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