1<?xml version="1.0" encoding="ISO-8859-1"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!-- 4 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 5 This file is generated from xml source: DO NOT EDIT 6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 7 --> 8<title>mod_proxy_balancer - Apache HTTP Server</title> 9<link href="/style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> 10<link href="/style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> 11<link href="/style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link rel="stylesheet" type="text/css" href="/style/css/prettify.css" /> 12<script src="/style/scripts/prettify.js" type="text/javascript"> 13</script> 14 15<link href="/images/favicon.ico" rel="shortcut icon" /></head> 16<body> 17<div id="page-header"> 18<p class="menu"><a href="/mod/">Modules</a> | <a href="/mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="/glossary.html">Glossary</a> | <a href="/sitemap.html">Sitemap</a></p> 19<p class="apache">Apache HTTP Server Version 2.2</p> 20<img alt="" src="/images/feather.gif" /></div> 21<div class="up"><a href="./"><img title="<-" alt="<-" src="/images/left.gif" /></a></div> 22<div id="path"> 23<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.2</a> > <a href="./">Modules</a></div> 24<div id="page-content"> 25<div id="preamble"><h1>Apache Module mod_proxy_balancer</h1> 26<div class="toplang"> 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> 29</div> 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> 35<h3>Summary</h3> 36 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 39 </p> 40 41 <p>Thus, in order to get the ability of load balancing, 42 <code class="module"><a href="/mod/mod_proxy.html">mod_proxy</a></code> and <code class="module"><a href="/mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> 43 have to be present in the server.</p> 44 45 <div class="warning"><h3>Warning</h3> 46 <p>Do not enable proxying until you have <a href="mod_proxy.html#access">secured your server</a>. Open proxy 47 servers are dangerous both to your network and to the Internet at 48 large.</p> 49 </div> 50</div> 51<div id="quickview"><h3 class="directives">Directives</h3> 52<p>This module provides no 53 directives.</p> 54<h3>Topics</h3> 55<ul id="topics"> 56<li><img alt="" src="/images/down.gif" /> <a href="#scheduler">Load balancer scheduler algorithm</a></li> 57<li><img alt="" src="/images/down.gif" /> <a href="#stickyness">Load balancer stickyness</a></li> 58<li><img alt="" src="/images/down.gif" /> <a href="#example">Examples of a balancer configuration</a></li> 59<li><img alt="" src="/images/down.gif" /> <a href="#requests">Request Counting Algorithm</a></li> 60<li><img alt="" src="/images/down.gif" /> <a href="#traffic">Weighted Traffic Counting Algorithm</a></li> 61<li><img alt="" src="/images/down.gif" /> <a href="#busyness">Pending Request Counting Algorithm</a></li> 62<li><img alt="" src="/images/down.gif" /> <a href="#environment">Exported Environment Variables</a></li> 63<li><img alt="" src="/images/down.gif" /> <a href="#balancer_manager">Enabling Balancer Manager Support</a></li> 64<li><img alt="" src="/images/down.gif" /> <a href="#stickyness_implementation">Details on load balancer stickyness</a></li> 65<li><img alt="" src="/images/down.gif" /> <a href="#stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></li> 66</ul><h3>See also</h3> 67<ul class="seealso"> 68<li><code class="module"><a href="/mod/mod_proxy.html">mod_proxy</a></code></li> 69</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 70<div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 71<div class="section"> 72<h2><a name="scheduler" id="scheduler">Load balancer scheduler algorithm</a></h2> 73 74 <p>At present, there are 3 load balancer scheduler algorithms available 75 for use: Request Counting, Weighted Traffic Counting and Pending Request 76 Counting. These are controlled via the <code>lbmethod</code> value of 77 the Balancer definition. See the <code class="directive"><a href="/mod/mod_proxy.html#proxypass">ProxyPass</a></code> 78 directive for more information.</p> 79</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 80<div class="section"> 81<h2><a name="stickyness" id="stickyness">Load balancer stickyness</a></h2> 82 83 <p>The balancer supports stickyness. When a request is proxied 84 to some back-end, then all following requests from the same user 85 should be proxied to the same back-end. Many load balancers implement 86 this feature via a table that maps client IP addresses to back-ends. 87 This approach is transparent to clients and back-ends, but suffers 88 from some problems: unequal load distribution if clients are themselves 89 hidden behind proxies, stickyness errors when a client uses a dynamic 90 IP address that changes during a session and loss of stickyness, if the 91 mapping table overflows.</p> 92 <p>The module <code class="module"><a href="/mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> implements stickyness 93 on top of two alternative means: cookies and URL encoding. Providing the 94 cookie can be either done by the back-end or by the Apache web server 95 itself. The URL encoding is usually done on the back-end.</p> 96</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 97<div class="section"> 98<h2><a name="example" id="example">Examples of a balancer configuration</a></h2> 99 100 <p>Before we dive into the technical details, here's an example of 101 how you might use <code class="module"><a href="/mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> to provide 102 load balancing between two back-end servers: 103 </p> 104 105 <div class="example"><p><code> 106 <Proxy balancer://mycluster><br /> 107 BalancerMember http://192.168.1.50:80<br /> 108 BalancerMember http://192.168.1.51:80<br /> 109 </Proxy><br /> 110 ProxyPass /test balancer://mycluster 111 </code></p></div> 112 113 <p>Another example of how to provide load balancing with stickyness 114 using <code class="module"><a href="/mod/mod_headers.html">mod_headers</a></code>, even if the back-end server does 115 not set a suitable session cookie: 116 </p> 117 118 <div class="example"><p><code> 119 Header add Set-Cookie "ROUTEID=.%{BALANCER_WORKER_ROUTE}e; path=/" 120 env=BALANCER_ROUTE_CHANGED<br /> 121 <Proxy balancer://mycluster><br /> 122 BalancerMember http://192.168.1.50:80 route=1<br /> 123 BalancerMember http://192.168.1.51:80 route=2<br /> 124 ProxySet stickysession=ROUTEID<br /> 125 </Proxy><br /> 126 ProxyPass /test balancer://mycluster 127 </code></p></div> 128</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 129<div class="section"> 130<h2><a name="requests" id="requests">Request Counting Algorithm</a></h2> 131 132 <p>Enabled via <code>lbmethod=byrequests</code>, the idea behind this 133 scheduler is that we distribute the requests among the 134 various workers to ensure that each gets their configured share 135 of the number of requests. It works as follows:</p> 136 137 <p><dfn>lbfactor</dfn> is <em>how much we expect this worker 138 to work</em>, or <em>the workers's work quota</em>. This is 139 a normalized value representing their "share" of the amount of 140 work to be done.</p> 141 142 <p><dfn>lbstatus</dfn> is <em>how urgent this worker has to work 143 to fulfill its quota of work</em>.</p> 144 145 <p>The <dfn>worker</dfn> is a member of the load balancer, 146 usually a remote host serving one of the supported protocols.</p> 147 148 <p>We distribute each worker's work quota to the worker, and then look 149 which of them needs to work most urgently (biggest lbstatus). This 150 worker is then selected for work, and its lbstatus reduced by the 151 total work quota we distributed to all workers. Thus the sum of all 152 lbstatus does not change(*) and we distribute the requests 153 as desired.</p> 154 155 <p>If some workers are disabled, the others will 156 still be scheduled correctly.</p> 157 158 <div class="example"><pre><code>for each worker in workers 159 worker lbstatus += worker lbfactor 160 total factor += worker lbfactor 161 if worker lbstatus > candidate lbstatus 162 candidate = worker 163 164candidate lbstatus -= total factor</code></pre></div> 165 166 <p>If a balancer is configured as follows:</p> 167 168 <table><tr><th>worker</th> 169 <th class="data">a</th> 170 <th class="data">b</th> 171 <th class="data">c</th> 172 <th class="data">d</th></tr> 173<tr><th>lbfactor</th> 174 <td class="data">25</td> 175 <td class="data">25</td> 176 <td class="data">25</td> 177 <td class="data">25</td></tr> 178<tr><th>lbstatus</th> 179 <td class="data">0</td> 180 <td class="data">0</td> 181 <td class="data">0</td> 182 <td class="data">0</td></tr> 183</table> 184 185 <p>And <var>b</var> gets disabled, the following schedule is produced:</p> 186 187 <table><tr><th>worker</th> 188 <th class="data">a</th> 189 <th class="data">b</th> 190 <th class="data">c</th> 191 <th class="data">d</th></tr> 192<tr><th>lbstatus</th> 193 <td class="data"><em>-50</em></td> 194 <td class="data">0</td> 195 <td class="data">25</td> 196 <td class="data">25</td></tr> 197<tr><th>lbstatus</th> 198 <td class="data">-25</td> 199 <td class="data">0</td> 200 <td class="data"><em>-25</em></td> 201 <td class="data">50</td></tr> 202<tr><th>lbstatus</th> 203 <td class="data">0</td> 204 <td class="data">0</td> 205 <td class="data">0</td> 206 <td class="data"><em>0</em></td></tr> 207<tr><td class="data" colspan="5">(repeat)</td></tr> 208</table> 209 210 <p>That is it schedules: <var>a</var> <var>c</var> <var>d</var> 211 <var>a</var> <var>c</var> <var>d</var> <var>a</var> <var>c</var> 212 <var>d</var> ... Please note that:</p> 213 214 <table><tr><th>worker</th> 215 <th class="data">a</th> 216 <th class="data">b</th> 217 <th class="data">c</th> 218 <th class="data">d</th></tr> 219<tr><th>lbfactor</th> 220 <td class="data">25</td> 221 <td class="data">25</td> 222 <td class="data">25</td> 223 <td class="data">25</td></tr> 224</table> 225 226 <p>Has the exact same behavior as:</p> 227 228 <table><tr><th>worker</th> 229 <th class="data">a</th> 230 <th class="data">b</th> 231 <th class="data">c</th> 232 <th class="data">d</th></tr> 233<tr><th>lbfactor</th> 234 <td class="data">1</td> 235 <td class="data">1</td> 236 <td class="data">1</td> 237 <td class="data">1</td></tr> 238</table> 239 240 <p>This is because all values of <dfn>lbfactor</dfn> are normalized 241 with respect to the others. For:</p> 242 243 <table><tr><th>worker</th> 244 <th class="data">a</th> 245 <th class="data">b</th> 246 <th class="data">c</th></tr> 247<tr><th>lbfactor</th> 248 <td class="data">1</td> 249 <td class="data">4</td> 250 <td class="data">1</td></tr> 251</table> 252 253 <p>worker <var>b</var> will, on average, get 4 times the requests 254 that <var>a</var> and <var>c</var> will.</p> 255 256 <p>The following asymmetric configuration works as one would expect:</p> 257 258 <table><tr><th>worker</th> 259 <th class="data">a</th> 260 <th class="data">b</th></tr> 261<tr><th>lbfactor</th> 262 <td class="data">70</td> 263 <td class="data">30</td></tr> 264<tr><td class="data" colspan="2"> </td></tr> 265<tr><th>lbstatus</th> 266 <td class="data"><em>-30</em></td> 267 <td class="data">30</td></tr> 268<tr><th>lbstatus</th> 269 <td class="data">40</td> 270 <td class="data"><em>-40</em></td></tr> 271<tr><th>lbstatus</th> 272 <td class="data"><em>10</em></td> 273 <td class="data">-10</td></tr> 274<tr><th>lbstatus</th> 275 <td class="data"><em>-20</em></td> 276 <td class="data">20</td></tr> 277<tr><th>lbstatus</th> 278 <td class="data"><em>-50</em></td> 279 <td class="data">50</td></tr> 280<tr><th>lbstatus</th> 281 <td class="data">20</td> 282 <td class="data"><em>-20</em></td></tr> 283<tr><th>lbstatus</th> 284 <td class="data"><em>-10</em></td> 285 <td class="data">10</td></tr> 286<tr><th>lbstatus</th> 287 <td class="data"><em>-40</em></td> 288 <td class="data">40</td></tr> 289<tr><th>lbstatus</th> 290 <td class="data">30</td> 291 <td class="data"><em>-30</em></td></tr> 292<tr><th>lbstatus</th> 293 <td class="data"><em>0</em></td> 294 <td class="data">0</td></tr> 295<tr><td class="data" colspan="3">(repeat)</td></tr> 296</table> 297 298 <p>That is after 10 schedules, the schedule repeats and 7 <var>a</var> 299 are selected with 3 <var>b</var> interspersed.</p> 300</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 301<div class="section"> 302<h2><a name="traffic" id="traffic">Weighted Traffic Counting Algorithm</a></h2> 303 304 <p>Enabled via <code>lbmethod=bytraffic</code>, the idea behind this 305 scheduler is very similar to the Request Counting method, with 306 the following changes:</p> 307 308 <p><dfn>lbfactor</dfn> is <em>how much traffic, in bytes, we want 309 this worker to handle</em>. This is also a normalized value 310 representing their "share" of the amount of work to be done, 311 but instead of simply counting the number of requests, we take 312 into account the amount of traffic this worker has seen.</p> 313 314 <p>If a balancer is configured as follows:</p> 315 316 <table><tr><th>worker</th> 317 <th class="data">a</th> 318 <th class="data">b</th> 319 <th class="data">c</th></tr> 320<tr><th>lbfactor</th> 321 <td class="data">1</td> 322 <td class="data">2</td> 323 <td class="data">1</td></tr> 324</table> 325 326 <p>Then we mean that we want <var>b</var> to process twice the 327 amount of bytes than <var>a</var> or <var>c</var> should. It does 328 not necessarily mean that <var>b</var> would handle twice as 329 many requests, but it would process twice the I/O. Thus, the 330 size of the request and response are applied to the weighting 331 and selection algorithm.</p> 332 333</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 334<div class="section"> 335<h2><a name="busyness" id="busyness">Pending Request Counting Algorithm</a></h2> 336 337 338 339 <p>Enabled via <code>lbmethod=bybusyness</code>, this scheduler keeps 340 track of how many requests each worker is assigned at present. A new 341 request is automatically assigned to the worker with the lowest 342 number of active requests. This is useful in the case of workers 343 that queue incoming requests independently of Apache, to ensure that 344 queue length stays even and a request is always given to the worker 345 most likely to service it fastest.</p> 346 347 <p>In the case of multiple least-busy workers, the statistics (and 348 weightings) used by the Request Counting method are used to break the 349 tie. Over time, the distribution of work will come to resemble that 350 characteristic of <code>byrequests</code>.</p> 351 352 <p>This algorithm is available in Apache HTTP Server 2.2.10 and later.</p> 353 354</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 355<div class="section"> 356<h2><a name="environment" id="environment">Exported Environment Variables</a></h2> 357 358 <p>At present there are 6 environment variables exported:</p> 359 360 <dl> 361 362 <dt><var><a name="balancer_session_sticky" id="balancer_session_sticky">BALANCER_SESSION_STICKY</a></var></dt> 363 <dd> 364 <p>This is assigned the <var>stickysession</var> value used for the current 365 request. It is the name of the cookie or request parameter used for sticky sessions</p> 366 </dd> 367 368 369 <dt><var><a name="balancer_session_route" id="balancer_session_route">BALANCER_SESSION_ROUTE</a></var></dt> 370 <dd> 371 <p>This is assigned the <var>route</var> parsed from the current 372 request.</p> 373 </dd> 374 375 376 <dt><var><a name="balancer_name" id="balancer_name">BALANCER_NAME</a></var></dt> 377 <dd> 378 <p>This is assigned the name of the balancer used for the current 379 request. The value is something like <code>balancer://foo</code>.</p> 380 </dd> 381 382 383 <dt><var><a name="balancer_worker_name" id="balancer_worker_name">BALANCER_WORKER_NAME</a></var></dt> 384 <dd> 385 <p>This is assigned the name of the worker used for the current request. 386 The value is something like <code>http://hostA:1234</code>.</p> 387 </dd> 388 389 390 <dt><var><a name="balancer_worker_route" id="balancer_worker_route">BALANCER_WORKER_ROUTE</a></var></dt> 391 <dd> 392 <p>This is assigned the <var>route</var> of the worker that will be 393 used for the current request.</p> 394 </dd> 395 396 397 <dt><var><a name="balancer_route_changed" id="balancer_route_changed">BALANCER_ROUTE_CHANGED</a></var></dt> 398 <dd> 399 <p>This is set to 1 if the session route does not match the 400 worker route (BALANCER_SESSION_ROUTE != BALANCER_WORKER_ROUTE) or the 401 session does not yet have an established route. This can be used to 402 determine when/if the client needs to be sent an updated route 403 when sticky sessions are used.</p> 404 </dd> 405 </dl> 406 407</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 408<div class="section"> 409<h2><a name="balancer_manager" id="balancer_manager">Enabling Balancer Manager Support</a></h2> 410 411 <p>This module <em>requires</em> the service of 412 <code class="module"><a href="/mod/mod_status.html">mod_status</a></code>. 413 Balancer manager enables dynamic update of balancer 414 members. You can use balancer manager to change the balance 415 factor or a particular member, or put it in the off line 416 mode. 417 </p> 418 419 <p>Thus, in order to get the ability of load balancer management, 420 <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> 421 have to be present in the server.</p> 422 423 <p>To enable load balancer management for browsers from the example.com 424 domain add this code to your <code>httpd.conf</code> 425 configuration file</p> 426<div class="example"><p><code> 427 <Location /balancer-manager><br /> 428 SetHandler balancer-manager<br /> 429<br /> 430 Order Deny,Allow<br /> 431 Deny from all<br /> 432 Allow from .example.com<br /> 433 </Location> 434</code></p></div> 435 436 <p>You can now access load balancer manager by using a Web browser 437 to access the page 438 <code>http://your.server.name/balancer-manager</code></p> 439</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 440<div class="section"> 441<h2><a name="stickyness_implementation" id="stickyness_implementation">Details on load balancer stickyness</a></h2> 442 443 <p>When using cookie based stickyness, you need to configure the 444 name of the cookie that contains the information about which back-end 445 to use. This is done via the <var>stickysession</var> attribute added 446 to either <code class="directive"><a href="/mod/mod_proxy.html#proxypass">ProxyPass</a></code> or 447 <code class="directive"><a href="/mod/mod_proxy.html#proxyset">ProxySet</a></code>. The name of 448 the cookie is case-sensitive. The balancer extracts the value of the 449 cookie and looks for a member worker with <var>route</var> equal 450 to that value. The <var>route</var> must also be set in either 451 <code class="directive"><a href="/mod/mod_proxy.html#proxypass">ProxyPass</a></code> or 452 <code class="directive"><a href="/mod/mod_proxy.html#proxyset">ProxySet</a></code>. The cookie can either 453 be set by the back-end, or as shown in the above 454 <a href="#example">example</a> by the Apache web server itself.</p> 455 <p>Some back-ends use a slightly different form of stickyness cookie, 456 for instance Apache Tomcat. Tomcat adds the name of the Tomcat instance 457 to the end of its session id cookie, separated with a dot (<code>.</code>) 458 from the session id. Thus if the Apache web server finds a dot in the value 459 of the stickyness cookie, it only uses the part behind the dot to search 460 for the route. In order to let Tomcat know about its instance name, you 461 need to set the attribute <code>jvmRoute</code> inside the Tomcat 462 configuration file <code>conf/server.xml</code> to the value of the 463 <var>route</var> of the worker that connects to the respective Tomcat. 464 The name of the session cookie used by Tomcat (and more generally by Java 465 web applications based on servlets) is <code>JSESSIONID</code> 466 (upper case) but can be configured to something else.</p> 467 <p>The second way of implementing stickyness is URL encoding. 468 The web server searches for a query parameter in the URL of the request. 469 The name of the parameter is specified again using <var>stickysession</var>. 470 The value of the parameter is used to lookup a member worker with <var>route</var> 471 equal to that value. Since it is not easy to extract and manipulate all 472 URL links contained in responses, generally the work of adding the parameters 473 to each link is done by the back-end generating the content. 474 In some cases it might be feasible doing 475 this via the web server using <code class="module"><a href="/mod/mod_substitute.html">mod_substitute</a></code>. 476 This can have negative impact on performance though.</p> 477 <p>The Java standards implement URL encoding slightly different. They use 478 a path info appended to the URL using a semicolon (<code>;</code>) 479 as the separator and add the session id behind. As in the cookie case, 480 Apache Tomcat can include the configured <code>jvmRoute</code> in this path 481 info. To let Apache find this sort of path info, you need to set 482 <code>scolonpathdelim</code> to <code>On</code> in 483 <code class="directive"><a href="/mod/mod_proxy.html#proxypass">ProxyPass</a></code> or 484 <code class="directive"><a href="/mod/mod_proxy.html#proxyset">ProxySet</a></code>.</p> 485 <p>Finally you can support cookies and URL encoding at the same time, by 486 configuring the name of the cookie and the name of the URL parameter 487 separated by a vertical bar (<code>|</code>) as in the following example:</p> 488 <div class="example"><p><code> 489 ProxyPass /test balancer://mycluster stickysession=JSESSIONID|jsessionid scolonpathdelim=On<br /> 490 <Proxy balancer://mycluster><br /> 491 BalancerMember http://192.168.1.50:80 route=node1<br /> 492 BalancerMember http://192.168.1.51:80 route=node2<br /> 493 </Proxy><br /> 494 </code></p></div> 495 <p>If the cookie and the request parameter both provide routing information 496 for the same request, the information from the request parameter is used.</p> 497</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 498<div class="section"> 499<h2><a name="stickyness_troubleshooting" id="stickyness_troubleshooting">Troubleshooting load balancer stickyness</a></h2> 500 501 <p>If you experience stickyness errors, e.g. users loose their 502 application sessions and need to login again, you first want to 503 check whether this is because the back-ends are sometimes unavailable 504 or whether your configuration is wrong. To find out about possible 505 stability problems with the back-ends, check your Apache error log 506 for proxy error messages.</p> 507 <p>To verify your configuration, first check, whether the stickyness 508 is based on a cookie or on URL encoding. Next step would be logging 509 the appropriate data in the access log by using an enhanced 510 <code class="directive"><a href="/mod/mod_log_config.html#logformat">LogFormat</a></code>. 511 The following fields are useful:</p> 512 <dl> 513 <dt><code>%{MYCOOKIE}C</code></dt> 514 <dd>The value contained in the cookie with name <code>MYCOOKIE</code>. 515 The name should be the same given in the <var>stickysession</var> 516 attribute.</dd> 517 <dt><code>%{Set-Cookie}o</code></dt> 518 <dd>This logs any cookie set by the back-end. You can track, 519 whether the back-end sets the session cookie you expect, and 520 to which value it is set.</dd> 521 <dt><code>%{BALANCER_SESSION_STICKY}e</code></dt> 522 <dd>The name of the cookie or request parameter used 523 to lookup the routing information.</dd> 524 <dt><code>%{BALANCER_SESSION_ROUTE}e</code></dt> 525 <dd>The route information found in the request.</dd> 526 <dt><code>%{BALANCER_WORKER_ROUTE}e</code></dt> 527 <dd>The route of the worker chosen.</dd> 528 <dt><code>%{BALANCER_ROUTE_CHANGED}e</code></dt> 529 <dd>Set to <code>1</code> if the route in the request 530 is different from the route of the worker, i.e. 531 the request couldn't be handled sticky.</dd> 532 </dl> 533 <p>Common reasons for loss of session are session timeouts, 534 which are usually configurable on the back-end server.</p> 535 <p>The balancer also logs detailed information about handling 536 stickyness to the error log, if the log level is set to 537 <code>debug</code> or higher. This is an easy way to 538 troubleshoot stickyness problems, but the log volume might 539 be to high for production servers under high load.</p> 540</div> 541</div> 542<div class="bottomlang"> 543<p><span>Available Languages: </span><a href="/en/mod/mod_proxy_balancer.html" title="English"> en </a> | 544<a href="/ja/mod/mod_proxy_balancer.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a></p> 545</div><div class="top"><a href="#page-header"><img src="/images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. 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