From 78bd325023fa08ea3528fb5a68b463bbabfaf5bb Mon Sep 17 00:00:00 2001 From: Rainer Jung Date: Thu, 30 Sep 2010 16:09:05 +0000 Subject: [PATCH] Add detailed information about how to use session stickyness. git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1003131 13f79535-47bb-0310-9956-ffa450edef68 --- docs/manual/mod/mod_proxy_balancer.xml | 127 ++++++++++++++++++++++++- 1 file changed, 122 insertions(+), 5 deletions(-) diff --git a/docs/manual/mod/mod_proxy_balancer.xml b/docs/manual/mod/mod_proxy_balancer.xml index b1d422eca7..b2d1d948ad 100644 --- a/docs/manual/mod/mod_proxy_balancer.xml +++ b/docs/manual/mod/mod_proxy_balancer.xml @@ -59,11 +59,27 @@
Load balancer scheduler algorithm

At present, there are 3 load balancer scheduler algorithms available - for use: Request Counting, Weighted Traffic Counting and Pending Request + 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.

+
+
+ 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.

@@ -82,7 +98,7 @@

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

@@ -106,8 +122,8 @@
BALANCER_SESSION_STICKY
-

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

+

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

@@ -151,7 +167,7 @@
-
+
Enabling Balancer Manager Support

This module requires the service of mod_status. @@ -183,6 +199,107 @@ http://your.server.name/balancer-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 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 + 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.

+
+ BalancerNonce Set the nonce used in the balancer-manager application -- 2.50.1