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Posted to cvs@httpd.apache.org by rj...@apache.org on 2010/09/30 18:09:05 UTC
svn commit: r1003131 -
/httpd/httpd/trunk/docs/manual/mod/mod_proxy_balancer.xml
Author: rjung
Date: Thu Sep 30 16:09:05 2010
New Revision: 1003131
URL: http://svn.apache.org/viewvc?rev=1003131&view=rev
Log:
Add detailed information about how to use
session stickyness.
Modified:
httpd/httpd/trunk/docs/manual/mod/mod_proxy_balancer.xml
Modified: httpd/httpd/trunk/docs/manual/mod/mod_proxy_balancer.xml
URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/docs/manual/mod/mod_proxy_balancer.xml?rev=1003131&r1=1003130&r2=1003131&view=diff
==============================================================================
--- httpd/httpd/trunk/docs/manual/mod/mod_proxy_balancer.xml (original)
+++ httpd/httpd/trunk/docs/manual/mod/mod_proxy_balancer.xml Thu Sep 30 16:09:05 2010
@@ -59,11 +59,27 @@
<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
+ 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">
@@ -82,7 +98,7 @@
</example>
<p>Another example of how to provide load balancing with stickyness
- using <module>mod_headers</module>, even if the backend server does
+ using <module>mod_headers</module>, even if the back-end server does
not set a suitable session cookie:
</p>
@@ -106,8 +122,8 @@
<!-- ============= 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 in the current
- request. It is the cookie or parameter name used for sticky sessions</p>
+ <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 ================ -->
@@ -151,7 +167,7 @@
</section>
-<section id="enable">
+<section id="balancer_manager">
<title>Enabling Balancer Manager Support</title>
<p>This module <em>requires</em> the service of
<module>mod_status</module>.
@@ -183,6 +199,107 @@
<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> or
+ <module>mod_sed</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
+ <Proxy balancer://mycluster><br />
+ BalancerMember http://192.168.1.50:80 route=node1<br />
+ BalancerMember http://192.168.1.51:80 route=node2<br />
+ </Proxy><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>
+
<directivesynopsis>
<name>BalancerNonce</name>
<description>Set the nonce used in the balancer-manager application</description>
Re: svn commit: r1003131 - /httpd/httpd/trunk/docs/manual/mod/mod_proxy_balancer.xml
Posted by Rainer Jung <ra...@kippdata.de>.
On 30.09.2010 18:09, rjung@apache.org wrote:
> Author: rjung
> Date: Thu Sep 30 16:09:05 2010
> New Revision: 1003131
>
> URL: http://svn.apache.org/viewvc?rev=1003131&view=rev
> Log:
> Add detailed information about how to use
> session stickyness.
>
> Modified:
> httpd/httpd/trunk/docs/manual/mod/mod_proxy_balancer.xml
I plan to add this to the 2.2 docs as well, description works there
as-is. Any reviews welcome.
Regards,
Rainer