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  10 <title>mod_proxy_hcheck - Apache HTTP Server Version 2.5</title>
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  27 <div id="preamble"><h1>Apache Module mod_proxy_hcheck</h1>
  28 <div class="toplang">
  29 <p><span>Available Languages: </span><a href="../en/mod/mod_proxy_hcheck.html" title="English">&nbsp;en&nbsp;</a></p>
  30 </div>
  31 <table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Dynamic health check of Balancer members (workers) for
  32 <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></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_hcheck_module</td></tr>
  35 <tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_proxy_hcheck.c</td></tr>
  36 <tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.4.21 and later</td></tr></table>
  37 <h3>Summary</h3>
  38
  39     <p>This module provides for dynamic health checking of balancer
  40         members (workers). This can be enabled on a worker-by-worker
  41         basis. The health check is done independently of the
  42         actual reverse proxy requests.</p>
  43
  44     <p>This module <em>requires</em> the service of <code class="module"><a href="../mod/mod_watchdog.html">mod_watchdog</a></code>.</p>
  45
  46 <div class="note"><h3>Parameters</h3>
  47   <p>The health check mechanism is enabled via the use of additional
  48         BalancerMember parameters, which are configured in the standard
  49         way via <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code>:</p>
  50
  51   <p>A new BalancerMember status state (flag) is defined via this module: "<code>C</code>".
  52      When the worker is taken offline due to failures as determined by the health
  53      check module, this flag is set, and can be seen (and modified) via the
  54      <code>balancer-manager</code>.</p>
  55
  56     <table>
  57     <tr><th>Parameter</th>
  58         <th>Default</th>
  59         <th>Description</th></tr>
  60     <tr><td>hcmethod</td>
  61         <td>None</td>
  62         <td>No dynamic health check performed. Choices are:
  63                 <table>
  64                         <tr><th>Method</th><th>Description</th><th>Note</th></tr>
  65                         <tr><td>None</td><td>No dynamic health checking done</td><td /></tr>
  66                         <tr><td>TCP</td><td>Check that a socket to the backend can be created: e.g. "are you up"</td><td /></tr>
  67                         <tr><td>OPTIONS</td><td>Send an <code>HTTP OPTIONS</code> request to the backend</td><td>*</td></tr>
  68                         <tr><td>HEAD</td><td>Send an <code>HTTP HEAD</code> request to the backend</td><td>*</td></tr>
  69                         <tr><td>GET</td><td>Send an <code>HTTP GET</code> request to the backend</td><td>*</td></tr>
  70
  71                                 <tr><td colspan="3" /></tr>
  72                                 <tr><td colspan="3">*: Unless <code>hcexpr</code> is used, a 2xx or 3xx HTTP status will be interpreted as <em>passing</em> the health check</td></tr>
  73                 </table>
  74         </td></tr>
  75     <tr><td>hcpasses</td>
  76         <td>1</td>
  77         <td>Number of successful health check tests before worker is re-enabled</td></tr>
  78     <tr><td>hcfails</td>
  79         <td>1</td>
  80         <td>Number of failed health check tests before worker is disabled</td></tr>
  81     <tr><td>hcinterval</td>
  82         <td>30</td>
  83         <td>Period of health checks in seconds (e.g. performed every 30 seconds)</td></tr>
  84     <tr><td>hcuri</td>
  85         <td>&nbsp;</td>
  86         <td>Additional URI to be appended to the worker URL for the health check.</td></tr>
  87     <tr><td>hctemplate</td>
  88         <td>&nbsp;</td>
  89         <td>Name of template, created via <code class="directive">ProxyHCTemplate</code> to use for setting health check parameters for this worker</td></tr>
  90     <tr><td>hcexpr</td>
  91         <td>&nbsp;</td>
  92         <td>Name of expression, created via <code class="directive">ProxyHCExpr</code>, used to check response headers for health.<br />
  93             <em>If not used, 2xx thru 3xx status codes imply success</em></td></tr>
  94     </table>
  95 </div>
  96
  97 </div>
  98 <div id="quickview"><h3>Topics</h3>
  99 <ul id="topics">
 100 <li><img alt="" src="../images/down.gif" /> <a href="#examples">Usage examples</a></li>
 101 </ul><h3 class="directives">Directives</h3>
 102 <ul id="toc">
 103 <li><img alt="" src="../images/down.gif" /> <a href="#proxyhcexpr">ProxyHCExpr</a></li>
 104 <li><img alt="" src="../images/down.gif" /> <a href="#proxyhctemplate">ProxyHCTemplate</a></li>
 105 <li><img alt="" src="../images/down.gif" /> <a href="#proxyhctpsize">ProxyHCTPsize</a></li>
 106 </ul>
 107 <h3>Bugfix checklist</h3><ul class="seealso"><li><a href="https://www.apache.org/dist/httpd/CHANGES_2.4">httpd changelog</a></li><li><a href="https://bz.apache.org/bugzilla/buglist.cgi?bug_status=__open__&amp;list_id=144532&amp;product=Apache%20httpd-2&amp;query_format=specific&amp;order=changeddate%20DESC%2Cpriority%2Cbug_severity&amp;component=mod_proxy_hcheck">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&amp;component=mod_proxy_hcheck">Report a bug</a></li></ul><h3>See also</h3>
 108 <ul class="seealso">
 109 <li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
 110 <li><a href="#comments_section">Comments</a></li></ul></div>
 111 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 112 <div class="section">
 113 <h2><a name="examples" id="examples">Usage examples</a></h2>
 114
 115
 116     <p>The following example shows how one might configured health checking
 117         for various backend servers:</p>
 118
 119
 120         <pre class="prettyprint lang-config">ProxyHCExpr ok234 {%{REQUEST_STATUS} =~ /^[234]/}
 121 ProxyHCExpr gdown {%{REQUEST_STATUS} =~ /^[5]/}
 122 ProxyHCExpr in_maint {hc('body') !~ /Under maintenance/}
 123
 124 &lt;Proxy balancer://foo&gt;
 125   BalancerMember http://www.example.com/  hcmethod=GET hcexpr=in_maint hcuri=/status.php
 126   BalancerMember http://www2.example.com/  hcmethod=HEAD hcexpr=ok234 hcinterval=10
 127   BalancerMember http://www3.example.com/ hcmethod=TCP hcinterval=5 hcpasses=2 hcfails=3
 128   BalancerMember http://www4.example.com/
 129 &lt;/Proxy&gt;
 130
 131 ProxyPass "/" "balancer://foo"
 132 ProxyPassReverse "/" "balancer://foo"</pre>
 133
 134
 135 <p>In this scenario, <code>http://www.example.com/</code> is health checked by sending a <code>GET /status.php</code>
 136 request to that server and seeing that the returned page does not include the string <em>Under maintenance</em>. If
 137 it does, that server is put in health-check fail mode, and disabled. This dynamic check is performed
 138 every 30 seconds, which is the default.</p>
 139
 140 <p><code>http://www2.example.com/</code> is checked by sending a simple <code>HEAD</code> request every
 141 10 seconds and making sure that the response status is 2xx, 3xx or 4xx. <code>http://www3.example.com/</code> is checked
 142 every 5 seconds by simply ensuring that the socket to that server is up. If the backend is marked as
 143 "down" and it passes 2 health check, it will be re-enabled and added back into the load balancer.
 144 It takes 3 back-to-back health check failures to disable the server and move it out
 145 of rotation. Finally, <code>http://www4.example.com/</code> is
 146 not dynamically checked at all.</p>
 147
 148 </div>
 149 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 150 <div class="directive-section"><h2><a name="ProxyHCExpr" id="ProxyHCExpr">ProxyHCExpr</a> <a name="proxyhcexpr" id="proxyhcexpr">Directive</a></h2>
 151 <table class="directive">
 152 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Creates a named condition expression to use to determine health of the backend based on its response.</td></tr>
 153 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyHCExpr name {ap_expr expression}</code></td></tr>
 154 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 155 <tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
 156 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 157 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_hcheck</td></tr>
 158 </table>
 159     <p>The <code class="directive">ProxyHCExpr</code> directive allows
 160        for creating a named condition expression that checks the response
 161        headers of the backend server to determine its health. This named
 162        condition can then be assigned to balancer members via the <code>hcexpr</code>
 163        parameter</p>
 164
 165     <div class="example"><h3>ProxyHCExpr: Allow for 2xx/3xx/4xx as passing</h3><pre class="prettyprint lang-config">ProxyHCExpr ok234 {%{REQUEST_STATUS} =~ /^[234]/}
 166 ProxyPass "/apps"     "http://backend.example.com/" hcexpr=ok234</pre>
 167 </div>
 168
 169     <div class="note">
 170     The <a href="../expr.html">expression</a> can use curly-parens ("{}") as
 171     quoting deliminators in addition to normal quotes.
 172     </div>
 173
 174     <p>If using a health check method (eg: <code>GET</code>) which results in a response
 175     body, that body itself can be checked via <code>ap_expr</code> using the <code>hc()</code>
 176     expression function, which is unique to this module.</p>
 177
 178     <p>In the following example, we send the backend a <code>GET</code> request
 179     and if the response body contains the phrase <em>Under maintenance</em>,
 180     we want to disable the backend.</p>
 181
 182     <div class="example"><h3>ProxyHCExpr: Checking response body</h3><pre class="prettyprint lang-config">ProxyHCExpr in_maint {hc('body') !~ /Under maintenance/}
 183 ProxyPass "/apps"     "http://backend.example.com/" hcexpr=in_maint hcmethod=get hcuri=/status.php</pre>
 184 </div>
 185
 186     <p><em>NOTE:</em> Since response body can quite large, it is best if used against specific status pages.</p>
 187
 188 </div>
 189 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 190 <div class="directive-section"><h2><a name="ProxyHCTemplate" id="ProxyHCTemplate">ProxyHCTemplate</a> <a name="proxyhctemplate" id="proxyhctemplate">Directive</a></h2>
 191 <table class="directive">
 192 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Creates a named template for setting various health check parameters</td></tr>
 193 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyHCTemplate name parameter=setting &lt;...&gt;</code></td></tr>
 194 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
 195 <tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
 196 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 197 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_hcheck</td></tr>
 198 </table>
 199     <p>The <code class="directive">ProxyHCTemplate</code> directive allows
 200        for creating a named set (template) of health check parameters
 201        that can then be assigned to balancer members via the <code>hctemplate</code>
 202        parameter</p>
 203
 204     <div class="example"><h3>ProxyHCTemplate</h3><pre class="prettyprint lang-config">ProxyHCTemplate tcp5 hcmethod=tcp hcinterval=5
 205 ProxyPass "/apps"     "http://backend.example.com/" hctemplate=tcp5</pre>
 206 </div>
 207
 208
 209 </div>
 210 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 211 <div class="directive-section"><h2><a name="ProxyHCTPsize" id="ProxyHCTPsize">ProxyHCTPsize</a> <a name="proxyhctpsize" id="proxyhctpsize">Directive</a></h2>
 212 <table class="directive">
 213 <tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Sets the total server-wide size of the threadpool used for the health check workers.</td></tr>
 214 <tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyHCTPsize &lt;size&gt;</code></td></tr>
 215 <tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
 216 <tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
 217 <tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_hcheck</td></tr>
 218 </table>
 219     <p>If Apache httpd and APR are built with thread support, the health check
 220        module will offload the work of the actual checking to a threadpool
 221        associated with the Watchdog process, allowing for parallel checks.
 222        The <code class="directive">ProxyHCTPsize</code> directive
 223        determines the size of this threadpool. If set to <code>0</code>, no threadpool
 224        is used at all, resulting in serialized health checks. The default size is 16.</p>
 225
 226     <div class="example"><h3>ProxyHCTPsize</h3><pre class="prettyprint lang-config">ProxyHCTPsize 32</pre>
 227 </div>
 228
 229
 230 </div>
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