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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.3</a> > <a href="./">Modules</a></div>
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<div id="preamble"><h1>Apache Module mod_proxy_balancer</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_proxy_balancer.html" title="English"> en </a> |
<a href="../ja/mod/mod_proxy_balancer.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p>
</div>
<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> extension for load balancing </td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module�Identifier:</a></th><td>proxy_balancer_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source�File:</a></th><td>mod_proxy_balancer.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.1 and later</td></tr></table>
<h3>Summary</h3>
<p>This module <em>requires</em> the service of <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>. It provides load balancing support for
<code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols
</p>
<p>Load balancing scheduler algorithm is provided by not this
module but other modules such as:
<code class="module"><a href="../mod/mod_lbmethod_byrequests.html">mod_lbmethod_byrequests</a></code>,
<code class="module"><a href="../mod/mod_lbmethod_bytraffic.html">mod_lbmethod_bytraffic</a></code> and
<code class="module"><a href="../mod/mod_lbmethod_bybusyness.html">mod_lbmethod_bybusyness</a></code>.
</p>
<p>Thus, in order to get the ability of load balancing,
<code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>, <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>
and at least one of load balancing scheduler algorithm modules have
to be present in the server.</p>
<div class="warning"><h3>Warning</h3>
<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>
</div>
</div>
<div id="quickview"><h3 class="directives">Directives</h3>
<p>This module provides no
directives.</p>
<h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#example">Examples of a balancer configuration</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#environment">Exported Environment Variables</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#enable">Enabling Balancer Manager Support</a></li>
</ul><h3>See also</h3>
<ul class="seealso">
<li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2>
<p>At present, there are 3 load balancer scheduler algorithms available
for use: Request Counting, Weighted Traffic Counting and Pending Request
Counting. These are controlled via the <code>lbmethod</code> value of
the Balancer definition. See the <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code>
directive for more information.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="example" id="example">Examples of a balancer configuration</a></h2>
<p>Before we dive into the technical details, here's an example of
how you might use <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> to provide
load balancing between two back-end servers:
</p>
<div class="example"><p><code>
<Proxy balancer://mycluster><br />
BalancerMember http://192.168.1.50:80<br />
BalancerMember http://192.168.1.51:80<br />
</Proxy><br />
ProxyPass /test balancer://mycluster/
</code></p></div>
<p>Another example of how to provide load balancing with stickyness
using <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code>, even if the backend server does
not set a suitable session cookie:
</p>
<div class="example"><p><code>
Header add Set-Cookie: "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/"
env=BALANCER_ROUTE_CHANGED<br />
<Proxy balancer://mycluster><br />
BalancerMember http://192.168.1.50:80 route=1<br />
BalancerMember http://192.168.1.51:80 route=2<br />
ProxySet stickysession=ROUTEID<br />
</Proxy><br />
ProxyPass /test balancer://mycluster/
</code></p></div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="environment" id="environment">Exported Environment Variables</a></h2>
<p>At present there are 6 environment variables exported:</p>
<dl>
<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>
<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>
<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>
<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>
<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>
<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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="enable" id="enable">Enabling Balancer Manager Support</a></h2>
<p>This module <em>requires</em> the service of
<code class="module"><a href="../mod/mod_status.html">mod_status</a></code>.
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,
<code class="module"><a href="../mod/mod_status.html">mod_status</a></code> and <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>
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>
<div class="example"><p><code>
<Location /balancer-manager><br />
SetHandler balancer-manager<br />
<br />
Order Deny,Allow<br />
Deny from all<br />
Allow from .example.com<br />
</Location>
</code></p></div>
<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>
</div>
</div>
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