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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></table>
36 <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
37 <code>HTTP</code>, <code>FTP</code> and <code>AJP13</code> protocols
40 <p>Load balancing scheduler algorithm is provided by not this
41 module but other modules such as:
42 <code class="module"><a href="../mod/mod_lbmethod_byrequests.html">mod_lbmethod_byrequests</a></code>,
43 <code class="module"><a href="../mod/mod_lbmethod_bytraffic.html">mod_lbmethod_bytraffic</a></code>,
44 <code class="module"><a href="../mod/mod_lbmethod_bybusyness.html">mod_lbmethod_bybusyness</a></code> and
45 <code class="module"><a href="../mod/mod_lbmethod_heartbeat.html">mod_lbmethod_heartbeat</a></code>.
48 <p>Thus, in order to get the ability of load balancing,
49 <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>
50 and at least one of load balancing scheduler algorithm modules have
51 to be present in the server.</p>
53 <div class="warning"><h3>Warning</h3>
54 <p>Do not enable proxying until you have <a href="mod_proxy.html#access">secured your server</a>. Open proxy
55 servers are dangerous both to your network and to the Internet at
59 <div id="quickview"><h3 class="directives">Directives</h3>
60 <p>This module provides no
64 <li><img alt="" src="../images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li>
65 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness">Load balancer stickyness</a></li>
66 <li><img alt="" src="../images/down.gif" /> <a href="#example">Examples of a balancer configuration</a></li>
67 <li><img alt="" src="../images/down.gif" /> <a href="#environment">Exported Environment Variables</a></li>
68 <li><img alt="" src="../images/down.gif" /> <a href="#balancer_manager">Enabling Balancer Manager Support</a></li>
69 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness_implementation">Details on load balancer stickyness</a></li>
70 <li><img alt="" src="../images/down.gif" /> <a href="#stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></li>
71 </ul><h3>See also</h3>
73 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
74 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
75 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
77 <h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2>
79 <p>At present, there are 3 load balancer scheduler algorithms available
80 for use: Request Counting, Weighted Traffic Counting and Pending Request
81 Counting. These are controlled via the <code>lbmethod</code> value of
82 the Balancer definition. See the <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code>
83 directive for more information, especially regarding how to
84 configure the Balancer and BalancerMembers.</p>
85 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
87 <h2><a name="stickyness" id="stickyness">Load balancer stickyness</a></h2>
89 <p>The balancer supports stickyness. When a request is proxied
90 to some back-end, then all following requests from the same user
91 should be proxied to the same back-end. Many load balancers implement
92 this feature via a table that maps client IP addresses to back-ends.
93 This approach is transparent to clients and back-ends, but suffers
94 from some problems: unequal load distribution if clients are themselves
95 hidden behind proxies, stickyness errors when a client uses a dynamic
96 IP address that changes during a session and loss of stickyness, if the
97 mapping table overflows.</p>
98 <p>The module <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> implements stickyness
99 on top of two alternative means: cookies and URL encoding. Providing the
100 cookie can be either done by the back-end or by the Apache web server
101 itself. The URL encoding is usually done on the back-end.</p>
102 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
103 <div class="section">
104 <h2><a name="example" id="example">Examples of a balancer configuration</a></h2>
106 <p>Before we dive into the technical details, here's an example of
107 how you might use <code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> to provide
108 load balancing between two back-end servers:
111 <pre class="prettyprint lang-config">
112 <Proxy balancer://mycluster>
113 BalancerMember http://192.168.1.50:80
114 BalancerMember http://192.168.1.51:80
116 ProxyPass /test balancer://mycluster
117 ProxyPassReverse /test balancer://mycluster
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">
127 Header add Set-Cookie "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/" env=BALANCER_ROUTE_CHANGED
128 <Proxy balancer://mycluster>
129 BalancerMember http://192.168.1.50:80 route=1
130 BalancerMember http://192.168.1.51:80 route=2
131 ProxySet stickysession=ROUTEID
133 ProxyPass /test balancer://mycluster
134 ProxyPassReverse /test balancer://mycluster
137 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
138 <div class="section">
139 <h2><a name="environment" id="environment">Exported Environment Variables</a></h2>
141 <p>At present there are 6 environment variables exported:</p>
145 <dt><var><a name="balancer_session_sticky" id="balancer_session_sticky">BALANCER_SESSION_STICKY</a></var></dt>
147 <p>This is assigned the <var>stickysession</var> value used for the current
148 request. It is the name of the cookie or request parameter used for sticky sessions</p>
152 <dt><var><a name="balancer_session_route" id="balancer_session_route">BALANCER_SESSION_ROUTE</a></var></dt>
154 <p>This is assigned the <var>route</var> parsed from the current
159 <dt><var><a name="balancer_name" id="balancer_name">BALANCER_NAME</a></var></dt>
161 <p>This is assigned the name of the balancer used for the current
162 request. The value is something like <code>balancer://foo</code>.</p>
166 <dt><var><a name="balancer_worker_name" id="balancer_worker_name">BALANCER_WORKER_NAME</a></var></dt>
168 <p>This is assigned the name of the worker used for the current request.
169 The value is something like <code>http://hostA:1234</code>.</p>
173 <dt><var><a name="balancer_worker_route" id="balancer_worker_route">BALANCER_WORKER_ROUTE</a></var></dt>
175 <p>This is assigned the <var>route</var> of the worker that will be
176 used for the current request.</p>
180 <dt><var><a name="balancer_route_changed" id="balancer_route_changed">BALANCER_ROUTE_CHANGED</a></var></dt>
182 <p>This is set to 1 if the session route does not match the
183 worker route (BALANCER_SESSION_ROUTE != BALANCER_WORKER_ROUTE) or the
184 session does not yet have an established route. This can be used to
185 determine when/if the client needs to be sent an updated route
186 when sticky sessions are used.</p>
190 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
191 <div class="section">
192 <h2><a name="balancer_manager" id="balancer_manager">Enabling Balancer Manager Support</a></h2>
194 <p>This module <em>requires</em> the service of
195 <code class="module"><a href="../mod/mod_status.html">mod_status</a></code>.
196 Balancer manager enables dynamic update of balancer
197 members. You can use balancer manager to change the balance
198 factor or a particular member, or put it in the off line
202 <p>Thus, in order to get the ability of load balancer management,
203 <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>
204 have to be present in the server.</p>
206 <p>To enable load balancer management for browsers from the example.com
207 domain add this code to your <code>httpd.conf</code>
208 configuration file</p>
209 <pre class="prettyprint lang-config">
210 <Location /balancer-manager>
211 SetHandler balancer-manager
212 Require host example.com
217 <p>You can now access load balancer manager by using a Web browser
219 <code>http://your.server.name/balancer-manager</code>. Please note
220 that only Balancers defined outside of <code><Location ...></code>
221 containers can be dynamically controlled by the Manager.</p>
222 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
223 <div class="section">
224 <h2><a name="stickyness_implementation" id="stickyness_implementation">Details on load balancer stickyness</a></h2>
226 <p>When using cookie based stickyness, you need to configure the
227 name of the cookie that contains the information about which back-end
228 to use. This is done via the <var>stickysession</var> attribute added
229 to either <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
230 <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. The name of
231 the cookie is case-sensitive. The balancer extracts the value of the
232 cookie and looks for a member worker with <var>route</var> equal
233 to that value. The <var>route</var> must also be set in either
234 <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
235 <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>. The cookie can either
236 be set by the back-end, or as shown in the above
237 <a href="#example">example</a> by the Apache web server itself.</p>
238 <p>Some back-ends use a slightly different form of stickyness cookie,
239 for instance Apache Tomcat. Tomcat adds the name of the Tomcat instance
240 to the end of its session id cookie, separated with a dot (<code>.</code>)
241 from the session id. Thus if the Apache web server finds a dot in the value
242 of the stickyness cookie, it only uses the part behind the dot to search
243 for the route. In order to let Tomcat know about its instance name, you
244 need to set the attribute <code>jvmRoute</code> inside the Tomcat
245 configuration file <code>conf/server.xml</code> to the value of the
246 <var>route</var> of the worker that connects to the respective Tomcat.
247 The name of the session cookie used by Tomcat (and more generally by Java
248 web applications based on servlets) is <code>JSESSIONID</code>
249 (upper case) but can be configured to something else.</p>
250 <p>The second way of implementing stickyness is URL encoding.
251 The web server searches for a query parameter in the URL of the request.
252 The name of the parameter is specified again using <var>stickysession</var>.
253 The value of the parameter is used to lookup a member worker with <var>route</var>
254 equal to that value. Since it is not easy to extract and manipulate all
255 URL links contained in responses, generally the work of adding the parameters
256 to each link is done by the back-end generating the content.
257 In some cases it might be feasible doing
258 this via the web server using <code class="module"><a href="../mod/mod_substitute.html">mod_substitute</a></code> or
259 <code class="module"><a href="../mod/mod_sed.html">mod_sed</a></code>. This can have negative impact on performance though.</p>
260 <p>The Java standards implement URL encoding slightly different. They use
261 a path info appended to the URL using a semicolon (<code>;</code>)
262 as the separator and add the session id behind. As in the cookie case,
263 Apache Tomcat can include the configured <code>jvmRoute</code> in this path
264 info. To let Apache find this sort of path info, you neet to set
265 <code>scolonpathdelim</code> to <code>On</code> in
266 <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code> or
267 <code class="directive"><a href="../mod/mod_proxy.html#proxyset">ProxySet</a></code>.</p>
268 <p>Finally you can support cookies and URL encoding at the same time, by
269 configuring the name of the cookie and the name of the URL parameter
270 separated by a vertical bar (<code>|</code>) as in the following example:</p>
271 <pre class="prettyprint lang-config">
272 ProxyPass /test balancer://mycluster stickysession=JSESSIONID|jsessionid scolonpathdelim=On
273 <Proxy balancer://mycluster>
274 BalancerMember http://192.168.1.50:80 route=node1
275 BalancerMember http://192.168.1.51:80 route=node2
279 <p>If the cookie and the request parameter both provide routing information
280 for the same request, the information from the request parameter is used.</p>
281 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
282 <div class="section">
283 <h2><a name="stickyness_troubleshooting" id="stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></h2>
285 <p>If you experience stickyness errors, e.g. users loose their
286 application sessions and need to login again, you first want to
287 check whether this is because the back-ends are sometimes unavailable
288 or whether your configuration is wrong. To find out about possible
289 stability problems with the back-ends, check your Apache error log
290 for proxy error messages.</p>
291 <p>To verify your configuration, first check, whether the stickyness
292 is based on a cookie or on URL encoding. Next step would be logging
293 the appropriate data in the access log by using an enhanced
294 <code class="directive"><a href="../mod/mod_log_config.html#logformat">LogFormat</a></code>.
295 The following fields are useful:</p>
297 <dt><code>%{MYCOOKIE}C</code></dt>
298 <dd>The value contained in the cookie with name <code>MYCOOKIE</code>.
299 The name should be the same given in the <var>stickysession</var>
301 <dt><code>%{Set-Cookie}o</code></dt>
302 <dd>This logs any cookie set by the back-end. You can track,
303 whether the back-end sets the session cookie you expect, and
304 to which value it is set.</dd>
305 <dt><code>%{BALANCER_SESSION_STICKY}e</code></dt>
306 <dd>The name of the cookie or request parameter used
307 to lookup the routing information.</dd>
308 <dt><code>%{BALANCER_SESSION_ROUTE}e</code></dt>
309 <dd>The route information found in the request.</dd>
310 <dt><code>%{BALANCER_WORKER_ROUTE}e</code></dt>
311 <dd>The route of the worker chosen.</dd>
312 <dt><code>%{BALANCER_ROUTE_CHANGED}e</code></dt>
313 <dd>Set to <code>1</code> if the route in the request
314 is different from the route of the worker, i.e.
315 the request couldn't be handled sticky.</dd>
317 <p>Common reasons for loss of session are session timeouts,
318 which are usually configurable on the back-end server.</p>
319 <p>The balancer also logs detailed information about handling
320 stickyness to the error log, if the log level is set to
321 <code>debug</code> or higher. This is an easy way to
322 troubleshoot stickyness problems, but the log volume might
323 be to high for production servers under high load.</p>
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