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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>. 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,
    <module>mod_proxy</module> and <module>mod_proxy_balancer</module>
    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.</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>

    <example>
    &lt;Proxy balancer://mycluster&gt;<br />
        BalancerMember http://192.168.1.50:80<br />
        BalancerMember http://192.168.1.51:80<br />
    &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">
    <title>Request Counting Algorithm</title>
    <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>

    <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>
    </example>

    <p>If a balancer is configured as follows:</p>
    
    <table style="data">
    <tr><th>worker</th>
        <th>a</th>
        <th>b</th>
        <th>c</th>
        <th>d</th></tr>
    <tr><th>lbfactor</th>
        <td>25</td>
        <td>25</td>
        <td>25</td>
        <td>25</td></tr>
    <tr><th>lbstatus</th>
        <td>0</td>
        <td>0</td>
        <td>0</td>
        <td>0</td></tr>
    </table>

    <p>And <var>b</var> gets disabled, the following schedule is produced:</p>

    <table style="data">
    <tr><th>worker</th>
        <th>a</th>
        <th>b</th>
        <th>c</th>
        <th>d</th></tr>
    <tr><th>lbstatus</th>
        <td><em>-50</em></td>
        <td>0</td>
        <td>25</td>
        <td>25</td></tr>
    <tr><th>lbstatus</th>
        <td>-25</td>
        <td>0</td>
        <td><em>-25</em></td>
        <td>50</td></tr>
    <tr><th>lbstatus</th>
        <td>0</td>
        <td>0</td>
        <td>0</td>
        <td><em>0</em></td></tr>
    <tr><td 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 style="data">
    <tr><th>worker</th>
        <th>a</th>
        <th>b</th>
        <th>c</th>
        <th>d</th></tr>
    <tr><th>lbfactor</th>
        <td>25</td>
        <td>25</td>
        <td>25</td>
        <td>25</td></tr>
    </table>

    <p>Has the exact same behavior as:</p>

    <table style="data">
    <tr><th>worker</th>
        <th>a</th>
        <th>b</th>
        <th>c</th>
        <th>d</th></tr>
    <tr><th>lbfactor</th>
        <td>1</td>
        <td>1</td>
        <td>1</td>
        <td>1</td></tr>
    </table>

    <p>This is because all values of <dfn>lbfactor</dfn> are normalized
    with respect to the others. For:</p>

    <table style="data">
    <tr><th>worker</th>
        <th>a</th>
        <th>b</th>
        <th>c</th></tr>
    <tr><th>lbfactor</th>
        <td>1</td>
        <td>4</td>
        <td>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 style="data">
    <tr><th>worker</th>
        <th>a</th>
        <th>b</th></tr>
    <tr><th>lbfactor</th>
        <td>70</td>
        <td>30</td></tr>
    <tr><td colspan="2">&nbsp;</td></tr>
    <tr><th>lbstatus</th>
        <td><em>-30</em></td>
        <td>30</td></tr>
    <tr><th>lbstatus</th>
        <td>40</td>
        <td><em>-40</em></td></tr>
    <tr><th>lbstatus</th>
        <td><em>10</em></td>
        <td>-10</td></tr>
    <tr><th>lbstatus</th>
        <td><em>-20</em></td>
        <td>20</td></tr>
    <tr><th>lbstatus</th>
        <td><em>-50</em></td>
        <td>50</td></tr>
    <tr><th>lbstatus</th>
        <td>20</td>
        <td><em>-20</em></td></tr>
    <tr><th>lbstatus</th>
        <td><em>-10</em></td>
        <td>10</td></tr>
    <tr><th>lbstatus</th>
        <td><em>-40</em></td>
        <td>40</td></tr>
    <tr><th>lbstatus</th>
        <td>30</td>
        <td><em>-30</em></td></tr>
    <tr><th>lbstatus</th>
        <td><em>0</em></td>
        <td>0</td></tr>
    <tr><td 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>
</section>

<section id="traffic">
    <title>Weighted Traffic Counting Algorithm</title>
    <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 style="data">
    <tr><th>worker</th>
        <th>a</th>
        <th>b</th>
        <th>c</th></tr>
    <tr><th>lbfactor</th>
        <td>1</td>
        <td>2</td>
        <td>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>

</section>

<section id="busyness">

    <title>Pending Request Counting Algorithm</title>

    <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>

</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 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,
    <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>
<example>
    &lt;Location /balancer-manager&gt;<br />
    SetHandler balancer-manager<br />
<br />
    Order Deny,Allow<br />
    Deny from all<br />
    Allow from .example.com<br />
    &lt;/Location&gt;
</example>

    <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>
</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>
