This module requires the service of HTTP, FTP and AJP13 protocols
Load balancing scheduler algorithm is provided by not this
module but other modules such as:
Thus, in order to get the ability of load balancing,
Do not enable proxying until you have secured your server. Open proxy servers are dangerous both to your network and to the Internet at large.
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
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
Before we dive into the technical details, here's an example of
how you might use
Another example of how to provide load balancing with stickyness
using
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 request.
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 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.
This module requires the service of
Thus, in order to get the ability of load balancer management,
To enable load balancer management for browsers from the example.com
domain add this code to your httpd.conf
configuration file
You can now access load balancer manager by using a Web browser
to access the page
http://your.server.name/balancer-manager. Please note
that only Balancers defined outside of <Location ...>
containers can be dynamically controlled by the Manager.
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
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
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
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:
If the cookie and the request parameter both provide routing information for the same request, the information from the request parameter is used.
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.
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
%{MYCOOKIE}CMYCOOKIE.
The name should be the same given in the stickysession
attribute.%{Set-Cookie}o%{BALANCER_SESSION_STICKY}e%{BALANCER_SESSION_ROUTE}e%{BALANCER_WORKER_ROUTE}e%{BALANCER_ROUTE_CHANGED}e1 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.