>lbstatus</th> |
| <td><em>10</em></td> |
| <td>-10</td></tr> |
| <tr><th>lbstatus</th> |
| <td><em>-20</em></td> |
| <td>20</td></tr> |
| <tr><th>lbstatus</th> |
| <td><em>-50</em></td> |
| <td>50</td></tr> |
| <tr><th>lbstatus</th> |
| <td>20</td> |
| <td><em>-20</em></td></tr> |
| <tr><th>lbstatus</th> |
| <td><em>-10</em></td> |
| <td>10</td></tr> |
| <tr><th>lbstatus</th> |
| <td><em>-40</em></td> |
| <td>40</td></tr> |
| <tr><th>lbstatus</th> |
| <td>30</td> |
| <td><em>-30</em></td></tr> |
| <tr><th>lbstatus</th> |
| <td><em>0</em></td> |
| <td>0</td></tr> |
| <tr><td colspan="3">(repeat)</td></tr> |
| </table> |
| |
| <p>That is after 10 schedules, the schedule repeats and 7 <var>a</var> |
| are selected with 3 <var>b</var> interspersed.</p> |
| </section> |
| |
| <section id="traffic"> |
| <title>Weighted Traffic Counting Algorithm</title> |
| <p>Enabled via <code>lbmethod=bytraffic</code>, the idea behind this |
| scheduler is very similar to the Request Counting method, with |
| the following changes:</p> |
| |
| <p><dfn>lbfactor</dfn> is <em>how much traffic, in bytes, we want |
| this worker to handle</em>. This is also a normalized value |
| representing their "share" of the amount of work to be done, |
| but instead of simply counting the number of requests, we take |
| into account the amount of traffic this worker has seen.</p> |
| |
| <p>If a balancer is configured as follows:</p> |
| |
| <table style="data"> |
| <tr><th>worker</th> |
| <th>a</th> |
| <th>b</th> |
| <th>c</th></tr> |
| <tr><th>lbfactor</th> |
| <td>1</td> |
| <td>2</td> |
| <td>1</td></tr> |
| </table> |
| |
| <p>Then we mean that we want <var>b</var> to process twice the |
| amount of bytes than <var>a</var> or <var>c</var> should. It does |
| not necessarily mean that <var>b</var> would handle twice as |
| many requests, but it would process twice the I/O. Thus, the |
| size of the request and response are applied to the weighting |
| and selection algorithm.</p> |
| |
| </section> |
| |
| <section id="enable"> |
| <title>Enabling Balancer Manager Support</title> |
| <p>This module <em>requires</em> the service of |
| <module>mod_status</module>. |
| Balancer manager enables dynamic update of balancer |
| members. You can use balancer manager to change the balance |
| factor or a particular member, or put it in the off line |
| mode. |
| </p> |
| |
| <p>Thus, in order to get the ability of load balancer management, |
| <module>mod_status</module> and <module>mod_proxy_balancer</module> |
| have to be present in the server.</p> |
| |
| <p>To enable load balancer management for browsers from the foo.com |
| domain add this code to your <code>httpd.conf</code> |
| configuration file</p> |
| <example> |
| <Location /balancer-manager><br /> |
| SetHandler balancer-manager<br /> |
| <br /> |
| Order Deny,Allow<br /> |
| Deny from all<br /> |
| Allow from .foo.com<br /> |
| </Location> |
| </example> |
| |
| <p>You can now access load balancer manager by using a Web browser |
| to access the page |
| <code>http://your.server.name/balancer-manager</code></p> |
| </section> |
| |
| </modulesynopsis> |
