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<div class="retired"><h4>Please note</h4>
<p> This document refers to a legacy release (<strong>2.2</strong>) of Apache httpd. The active release (<strong>2.4</strong>) is documented <a href="http://httpd.apache.org/docs/current">here</a>. If you have not already upgraded, please follow <a href="http://httpd.apache.org/docs/current/upgrading.html">this link</a> for more information.</p>
<p>You may follow <a href="http://httpd.apache.org/docs/current/mod/mod_proxy_balancer.html">this link</a> to go to the current version of this document.</p></div><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>
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<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>Thus, in order to get the ability of load balancing,
<code class="module"><a href="../mod/mod_proxy.html">mod_proxy</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>
<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>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="#stickyness">Load balancer stickyness</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="#requests">Request Counting Algorithm</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#traffic">Weighted Traffic Counting Algorithm</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#busyness">Pending Request Counting Algorithm</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="#balancer_manager">Enabling Balancer Manager Support</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#stickyness_implementation">Details on load balancer stickyness</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></li>
</ul><h3 class="directives">Directives</h3>
<p>This module provides no
directives.</p>
<h3>See also</h3>
<ul class="seealso">
<li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></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="stickyness" id="stickyness">Load balancer stickyness</a></h2>
<p>The balancer supports stickyness. When a request is proxied
to some back-end, then all following requests from the same user
should be proxied to the same back-end. Many load balancers implement
this feature via a table that maps client IP addresses to back-ends.
This approach is transparent to clients and back-ends, but suffers
from some problems: unequal load distribution if clients are themselves
hidden behind proxies, stickyness errors when a client uses a dynamic
IP address that changes during a session and loss of stickyness, if the
mapping table overflows.</p>
<p>The module <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> implements stickyness
on top of two alternative means: cookies and URL encoding. Providing the
cookie can be either done by the back-end or by the Apache web server
itself. The URL encoding is usually done on the back-end.</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 back-end 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="requests" id="requests">Request Counting Algorithm</a></h2>
<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>
<div class="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></div>
<p>If a balancer is configured as follows:</p>
<table><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th>
<th class="data">c</th>
<th class="data">d</th></tr>
<tr><th>lbfactor</th>
<td class="data">25</td>
<td class="data">25</td>
<td class="data">25</td>
<td class="data">25</td></tr>
<tr><th>lbstatus</th>
<td class="data">0</td>
<td class="data">0</td>
<td class="data">0</td>
<td class="data">0</td></tr>
</table>
<p>And <var>b</var> gets disabled, the following schedule is produced:</p>
<table><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th>
<th class="data">c</th>
<th class="data">d</th></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-50</em></td>
<td class="data">0</td>
<td class="data">25</td>
<td class="data">25</td></tr>
<tr><th>lbstatus</th>
<td class="data">-25</td>
<td class="data">0</td>
<td class="data"><em>-25</em></td>
<td class="data">50</td></tr>
<tr><th>lbstatus</th>
<td class="data">0</td>
<td class="data">0</td>
<td class="data">0</td>
<td class="data"><em>0</em></td></tr>
<tr><td class="data" 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><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th>
<th class="data">c</th>
<th class="data">d</th></tr>
<tr><th>lbfactor</th>
<td class="data">25</td>
<td class="data">25</td>
<td class="data">25</td>
<td class="data">25</td></tr>
</table>
<p>Has the exact same behavior as:</p>
<table><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th>
<th class="data">c</th>
<th class="data">d</th></tr>
<tr><th>lbfactor</th>
<td class="data">1</td>
<td class="data">1</td>
<td class="data">1</td>
<td class="data">1</td></tr>
</table>
<p>This is because all values of <dfn>lbfactor</dfn> are normalized
with respect to the others. For:</p>
<table><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th>
<th class="data">c</th></tr>
<tr><th>lbfactor</th>
<td class="data">1</td>
<td class="data">4</td>
<td class="data">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><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th></tr>
<tr><th>lbfactor</th>
<td class="data">70</td>
<td class="data">30</td></tr>
<tr><td class="data" colspan="2"> </td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-30</em></td>
<td class="data">30</td></tr>
<tr><th>lbstatus</th>
<td class="data">40</td>
<td class="data"><em>-40</em></td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>10</em></td>
<td class="data">-10</td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-20</em></td>
<td class="data">20</td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-50</em></td>
<td class="data">50</td></tr>
<tr><th>lbstatus</th>
<td class="data">20</td>
<td class="data"><em>-20</em></td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-10</em></td>
<td class="data">10</td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>-40</em></td>
<td class="data">40</td></tr>
<tr><th>lbstatus</th>
<td class="data">30</td>
<td class="data"><em>-30</em></td></tr>
<tr><th>lbstatus</th>
<td class="data"><em>0</em></td>
<td class="data">0</td></tr>
<tr><td class="data" 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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="traffic" id="traffic">Weighted Traffic Counting Algorithm</a></h2>
<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><tr><th>worker</th>
<th class="data">a</th>
<th class="data">b</th>
<th class="data">c</th></tr>
<tr><th>lbfactor</th>
<td class="data">1</td>
<td class="data">2</td>
<td class="data">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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="busyness" id="busyness">Pending Request Counting Algorithm</a></h2>
<p>Enabled via <code>lbmethod=bybusyness</code>, this scheduler keeps
track of how many requests each worker is assigned at present. A new
request is automatically assigned to the worker with the lowest
number of active requests. This is useful in the case of workers
that queue incoming requests independently of Apache, to ensure that
queue length stays even and a request is always given to the worker
most likely to service it fastest.</p>
<p>In the case of multiple least-busy workers, the statistics (and
weightings) used by the Request Counting method are used to break the
tie. Over time, the distribution of work will come to resemble that
characteristic of <code>byrequests</code>.</p>
<p>This algorithm is available in Apache HTTP Server 2.2.10 and later.</p>
</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 for the current
request. It is the name of the cookie or request parameter 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="balancer_manager" id="balancer_manager">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 of 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 class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="stickyness_implementation" id="stickyness_implementation">Details on load balancer stickyness</a></h2>
<p>When using cookie based stickyness, you need to configure the
name of the cookie that contains the information about which back-end
to use. This is done via the <var>stickysession</var> attribute added
to either <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
<code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. The name of
the cookie is case-sensitive. The balancer extracts the value of the
cookie and looks for a member worker with <var>route</var> equal
to that value. The <var>route</var> must also be set in either
<code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
<code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. The cookie can either
be set by the back-end, or as shown in the above
<a href="#example">example</a> by the Apache web server itself.</p>
<p>Some back-ends use a slightly different form of stickyness cookie,
for instance Apache Tomcat. Tomcat adds the name of the Tomcat instance
to the end of its session id cookie, separated with a dot (<code>.</code>)
from the session id. Thus if the Apache web server finds a dot in the value
of the stickyness cookie, it only uses the part behind the dot to search
for the route. In order to let Tomcat know about its instance name, you
need to set the attribute <code>jvmRoute</code> inside the Tomcat
configuration file <code>conf/server.xml</code> to the value of the
<var>route</var> of the worker that connects to the respective Tomcat.
The name of the session cookie used by Tomcat (and more generally by Java
web applications based on servlets) is <code>JSESSIONID</code>
(upper case) but can be configured to something else.</p>
<p>The second way of implementing stickyness is URL encoding.
The web server searches for a query parameter in the URL of the request.
The name of the parameter is specified again using <var>stickysession</var>.
The value of the parameter is used to lookup a member worker with <var>route</var>
equal to that value. Since it is not easy to extract and manipulate all
URL links contained in responses, generally the work of adding the parameters
to each link is done by the back-end generating the content.
In some cases it might be feasible doing
this via the web server using <code class="module"><a href="../mod/mod_substitute.html">mod_substitute</a></code>.
This can have negative impact on performance though.</p>
<p>The Java standards implement URL encoding slightly different. They use
a path info appended to the URL using a semicolon (<code>;</code>)
as the separator and add the session id behind. As in the cookie case,
Apache Tomcat can include the configured <code>jvmRoute</code> in this path
info. To let Apache find this sort of path info, you need to set
<code>scolonpathdelim</code> to <code>On</code> in
<code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
<code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>.</p>
<p>Finally you can support cookies and URL encoding at the same time, by
configuring the name of the cookie and the name of the URL parameter
separated by a vertical bar (<code>|</code>) as in the following example:</p>
<div class="example"><p><code>
ProxyPass /test balancer://mycluster stickysession=JSESSIONID|jsessionid scolonpathdelim=On<br />
<Proxy balancer://mycluster><br />
BalancerMember http://192.168.1.50:80 route=node1<br />
BalancerMember http://192.168.1.51:80 route=node2<br />
</Proxy><br />
</code></p></div>
<p>If the cookie and the request parameter both provide routing information
for the same request, the information from the request parameter is used.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="stickyness_troubleshooting" id="stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></h2>
<p>If you experience stickyness errors, e.g. users lose their
application sessions and need to login again, you first want to
check whether this is because the back-ends are sometimes unavailable
or whether your configuration is wrong. To find out about possible
stability problems with the back-ends, check your Apache error log
for proxy error messages.</p>
<p>To verify your configuration, first check, whether the stickyness
is based on a cookie or on URL encoding. Next step would be logging
the appropriate data in the access log by using an enhanced
<code class="directive"><a href="../mod/mod_log_config.html#logformat">LogFormat</a></code>.
The following fields are useful:</p>
<dl>
<dt><code>%{MYCOOKIE}C</code></dt>
<dd>The value contained in the cookie with name <code>MYCOOKIE</code>.
The name should be the same given in the <var>stickysession</var>
attribute.</dd>
<dt><code>%{Set-Cookie}o</code></dt>
<dd>This logs any cookie set by the back-end. You can track,
whether the back-end sets the session cookie you expect, and
to which value it is set.</dd>
<dt><code>%{BALANCER_SESSION_STICKY}e</code></dt>
<dd>The name of the cookie or request parameter used
to lookup the routing information.</dd>
<dt><code>%{BALANCER_SESSION_ROUTE}e</code></dt>
<dd>The route information found in the request.</dd>
<dt><code>%{BALANCER_WORKER_ROUTE}e</code></dt>
<dd>The route of the worker chosen.</dd>
<dt><code>%{BALANCER_ROUTE_CHANGED}e</code></dt>
<dd>Set to <code>1</code> if the route in the request
is different from the route of the worker, i.e.
the request couldn't be handled sticky.</dd>
</dl>
<p>Common reasons for loss of session are session timeouts,
which are usually configurable on the back-end server.</p>
<p>The balancer also logs detailed information about handling
stickyness to the error log, if the log level is set to
<code>debug</code> or higher. This is an easy way to
troubleshoot stickyness problems, but the log volume might
be to high for production servers under high load.</p>
</div>
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