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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>mod_proxy_balancer.c</sourcefile>

<identifier>proxy_balancer_module</identifier>

<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>Load balancing scheduler algorithm is provided by not this

module but other modules such as:

<module>mod_lbmethod_byrequests</module>,

<module>mod_lbmethod_bytraffic</module>,

<module>mod_lbmethod_bybusyness</module> and

<module>mod_lbmethod_heartbeat</module>.

</p>

<p>Thus, in order to get the ability of load balancing,

<module>mod_proxy</module>, <module>mod_proxy_balancer</module>

and at least one of load balancing scheduler algorithm modules 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 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 <directive module="mod_proxy">ProxyPass</directive>

directive for more information, especially regarding how to

configure the Balancer and BalancerMembers.</p>

</section>

<section id="stickyness">

<title>Load balancer stickyness</title>

<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 <module>mod_proxy_balancer</module> 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>

</section>

<section id="example">

<title>Examples 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>

<highlight language="config">

&lt;Proxy balancer://mycluster&gt;

BalancerMember http://192.168.1.50:80

BalancerMember http://192.168.1.51:80

&lt;/Proxy&gt;

ProxyPass /test balancer://mycluster

ProxyPassReverse /test balancer://mycluster

</highlight>

<p>Another example of how to provide load balancing with stickyness

using <module>mod_headers</module>, even if the back-end server does

not set a suitable session cookie:

</p>

<highlight language="config">

Header add Set-Cookie "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/" env=BALANCER_ROUTE_CHANGED

&lt;Proxy balancer://mycluster&gt;

BalancerMember http://192.168.1.50:80 route=1

BalancerMember http://192.168.1.51:80 route=2

ProxySet stickysession=ROUTEID

&lt;/Proxy&gt;

ProxyPass /test balancer://mycluster

ProxyPassReverse /test balancer://mycluster

</highlight>

</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 for the current

request. It is the name of the cookie or request parameter 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="balancer_manager">

<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 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,

<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>

<highlight language="config">

&lt;Location "/balancer-manager"&gt;

SetHandler balancer-manager

Require host example.com

&lt;/Location&gt;

</highlight>

<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>. Please note

that only Balancers defined outside of <code>&lt;Location ...&gt;</code>

containers can be dynamically controlled by the Manager.</p>

</section>

<section id="stickyness_implementation">

<title>Details on load balancer stickyness</title>

<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 <directive module="mod_proxy">ProxyPass</directive> or

<directive module="mod_proxy">ProxySet</directive>. 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

<directive module="mod_proxy">ProxyPass</directive> or

<directive module="mod_proxy">ProxySet</directive>. 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 <module>mod_substitute</module> or

<module>mod_sed</module>. 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 neet to set

<code>scolonpathdelim</code> to <code>On</code> in

<directive module="mod_proxy">ProxyPass</directive> or

<directive module="mod_proxy">ProxySet</directive>.</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>

<highlight language="config">

ProxyPass /test balancer://mycluster stickysession=JSESSIONID|jsessionid scolonpathdelim=On

&lt;Proxy balancer://mycluster&gt;

BalancerMember http://192.168.1.50:80 route=node1

BalancerMember http://192.168.1.51:80 route=node2

&lt;/Proxy&gt;

</highlight>

<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>

</section>

<section id="stickyness_troubleshooting">

<title>Troubleshooting load balancer stickyness</title>

<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

<directive module="mod_log_config">LogFormat</directive>.

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>

</section>

</modulesynopsis>