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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">ProxyPass</directive> directive for

more information.</p>

</section>

<section id="example">

<title>Example of a balancer configuration</title>

<p>Before we dive into the technical details, here's an example of

how you might use <module>mod_proxy_balancer</module> to provide

load balancing between two back-end servers:

</p>

<example>

&lt;Proxy balancer://mycluster&gt;<br />

BalancerMember http://192.168.1.50:80<br />

BalancerMember http://192.168.1.51:80<br />

&lt;/Proxy&gt;<br />

ProxyPass /test balancer://mycluster/

</example>

</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">&nbsp;</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="environment">

<title>Exported Environment Variables</title>

<p>At present there are 6 environment variables exported:</p>

<dl>

<!-- ============= BALANCER_SESSION_STICKY =============== -->

<dt><var><a name="balancer_session_sticky" id="balancer_session_sticky">BALANCER_SESSION_STICKY</a></var></dt>

<dd>

<p>This is assigned the <var>stickysession</var> value used in the current

request. It is the cookie or parameter name used for sticky sessions</p>

</dd>

<!-- ============= BALANCER_SESSION_ROUTE ================ -->

<dt><var><a name="balancer_session_route" id="balancer_session_route">BALANCER_SESSION_ROUTE</a></var></dt>

<dd>

<p>This is assigned the <var>route</var> parsed from the current

request.</p>

</dd>

<!-- ============= BALANCER_NAME ========================= -->

<dt><var><a name="balancer_name" id="balancer_name">BALANCER_NAME</a></var></dt>

<dd>

<p>This is assigned the name of the balancer used for the current

request. The value is something like <code>balancer://foo</code>.</p>

</dd>

<!-- ============= BALANCER_WORKER_NAME ================== -->

<dt><var><a name="balancer_worker_name" id="balancer_worker_name">BALANCER_WORKER_NAME</a></var></dt>

<dd>

<p>This is assigned the name of the worker used for the current request.

The value is something like <code>http://hostA:1234</code>.</p>

</dd>

<!-- ============= BALANCER_WORKER_ROUTE ================= -->

<dt><var><a name="balancer_worker_route" id="balancer_worker_route">BALANCER_WORKER_ROUTE</a></var></dt>

<dd>

<p>This is assigned the <var>route</var> of the worker that will be

used for the current request.</p>

</dd>

<!-- ============= BALANCER_ROUTE_CHANGED ================= -->

<dt><var><a name="balancer_route_changed" id="balancer_route_changed">BALANCER_ROUTE_CHANGED</a></var></dt>

<dd>

<p>This is set to 1 if the session route does not match the

worker route (BALANCER_SESSION_ROUTE != BALANCER_WORKER_ROUTE) or the

session does not yet have an established route. This can be used to

determine when/if the client needs to be sent an updated route

when sticky sessions are used.</p>

</dd>

</dl>

</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 example.com

domain add this code to your <code>httpd.conf</code>

configuration file</p>

<example>

&lt;Location /balancer-manager&gt;<br />

SetHandler balancer-manager<br />

<br />

Order Deny,Allow<br />

Deny from all<br />

Allow from .example.com<br />

&lt;/Location&gt;

</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>