Add detailed information about how to use

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

    <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:

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

    ProxyPass /test balancer://mycluster

    </example>

    <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>

    <example>

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

           env=BALANCER_ROUTE_CHANGED<br />

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

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

    ProxyPass /test balancer://mycluster

    </example>

</section>

<section id="requests">

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

    <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 ================ -->

</section>

<section id="enable">

<section id="balancer_manager">

    <title>Enabling Balancer Manager Support</title>

    <p>This module <em>requires</em> the service of 

    <module>mod_status</module>.

    <code>http://your.server.name/balancer-manager</code></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>.

    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>

    <example>

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

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

    BalancerMember http://192.168.1.50:80 route=node1<br />

    BalancerMember http://192.168.1.51:80 route=node2<br />

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

    </example>

    <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 loose 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>