mod_proxy_balancer - Apache HTTP Server Version 2.4

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Apache HTTP Server Version 2.4

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Apache Module mod_proxy_balancer

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Description:mod_proxy extension for load balancing 
Compatibility:Available in version 2.1 and later

    This module requires the service of mod_proxy and it provides load balancing for
    all the supported protocols. The most important ones are:
        HTTP, using mod_proxy_http
        FTP, using mod_proxy_ftp
        AJP13, using mod_proxy_ajp
        WebSocket, using mod_proxy_wstunnel
    The Load balancing scheduler algorithm is not provided by this
    module but from other ones such as:

    Thus, in order to get the ability of load balancing,
    mod_proxy, mod_proxy_balancer
    and at least one of load balancing scheduler algorithm modules have
    to be present in the server.

      Do not enable proxying until you have secured your server. Open proxy
      servers are dangerous both to your network and to the Internet at


 Load balancer scheduler algorithm
 Load balancer stickyness
 Examples of a balancer configuration
 Exported Environment Variables
 Enabling Balancer Manager Support
 Details on load balancer stickyness
 Troubleshooting load balancer stickyness
This module provides no
Bugfix checklisthttpd changelogKnown issuesReport a bugSee also


Load balancer scheduler algorithm
    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 lbmethod value of
    the Balancer definition. See the ProxyPass
    directive for more information, especially regarding how to
    configure the Balancer and BalancerMembers.

Load balancer stickyness
    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.
    The module mod_proxy_balancer 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.

Examples of a balancer configuration
    Before we dive into the technical details, here's an example of
    how you might use mod_proxy_balancer to provide
    load balancing between two back-end servers:

    <Proxy "balancer://mycluster">
    BalancerMember ""
    BalancerMember ""
ProxyPass "/test" "balancer://mycluster"
ProxyPassReverse "/test" "balancer://mycluster"

    Another example of how to provide load balancing with stickyness
    using mod_headers, even if the back-end server does
    not set a suitable session cookie:

<Proxy "balancer://mycluster">
    BalancerMember "" route=1
    BalancerMember "" route=2
    ProxySet stickysession=ROUTEID
ProxyPass "/test" "balancer://mycluster"
ProxyPassReverse "/test" "balancer://mycluster"

Exported Environment Variables
    At present there are 6 environment variables exported:

    This is assigned the stickysession value used for the current
    request.  It is the name of the cookie or request parameter used for sticky sessions

    This is assigned the route parsed from the current

    This is assigned the name of the balancer used for the current
    request. The value is something like balancer://foo.

    This is assigned the name of the worker used for the current request.
    The value is something like http://hostA:1234.

    This is assigned the route of the worker that will be
    used for the current request.

    This is set to 1 if the session route does not match 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.

Enabling Balancer Manager Support
    This module requires the service of
    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

    Thus, in order to get the ability of load balancer management,
    mod_status and mod_proxy_balancer
    have to be present in the server.

    To enable load balancer management for browsers from the
    domain add this code to your httpd.conf
    configuration file
<Location "/balancer-manager">
    SetHandler balancer-manager
    Require host

    You can now access load balancer manager by using a Web browser
    to access the page Please note
    that only Balancers defined outside of <Location ...>
    containers can be dynamically controlled by the Manager.

Details on load balancer stickyness
    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 stickysession attribute added
    to either ProxyPass or
    ProxySet. The name of
    the cookie is case-sensitive. The balancer extracts the value of the
    cookie and looks for a member worker with route equal
    to that value. The route must also be set in either
    ProxyPass or
    ProxySet. The cookie can either
    be set by the back-end, or as shown in the above
    example by the Apache web server itself.
    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 (.)
    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 jvmRoute inside the Tomcat
    configuration file conf/server.xml to the value of the
    route 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 JSESSIONID
    (upper case) but can be configured to something else.
    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 stickysession.
    The value of the parameter is used to lookup a member worker with route
    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 mod_substitute or
    mod_sed. This can have negative impact on performance though.
    The Java standards implement URL encoding slightly different. They use
    a path info appended to the URL using a semicolon (;)
    as the separator and add the session id behind. As in the cookie case,
    Apache Tomcat can include the configured jvmRoute in this path
    info. To let Apache find this sort of path info, you neet to set
    scolonpathdelim to On in
    ProxyPass or
    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 (|) as in the following example:
    ProxyPass "/test" "balancer://mycluster" stickysession=JSESSIONID|jsessionid scolonpathdelim=On
<Proxy "balancer://mycluster">
    BalancerMember "" route=node1
    BalancerMember "" route=node2

    If the cookie and the request parameter both provide routing information
    for the same request, the information from the request parameter is used.

Troubleshooting load balancer stickyness
    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.
    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
    The following fields are useful:
    The value contained in the cookie with name MYCOOKIE.
    The name should be the same given in the stickysession
    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.
    The name of the cookie or request parameter used
    to lookup the routing information.
    The route information found in the request.
    The route of the worker chosen.
    Set to 1 if the route in the request
    is different from the route of the worker, i.e.
    the request couldn't be handled sticky.
    Common reasons for loss of session are session timeouts,
    which are usually configurable on the back-end server.
    The balancer also logs detailed information about handling
    stickyness to the error log, if the log level is set to
    debug 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.

Available Languages:  en  |
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CommentsNotice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.

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