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Diffstat (limited to 'docs/manual/mod/mod_proxy_balancer.html.en')
| -rw-r--r-- | docs/manual/mod/mod_proxy_balancer.html.en | 178 |
1 files changed, 157 insertions, 21 deletions
diff --git a/docs/manual/mod/mod_proxy_balancer.html.en b/docs/manual/mod/mod_proxy_balancer.html.en index df826088..3c3d9f04 100644 --- a/docs/manual/mod/mod_proxy_balancer.html.en +++ b/docs/manual/mod/mod_proxy_balancer.html.en @@ -51,12 +51,15 @@ <h3>Topics</h3> <ul id="topics"> <li><img alt="" src="../images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li> -<li><img alt="" src="../images/down.gif" /> <a href="#example">Example of a balancer configuration</a></li> +<li><img alt="" src="../images/down.gif" /> <a href="#stickyness">Load balancer stickyness</a></li> +<li><img alt="" src="../images/down.gif" /> <a href="#example">Examples of a balancer configuration</a></li> <li><img alt="" src="../images/down.gif" /> <a href="#requests">Request Counting Algorithm</a></li> <li><img alt="" src="../images/down.gif" /> <a href="#traffic">Weighted Traffic Counting Algorithm</a></li> <li><img alt="" src="../images/down.gif" /> <a href="#busyness">Pending Request Counting Algorithm</a></li> <li><img alt="" src="../images/down.gif" /> <a href="#environment">Exported Environment Variables</a></li> -<li><img alt="" src="../images/down.gif" /> <a href="#enable">Enabling Balancer Manager Support</a></li> +<li><img alt="" src="../images/down.gif" /> <a href="#balancer_manager">Enabling Balancer Manager Support</a></li> +<li><img alt="" src="../images/down.gif" /> <a href="#stickyness_implementation">Details on load balancer stickyness</a></li> +<li><img alt="" src="../images/down.gif" /> <a href="#stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></li> </ul><h3>See also</h3> <ul class="seealso"> <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li> @@ -66,27 +69,59 @@ <h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2> <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 <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> directive for more information.</p> - </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> <div class="section"> -<h2><a name="example" id="example">Example of a balancer configuration</a></h2> - - <p>Before we dive into the technical details, here's an example of - how you might use <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> to provide - load balancing between two back-end servers: - </p> - - <div class="example"><p><code> - <Proxy balancer://mycluster><br /> - BalancerMember http://192.168.1.50:80<br /> - BalancerMember http://192.168.1.51:80<br /> - </Proxy><br /> - ProxyPass /test balancer://mycluster - </code></p></div> +<h2><a name="stickyness" id="stickyness">Load balancer stickyness</a></h2> + + <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 <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> 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> +</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> +<div class="section"> +<h2><a name="example" id="example">Examples of a balancer configuration</a></h2> + + <p>Before we dive into the technical details, here's an example of + how you might use <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> to provide + load balancing between two back-end servers: + </p> + + <div class="example"><p><code> + <Proxy balancer://mycluster><br /> + BalancerMember http://192.168.1.50:80<br /> + BalancerMember http://192.168.1.51:80<br /> + </Proxy><br /> + ProxyPass /test balancer://mycluster + </code></p></div> + + <p>Another example of how to provide load balancing with stickyness + using <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code>, even if the back-end server does + not set a suitable session cookie: + </p> + + <div class="example"><p><code> + Header add Set-Cookie "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/" + env=BALANCER_ROUTE_CHANGED<br /> + <Proxy balancer://mycluster><br /> + BalancerMember http://192.168.1.50:80 route=1<br /> + BalancerMember http://192.168.1.51:80 route=2<br /> + ProxySet stickysession=ROUTEID<br /> + </Proxy><br /> + ProxyPass /test balancer://mycluster + </code></p></div> </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> <div class="section"> <h2><a name="requests" id="requests">Request Counting Algorithm</a></h2> @@ -321,8 +356,8 @@ candidate lbstatus -= total factor</code></pre></div> <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> @@ -366,7 +401,7 @@ candidate lbstatus -= total factor</code></pre></div> </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> <div class="section"> -<h2><a name="enable" id="enable">Enabling Balancer Manager Support</a></h2> +<h2><a name="balancer_manager" id="balancer_manager">Enabling Balancer Manager Support</a></h2> <p>This module <em>requires</em> the service of <code class="module"><a href="../mod/mod_status.html">mod_status</a></code>. @@ -396,6 +431,107 @@ candidate lbstatus -= total factor</code></pre></div> <p>You can now access load balancer manager by using a Web browser to access the page <code>http://your.server.name/balancer-manager</code></p> +</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> +<div class="section"> +<h2><a name="stickyness_implementation" id="stickyness_implementation">Details on load balancer stickyness</a></h2> + + <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 <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or + <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. 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 + <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or + <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. 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 <code class="module"><a href="../mod/mod_substitute.html">mod_substitute</a></code>. + 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 + <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or + <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>.</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> + <div class="example"><p><code> + 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 /> + </code></p></div> + <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> +</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> +<div class="section"> +<h2><a name="stickyness_troubleshooting" id="stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></h2> + + <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 + <code class="directive"><a href="../mod/mod_log_config.html#logformat">LogFormat</a></code>. + 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> </div> </div> <div class="bottomlang"> |