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27 <div id="preamble"><h1>Apache Module mod_proxy_balancer</h1>
29 <p><span>Available Languages: </span><a href="../en/mod/mod_proxy_balancer.html" title="English"> en </a> |
30 <a href="../ja/mod/mod_proxy_balancer.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p>
32 <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>
33 <tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
34 <tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>proxy_balancer_module</td></tr>
35 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_proxy_balancer.c</td></tr></table>
38 <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
39 <code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols
42 <p>Load balancing scheduler algorithm is provided by not this
43 module but other modules such as:
44 <code class="module"><a href="../mod/mod_lbmethod_byrequests.html">mod_lbmethod_byrequests</a></code>,
45 <code class="module"><a href="../mod/mod_lbmethod_bytraffic.html">mod_lbmethod_bytraffic</a></code>,
46 <code class="module"><a href="../mod/mod_lbmethod_bybusyness.html">mod_lbmethod_bybusyness</a></code> and
47 <code class="module"><a href="../mod/mod_lbmethod_heartbeat.html">mod_lbmethod_heartbeat</a></code>.
50 <p>Thus, in order to get the ability of load balancing,
51 <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>
52 and at least one of load balancing scheduler algorithm modules have
53 to be present in the server.</p>
55 <div class="warning"><h3>Warning</h3>
56 <p>Do not enable proxying until you have <a href="mod_proxy.html#access">secured your server</a>. Open proxy
57 servers are dangerous both to your network and to the Internet at
61 <div id="quickview"><h3>Topics</h3>
63 <li><img alt="" src="../images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li>
64 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness">Load balancer stickyness</a></li>
65 <li><img alt="" src="../images/down.gif" /> <a href="#example">Examples of a balancer configuration</a></li>
66 <li><img alt="" src="../images/down.gif" /> <a href="#environment">Exported Environment Variables</a></li>
67 <li><img alt="" src="../images/down.gif" /> <a href="#balancer_manager">Enabling Balancer Manager Support</a></li>
68 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness_implementation">Details on load balancer stickyness</a></li>
69 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></li>
70 </ul><h3 class="directives">Directives</h3>
71 <p>This module provides no
75 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
76 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
77 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
79 <h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2>
81 <p>At present, there are 3 load balancer scheduler algorithms available
82 for use: Request Counting, Weighted Traffic Counting and Pending Request
83 Counting. These are controlled via the <code>lbmethod</code> value of
84 the Balancer definition. See the <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code>
85 directive for more information, especially regarding how to
86 configure the Balancer and BalancerMembers.</p>
87 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
89 <h2><a name="stickyness" id="stickyness">Load balancer stickyness</a></h2>
91 <p>The balancer supports stickyness. When a request is proxied
92 to some back-end, then all following requests from the same user
93 should be proxied to the same back-end. Many load balancers implement
94 this feature via a table that maps client IP addresses to back-ends.
95 This approach is transparent to clients and back-ends, but suffers
96 from some problems: unequal load distribution if clients are themselves
97 hidden behind proxies, stickyness errors when a client uses a dynamic
98 IP address that changes during a session and loss of stickyness, if the
99 mapping table overflows.</p>
100 <p>The module <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> implements stickyness
101 on top of two alternative means: cookies and URL encoding. Providing the
102 cookie can be either done by the back-end or by the Apache web server
103 itself. The URL encoding is usually done on the back-end.</p>
104 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
105 <div class="section">
106 <h2><a name="example" id="example">Examples of a balancer configuration</a></h2>
108 <p>Before we dive into the technical details, here's an example of
109 how you might use <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> to provide
110 load balancing between two back-end servers:
113 <pre class="prettyprint lang-config"><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"</pre>
121 <p>Another example of how to provide load balancing with stickyness
122 using <code class="module"><a href="../mod/mod_headers.html">mod_headers</a></code>, even if the back-end server does
123 not set a suitable session cookie:
126 <pre class="prettyprint lang-config">Header add Set-Cookie "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/" env=BALANCER_ROUTE_CHANGED
127 <Proxy balancer://mycluster>
128 BalancerMember http://192.168.1.50:80 route=1
129 BalancerMember http://192.168.1.51:80 route=2
130 ProxySet stickysession=ROUTEID
132 ProxyPass "/test" "balancer://mycluster"
133 ProxyPassReverse "/test" "balancer://mycluster"</pre>
135 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
136 <div class="section">
137 <h2><a name="environment" id="environment">Exported Environment Variables</a></h2>
139 <p>At present there are 6 environment variables exported:</p>
143 <dt><var><a name="balancer_session_sticky" id="balancer_session_sticky">BALANCER_SESSION_STICKY</a></var></dt>
145 <p>This is assigned the <var>stickysession</var> value used for the current
146 request. It is the name of the cookie or request parameter used for sticky sessions</p>
150 <dt><var><a name="balancer_session_route" id="balancer_session_route">BALANCER_SESSION_ROUTE</a></var></dt>
152 <p>This is assigned the <var>route</var> parsed from the current
157 <dt><var><a name="balancer_name" id="balancer_name">BALANCER_NAME</a></var></dt>
159 <p>This is assigned the name of the balancer used for the current
160 request. The value is something like <code>balancer://foo</code>.</p>
164 <dt><var><a name="balancer_worker_name" id="balancer_worker_name">BALANCER_WORKER_NAME</a></var></dt>
166 <p>This is assigned the name of the worker used for the current request.
167 The value is something like <code>http://hostA:1234</code>.</p>
171 <dt><var><a name="balancer_worker_route" id="balancer_worker_route">BALANCER_WORKER_ROUTE</a></var></dt>
173 <p>This is assigned the <var>route</var> of the worker that will be
174 used for the current request.</p>
178 <dt><var><a name="balancer_route_changed" id="balancer_route_changed">BALANCER_ROUTE_CHANGED</a></var></dt>
180 <p>This is set to 1 if the session route does not match the
181 worker route (BALANCER_SESSION_ROUTE != BALANCER_WORKER_ROUTE) or the
182 session does not yet have an established route. This can be used to
183 determine when/if the client needs to be sent an updated route
184 when sticky sessions are used.</p>
188 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
189 <div class="section">
190 <h2><a name="balancer_manager" id="balancer_manager">Enabling Balancer Manager Support</a></h2>
192 <p>This module <em>requires</em> the service of
193 <code class="module"><a href="../mod/mod_status.html">mod_status</a></code>.
194 Balancer manager enables dynamic update of balancer
195 members. You can use balancer manager to change the balance
196 factor of a particular member, or put it in the off line
200 <p>Thus, in order to get the ability of load balancer management,
201 <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>
202 have to be present in the server.</p>
204 <p>To enable load balancer management for browsers from the example.com
205 domain add this code to your <code>httpd.conf</code>
206 configuration file</p>
207 <pre class="prettyprint lang-config"><Location "/balancer-manager">
208 SetHandler balancer-manager
209 Require host example.com
210 </Location></pre>
213 <p>You can now access load balancer manager by using a Web browser
215 <code>http://your.server.name/balancer-manager</code>. Please note
216 that only Balancers defined outside of <code><Location ...></code>
217 containers can be dynamically controlled by the Manager.</p>
218 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
219 <div class="section">
220 <h2><a name="stickyness_implementation" id="stickyness_implementation">Details on load balancer stickyness</a></h2>
222 <p>When using cookie based stickyness, you need to configure the
223 name of the cookie that contains the information about which back-end
224 to use. This is done via the <var>stickysession</var> attribute added
225 to either <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
226 <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. The name of
227 the cookie is case-sensitive. The balancer extracts the value of the
228 cookie and looks for a member worker with <var>route</var> equal
229 to that value. The <var>route</var> must also be set in either
230 <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 cookie can either
232 be set by the back-end, or as shown in the above
233 <a href="#example">example</a> by the Apache web server itself.</p>
234 <p>Some back-ends use a slightly different form of stickyness cookie,
235 for instance Apache Tomcat. Tomcat adds the name of the Tomcat instance
236 to the end of its session id cookie, separated with a dot (<code>.</code>)
237 from the session id. Thus if the Apache web server finds a dot in the value
238 of the stickyness cookie, it only uses the part behind the dot to search
239 for the route. In order to let Tomcat know about its instance name, you
240 need to set the attribute <code>jvmRoute</code> inside the Tomcat
241 configuration file <code>conf/server.xml</code> to the value of the
242 <var>route</var> of the worker that connects to the respective Tomcat.
243 The name of the session cookie used by Tomcat (and more generally by Java
244 web applications based on servlets) is <code>JSESSIONID</code>
245 (upper case) but can be configured to something else.</p>
246 <p>The second way of implementing stickyness is URL encoding.
247 The web server searches for a query parameter in the URL of the request.
248 The name of the parameter is specified again using <var>stickysession</var>.
249 The value of the parameter is used to lookup a member worker with <var>route</var>
250 equal to that value. Since it is not easy to extract and manipulate all
251 URL links contained in responses, generally the work of adding the parameters
252 to each link is done by the back-end generating the content.
253 In some cases it might be feasible doing
254 this via the web server using <code class="module"><a href="../mod/mod_substitute.html">mod_substitute</a></code> or
255 <code class="module"><a href="../mod/mod_sed.html">mod_sed</a></code>. This can have negative impact on performance though.</p>
256 <p>The Java standards implement URL encoding slightly different. They use
257 a path info appended to the URL using a semicolon (<code>;</code>)
258 as the separator and add the session id behind. As in the cookie case,
259 Apache Tomcat can include the configured <code>jvmRoute</code> in this path
260 info. To let Apache find this sort of path info, you neet to set
261 <code>scolonpathdelim</code> to <code>On</code> in
262 <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
263 <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>.</p>
264 <p>Finally you can support cookies and URL encoding at the same time, by
265 configuring the name of the cookie and the name of the URL parameter
266 separated by a vertical bar (<code>|</code>) as in the following example:</p>
267 <pre class="prettyprint lang-config">ProxyPass "/test" "balancer://mycluster" stickysession=JSESSIONID|jsessionid scolonpathdelim=On
268 <Proxy balancer://mycluster>
269 BalancerMember http://192.168.1.50:80 route=node1
270 BalancerMember http://192.168.1.51:80 route=node2
273 <p>If the cookie and the request parameter both provide routing information
274 for the same request, the information from the request parameter is used.</p>
275 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
276 <div class="section">
277 <h2><a name="stickyness_troubleshooting" id="stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></h2>
279 <p>If you experience stickyness errors, e.g. users lose their
280 application sessions and need to login again, you first want to
281 check whether this is because the back-ends are sometimes unavailable
282 or whether your configuration is wrong. To find out about possible
283 stability problems with the back-ends, check your Apache error log
284 for proxy error messages.</p>
285 <p>To verify your configuration, first check, whether the stickyness
286 is based on a cookie or on URL encoding. Next step would be logging
287 the appropriate data in the access log by using an enhanced
288 <code class="directive"><a href="../mod/mod_log_config.html#logformat">LogFormat</a></code>.
289 The following fields are useful:</p>
291 <dt><code>%{MYCOOKIE}C</code></dt>
292 <dd>The value contained in the cookie with name <code>MYCOOKIE</code>.
293 The name should be the same given in the <var>stickysession</var>
295 <dt><code>%{Set-Cookie}o</code></dt>
296 <dd>This logs any cookie set by the back-end. You can track,
297 whether the back-end sets the session cookie you expect, and
298 to which value it is set.</dd>
299 <dt><code>%{BALANCER_SESSION_STICKY}e</code></dt>
300 <dd>The name of the cookie or request parameter used
301 to lookup the routing information.</dd>
302 <dt><code>%{BALANCER_SESSION_ROUTE}e</code></dt>
303 <dd>The route information found in the request.</dd>
304 <dt><code>%{BALANCER_WORKER_ROUTE}e</code></dt>
305 <dd>The route of the worker chosen.</dd>
306 <dt><code>%{BALANCER_ROUTE_CHANGED}e</code></dt>
307 <dd>Set to <code>1</code> if the route in the request
308 is different from the route of the worker, i.e.
309 the request couldn't be handled sticky.</dd>
311 <p>Common reasons for loss of session are session timeouts,
312 which are usually configurable on the back-end server.</p>
313 <p>The balancer also logs detailed information about handling
314 stickyness to the error log, if the log level is set to
315 <code>debug</code> or higher. This is an easy way to
316 troubleshoot stickyness problems, but the log volume might
317 be to high for production servers under high load.</p>
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