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25 <div id="preamble"><h1>Apache Module mod_proxy_balancer</h1>
27 <p><span>Available Languages: </span><a href="../en/mod/mod_proxy_balancer.html" title="English"> en </a> |
28 <a href="../ja/mod/mod_proxy_balancer.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p>
30 <table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> extension for load balancing </td></tr>
31 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
32 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>proxy_balancer_module</td></tr>
33 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_proxy_balancer.c</td></tr>
34 <tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in version 2.1 and later</td></tr></table>
37 <p>This module <em>requires</em> the service of <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>. It provides load balancing support for
38 <code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols
41 <p>Load balancing scheduler algorithm is provided by not this
42 module but other modules such as:
43 <code class="module"><a href="../mod/mod_lbmethod_byrequests.html">mod_lbmethod_byrequests</a></code>,
44 <code class="module"><a href="../mod/mod_lbmethod_bytraffic.html">mod_lbmethod_bytraffic</a></code>,
45 <code class="module"><a href="../mod/mod_lbmethod_bybusyness.html">mod_lbmethod_bybusyness</a></code> and
46 <code class="module"><a href="../mod/mod_lbmethod_heartbeat.html">mod_lbmethod_heartbeat</a></code>.
49 <p>Thus, in order to get the ability of load balancing,
50 <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>, <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>
51 and at least one of load balancing scheduler algorithm modules have
52 to be present in the server.</p>
54 <div class="warning"><h3>Warning</h3>
55 <p>Do not enable proxying until you have <a href="mod_proxy.html#access">secured your server</a>. Open proxy
56 servers are dangerous both to your network and to the Internet at
60 <div id="quickview"><h3 class="directives">Directives</h3>
61 <p>This module provides no
65 <li><img alt="" src="../images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li>
66 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness">Load balancer stickyness</a></li>
67 <li><img alt="" src="../images/down.gif" /> <a href="#example">Examples of a balancer configuration</a></li>
68 <li><img alt="" src="../images/down.gif" /> <a href="#environment">Exported Environment Variables</a></li>
69 <li><img alt="" src="../images/down.gif" /> <a href="#balancer_manager">Enabling Balancer Manager Support</a></li>
70 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness_implementation">Details on load balancer stickyness</a></li>
71 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></li>
72 </ul><h3>See also</h3>
74 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
75 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
76 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
78 <h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2>
80 <p>At present, there are 3 load balancer scheduler algorithms available
81 for use: Request Counting, Weighted Traffic Counting and Pending Request
82 Counting. These are controlled via the <code>lbmethod</code> value of
83 the Balancer definition. See the <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code>
84 directive for more information, especially regarding how to
85 configure the Balancer and BalancerMembers.</p>
86 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
88 <h2><a name="stickyness" id="stickyness">Load balancer stickyness</a></h2>
90 <p>The balancer supports stickyness. When a request is proxied
91 to some back-end, then all following requests from the same user
92 should be proxied to the same back-end. Many load balancers implement
93 this feature via a table that maps client IP addresses to back-ends.
94 This approach is transparent to clients and back-ends, but suffers
95 from some problems: unequal load distribution if clients are themselves
96 hidden behind proxies, stickyness errors when a client uses a dynamic
97 IP address that changes during a session and loss of stickyness, if the
98 mapping table overflows.</p>
99 <p>The module <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> implements stickyness
100 on top of two alternative means: cookies and URL encoding. Providing the
101 cookie can be either done by the back-end or by the Apache web server
102 itself. The URL encoding is usually done on the back-end.</p>
103 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
104 <div class="section">
105 <h2><a name="example" id="example">Examples of a balancer configuration</a></h2>
107 <p>Before we dive into the technical details, here's an example of
108 how you might use <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> to provide
109 load balancing between two back-end servers:
112 <pre class="prettyprint lang-config">
113 <Proxy balancer://mycluster>
114 BalancerMember http://192.168.1.50:80
115 BalancerMember http://192.168.1.51:80
117 ProxyPass /test balancer://mycluster
118 ProxyPassReverse /test balancer://mycluster
122 <p>Another example of how to provide load balancing with stickyness
123 using <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code>, even if the back-end server does
124 not set a suitable session cookie:
127 <pre class="prettyprint lang-config">
128 Header add Set-Cookie "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/" env=BALANCER_ROUTE_CHANGED
129 <Proxy balancer://mycluster>
130 BalancerMember http://192.168.1.50:80 route=1
131 BalancerMember http://192.168.1.51:80 route=2
132 ProxySet stickysession=ROUTEID
134 ProxyPass /test balancer://mycluster
135 ProxyPassReverse /test balancer://mycluster
138 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
139 <div class="section">
140 <h2><a name="environment" id="environment">Exported Environment Variables</a></h2>
142 <p>At present there are 6 environment variables exported:</p>
146 <dt><var><a name="balancer_session_sticky" id="balancer_session_sticky">BALANCER_SESSION_STICKY</a></var></dt>
148 <p>This is assigned the <var>stickysession</var> value used for the current
149 request. It is the name of the cookie or request parameter used for sticky sessions</p>
153 <dt><var><a name="balancer_session_route" id="balancer_session_route">BALANCER_SESSION_ROUTE</a></var></dt>
155 <p>This is assigned the <var>route</var> parsed from the current
160 <dt><var><a name="balancer_name" id="balancer_name">BALANCER_NAME</a></var></dt>
162 <p>This is assigned the name of the balancer used for the current
163 request. The value is something like <code>balancer://foo</code>.</p>
167 <dt><var><a name="balancer_worker_name" id="balancer_worker_name">BALANCER_WORKER_NAME</a></var></dt>
169 <p>This is assigned the name of the worker used for the current request.
170 The value is something like <code>http://hostA:1234</code>.</p>
174 <dt><var><a name="balancer_worker_route" id="balancer_worker_route">BALANCER_WORKER_ROUTE</a></var></dt>
176 <p>This is assigned the <var>route</var> of the worker that will be
177 used for the current request.</p>
181 <dt><var><a name="balancer_route_changed" id="balancer_route_changed">BALANCER_ROUTE_CHANGED</a></var></dt>
183 <p>This is set to 1 if the session route does not match the
184 worker route (BALANCER_SESSION_ROUTE != BALANCER_WORKER_ROUTE) or the
185 session does not yet have an established route. This can be used to
186 determine when/if the client needs to be sent an updated route
187 when sticky sessions are used.</p>
191 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
192 <div class="section">
193 <h2><a name="balancer_manager" id="balancer_manager">Enabling Balancer Manager Support</a></h2>
195 <p>This module <em>requires</em> the service of
196 <code class="module"><a href="../mod/mod_status.html">mod_status</a></code>.
197 Balancer manager enables dynamic update of balancer
198 members. You can use balancer manager to change the balance
199 factor or a particular member, or put it in the off line
203 <p>Thus, in order to get the ability of load balancer management,
204 <code class="module"><a href="../mod/mod_status.html">mod_status</a></code> and <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code>
205 have to be present in the server.</p>
207 <p>To enable load balancer management for browsers from the example.com
208 domain add this code to your <code>httpd.conf</code>
209 configuration file</p>
210 <pre class="prettyprint lang-config">
211 <Location /balancer-manager>
212 SetHandler balancer-manager
213 Require host example.com
218 <p>You can now access load balancer manager by using a Web browser
220 <code>http://your.server.name/balancer-manager</code>. Please note
221 that only Balancers defined outside of <code><Location ...></code>
222 containers can be dynamically controlled by the Manager.</p>
223 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
224 <div class="section">
225 <h2><a name="stickyness_implementation" id="stickyness_implementation">Details on load balancer stickyness</a></h2>
227 <p>When using cookie based stickyness, you need to configure the
228 name of the cookie that contains the information about which back-end
229 to use. This is done via the <var>stickysession</var> attribute added
230 to either <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
231 <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. The name of
232 the cookie is case-sensitive. The balancer extracts the value of the
233 cookie and looks for a member worker with <var>route</var> equal
234 to that value. The <var>route</var> must also be set in either
235 <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
236 <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. The cookie can either
237 be set by the back-end, or as shown in the above
238 <a href="#example">example</a> by the Apache web server itself.</p>
239 <p>Some back-ends use a slightly different form of stickyness cookie,
240 for instance Apache Tomcat. Tomcat adds the name of the Tomcat instance
241 to the end of its session id cookie, separated with a dot (<code>.</code>)
242 from the session id. Thus if the Apache web server finds a dot in the value
243 of the stickyness cookie, it only uses the part behind the dot to search
244 for the route. In order to let Tomcat know about its instance name, you
245 need to set the attribute <code>jvmRoute</code> inside the Tomcat
246 configuration file <code>conf/server.xml</code> to the value of the
247 <var>route</var> of the worker that connects to the respective Tomcat.
248 The name of the session cookie used by Tomcat (and more generally by Java
249 web applications based on servlets) is <code>JSESSIONID</code>
250 (upper case) but can be configured to something else.</p>
251 <p>The second way of implementing stickyness is URL encoding.
252 The web server searches for a query parameter in the URL of the request.
253 The name of the parameter is specified again using <var>stickysession</var>.
254 The value of the parameter is used to lookup a member worker with <var>route</var>
255 equal to that value. Since it is not easy to extract and manipulate all
256 URL links contained in responses, generally the work of adding the parameters
257 to each link is done by the back-end generating the content.
258 In some cases it might be feasible doing
259 this via the web server using <code class="module"><a href="../mod/mod_substitute.html">mod_substitute</a></code> or
260 <code class="module"><a href="../mod/mod_sed.html">mod_sed</a></code>. This can have negative impact on performance though.</p>
261 <p>The Java standards implement URL encoding slightly different. They use
262 a path info appended to the URL using a semicolon (<code>;</code>)
263 as the separator and add the session id behind. As in the cookie case,
264 Apache Tomcat can include the configured <code>jvmRoute</code> in this path
265 info. To let Apache find this sort of path info, you neet to set
266 <code>scolonpathdelim</code> to <code>On</code> in
267 <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
268 <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>.</p>
269 <p>Finally you can support cookies and URL encoding at the same time, by
270 configuring the name of the cookie and the name of the URL parameter
271 separated by a vertical bar (<code>|</code>) as in the following example:</p>
272 <pre class="prettyprint lang-config">
273 ProxyPass /test balancer://mycluster stickysession=JSESSIONID|jsessionid scolonpathdelim=On
274 <Proxy balancer://mycluster>
275 BalancerMember http://192.168.1.50:80 route=node1
276 BalancerMember http://192.168.1.51:80 route=node2
280 <p>If the cookie and the request parameter both provide routing information
281 for the same request, the information from the request parameter is used.</p>
282 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
283 <div class="section">
284 <h2><a name="stickyness_troubleshooting" id="stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></h2>
286 <p>If you experience stickyness errors, e.g. users loose their
287 application sessions and need to login again, you first want to
288 check whether this is because the back-ends are sometimes unavailable
289 or whether your configuration is wrong. To find out about possible
290 stability problems with the back-ends, check your Apache error log
291 for proxy error messages.</p>
292 <p>To verify your configuration, first check, whether the stickyness
293 is based on a cookie or on URL encoding. Next step would be logging
294 the appropriate data in the access log by using an enhanced
295 <code class="directive"><a href="../mod/mod_log_config.html#logformat">LogFormat</a></code>.
296 The following fields are useful:</p>
298 <dt><code>%{MYCOOKIE}C</code></dt>
299 <dd>The value contained in the cookie with name <code>MYCOOKIE</code>.
300 The name should be the same given in the <var>stickysession</var>
302 <dt><code>%{Set-Cookie}o</code></dt>
303 <dd>This logs any cookie set by the back-end. You can track,
304 whether the back-end sets the session cookie you expect, and
305 to which value it is set.</dd>
306 <dt><code>%{BALANCER_SESSION_STICKY}e</code></dt>
307 <dd>The name of the cookie or request parameter used
308 to lookup the routing information.</dd>
309 <dt><code>%{BALANCER_SESSION_ROUTE}e</code></dt>
310 <dd>The route information found in the request.</dd>
311 <dt><code>%{BALANCER_WORKER_ROUTE}e</code></dt>
312 <dd>The route of the worker chosen.</dd>
313 <dt><code>%{BALANCER_ROUTE_CHANGED}e</code></dt>
314 <dd>Set to <code>1</code> if the route in the request
315 is different from the route of the worker, i.e.
316 the request couldn't be handled sticky.</dd>
318 <p>Common reasons for loss of session are session timeouts,
319 which are usually configurable on the back-end server.</p>
320 <p>The balancer also logs detailed information about handling
321 stickyness to the error log, if the log level is set to
322 <code>debug</code> or higher. This is an easy way to
323 troubleshoot stickyness problems, but the log volume might
324 be to high for production servers under high load.</p>
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