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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>
<compatibility>Available in version 2.1 and later</compatibility>

<summary>
    <p>This module <em>requires</em> the service of <module
    >mod_proxy</module> and it provides load balancing for
    all the supported protocols. The most important ones are:</p>
    <ul>
        <li>HTTP, using <module>mod_proxy_http</module></li>
        <li>FTP, using <module>mod_proxy_ftp</module></li>
        <li>AJP13, using <module>mod_proxy_ajp</module></li>
        <li>WebSocket, using <module>mod_proxy_wstunnel</module></li>
    </ul>
    
    <p>The Load balancing scheduler algorithm is not provided by this
    module but from other ones such as:</p>
    <ul>
        <li><module>mod_lbmethod_byrequests</module></li>
        <li><module>mod_lbmethod_bytraffic</module></li>
        <li><module>mod_lbmethod_bybusyness</module></li>
        <li><module>mod_lbmethod_heartbeat</module></li>
    </ul>

    <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>
<seealso><directive module="mod_proxy">BalancerMember</directive></seealso>
<seealso><directive module="mod_proxy">BalancerGrowth</directive></seealso>
<seealso><directive module="mod_proxy">BalancerPersist</directive></seealso>
<seealso><directive module="mod_proxy">BalancerInherit</directive></seealso>

<section id="scheduler">
    <title>Load balancer scheduler algorithm</title>
    <p>At present, there are 4 load balancer scheduler algorithms available
    for use: Request Counting (<module>mod_lbmethod_byrequests</module>),
    Weighted Traffic Counting (<module>mod_lbmethod_bytraffic</module>),
    Pending Request Counting (<module>mod_lbmethod_bybusyness</module>) and
    Heartbeat Traffic Counting (<module>mod_lbmethod_heartbeat</module>).
    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>
