Apache Module mod_proxy_balancer

Available Languages: en | + fr | + ja

+ + + + +

Description:	`mod_proxy` extension for load balancing
Status:	Extension
Module Identifier:	proxy_balancer_module
Source File:	mod_proxy_balancer.c
Compatibility:	Available in version 2.1 and later

Summary

+ +

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.

+ +

Warning

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.

Directives

This module provides no + directives.

Bugfix checklist

Load balancer scheduler algorithm

+ +

At present, there are 4 load balancer scheduler algorithms available + for use: Request Counting (mod_lbmethod_byrequests), + Weighted Traffic Counting (mod_lbmethod_bytraffic), + Pending Request Counting (mod_lbmethod_bybusyness) and + Heartbeat Traffic Counting (mod_lbmethod_heartbeat). + 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 "http://192.168.1.50:80"
+    BalancerMember "http://192.168.1.51:80"
+</Proxy>
+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: +

+ +

Header add Set-Cookie "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/" env=BALANCER_ROUTE_CHANGED
+<Proxy "balancer://mycluster">
+    BalancerMember "http://192.168.1.50:80" route=1
+    BalancerMember "http://192.168.1.51:80" route=2
+    ProxySet stickysession=ROUTEID
+</Proxy>
+ProxyPass        "/test" "balancer://mycluster"
+ProxyPassReverse "/test" "balancer://mycluster"

+ +

Exported Environment Variables

+ +

At present there are 6 environment variables exported:

+ +

BALANCER_SESSION_STICKY: +
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
+
BALANCER_SESSION_ROUTE: +
This is assigned the route parsed from the current + request.
+
BALANCER_NAME: +
This is assigned the name of the balancer used for the current + request. The value is something like balancer://foo.
+
BALANCER_WORKER_NAME: +
This is assigned the name of the worker used for the current request. + The value is something like http://hostA:1234.
+
BALANCER_WORKER_ROUTE: +
This is assigned the route of the worker that will be + used for the current request.
+
BALANCER_ROUTE_CHANGED: +
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.
+

+ +

Enabling Balancer Manager Support

+ +

This module requires the service of + mod_status. + 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. +

+ +

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 example.com + domain add this code to your httpd.conf + configuration file

<Location "/balancer-manager">
+    SetHandler balancer-manager
+    Require host example.com
+</Location>

+ + +

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.

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 + ProxySet.

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 "http://192.168.1.50:80" route=node1
+    BalancerMember "http://192.168.1.51:80" route=node2
+</Proxy>

+ +

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 + LogFormat. + The following fields are useful:

%{MYCOOKIE}C: The value contained in the cookie with name MYCOOKIE. + The name should be the same given in the stickysession + attribute.
%{Set-Cookie}o: 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.
%{BALANCER_SESSION_STICKY}e: The name of the cookie or request parameter used + to lookup the routing information.
%{BALANCER_SESSION_ROUTE}e: The route information found in the request.
%{BALANCER_WORKER_ROUTE}e: The route of the worker chosen.
%{BALANCER_ROUTE_CHANGED}e: 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.