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<title>mod_proxy_hcheck - Apache HTTP Server Version 2.5</title>
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<div id="preamble"><h1>Apache Module mod_proxy_hcheck</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_proxy_hcheck.html" title="English"> en </a> |
<a href="../fr/mod/mod_proxy_hcheck.html" hreflang="fr" rel="alternate" title="Français"> fr </a></p>
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<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Dynamic health check of Balancer members (workers) for
<code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>proxy_hcheck_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_proxy_hcheck.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 2.4.21 and later</td></tr></table>
<h3>Summary</h3>
<p>This module provides for dynamic health checking of balancer
members (workers). This can be enabled on a worker-by-worker
basis. The health check is done independently of the
actual reverse proxy requests.</p>
<p>This module <em>requires</em> the service of <code class="module"><a href="../mod/mod_watchdog.html">mod_watchdog</a></code>.</p>
<div class="note"><h3>Parameters</h3>
<p>The health check mechanism is enabled via the use of additional
<code class="directive"><a href="../mod/mod_proxy.html#balancermember">BalancerMember</a></code> parameters, which are configured
in the standard way via <code class="directive"><a href="../mod/mod_proxy.html#proxypass">ProxyPass</a></code>:</p>
<p>A new BalancerMember <a href="mod_proxy.html#status_table">status</a> state (flag)
is defined via this module: "<code>C</code>".
When the worker is taken offline due to failures as determined by the health
check module, this flag is set, and can be seen (and modified) via the
<code>balancer-manager</code>.</p>
<table>
<tr><th>Parameter</th>
<th>Default</th>
<th>Description</th></tr>
<tr><td>hcmethod</td>
<td>None</td>
<td>No dynamic health check performed. Choices are:
<table>
<tr><th>Method</th><th>Description</th><th>Note</th></tr>
<tr><td>None</td><td>No dynamic health checking done</td><td /></tr>
<tr><td>TCP</td><td>Check that a socket to the backend can be created: e.g. "are you up"</td><td /></tr>
<tr><td>OPTIONS</td><td>Send an <code>HTTP OPTIONS</code> request to the backend</td><td>*</td></tr>
<tr><td>HEAD</td><td>Send an <code>HTTP HEAD</code> request to the backend</td><td>*</td></tr>
<tr><td>GET</td><td>Send an <code>HTTP GET</code> request to the backend</td><td>*</td></tr>
<tr><td colspan="3" /></tr>
<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>
</table>
</td></tr>
<tr><td>hcpasses</td>
<td>1</td>
<td>Number of successful health check tests before worker is re-enabled</td></tr>
<tr><td>hcfails</td>
<td>1</td>
<td>Number of failed health check tests before worker is disabled</td></tr>
<tr><td>hcinterval</td>
<td>30</td>
<td>Period of health checks in seconds (e.g. performed every 30 seconds).
Uses the <a href="directive-dict.html#Syntax">time-interval</a> directive syntax.</td></tr>
<tr><td>hcuri</td>
<td> </td>
<td>Additional URI to be appended to the worker URL for the health check.</td></tr>
<tr><td>hctemplate</td>
<td> </td>
<td>Name of template, created via <code class="directive"><a href="#proxyhctemplate">ProxyHCTemplate</a></code>,
to use for setting health check parameters for this worker</td></tr>
<tr><td>hcexpr</td>
<td> </td>
<td>Name of expression, created via <code class="directive"><a href="#proxyhcexpr">ProxyHCExpr</a></code>,
used to check response headers for health.<br />
<em>If not used, 2xx thru 3xx status codes imply success</em></td></tr>
</table>
</div>
</div>
<div id="quickview"><h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#examples">Usage examples</a></li>
</ul><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif" /> <a href="#proxyhcexpr">ProxyHCExpr</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#proxyhctemplate">ProxyHCTemplate</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#proxyhctpsize">ProxyHCTPsize</a></li>
</ul>
<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__&list_id=144532&product=Apache%20httpd-2&query_format=specific&order=changeddate%20DESC%2Cpriority%2Cbug_severity&component=mod_proxy_hcheck">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&component=mod_proxy_hcheck">Report a bug</a></li></ul><h3>See also</h3>
<ul class="seealso">
<li><code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code></li>
<li><a href="#comments_section">Comments</a></li></ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="examples" id="examples">Usage examples</a> <a title="Permanent link" href="#examples" class="permalink">¶</a></h2>
<p>The following example shows how one might configured health checking
for various backend servers:</p>
<pre class="prettyprint lang-config">ProxyHCExpr ok234 {%{REQUEST_STATUS} =~ /^[234]/}
ProxyHCExpr gdown {%{REQUEST_STATUS} =~ /^[5]/}
ProxyHCExpr in_maint {hc('body') !~ /Under maintenance/}
<Proxy balancer://foo>
BalancerMember http://www.example.com/ hcmethod=GET hcexpr=in_maint hcuri=/status.php
BalancerMember http://www2.example.com/ hcmethod=HEAD hcexpr=ok234 hcinterval=10
BalancerMember http://www3.example.com/ hcmethod=TCP hcinterval=5 hcpasses=2 hcfails=3
BalancerMember http://www4.example.com/
</Proxy>
ProxyPass "/" "balancer://foo"
ProxyPassReverse "/" "balancer://foo"</pre>
<p>In this scenario, <code>http://www.example.com/</code> is health checked by sending a <code>GET /status.php</code>
request to that server and seeing that the returned page does not include the string <em>Under maintenance</em>. If
it does, that server is put in health-check fail mode, and disabled. This dynamic check is performed
every 30 seconds, which is the default.</p>
<p><code>http://www2.example.com/</code> is checked by sending a simple <code>HEAD</code> request every
10 seconds and making sure that the response status is 2xx, 3xx or 4xx. <code>http://www3.example.com/</code> is checked
every 5 seconds by simply ensuring that the socket to that server is up. If the backend is marked as
"down" and it passes 2 health check, it will be re-enabled and added back into the load balancer.
It takes 3 back-to-back health check failures to disable the server and move it out
of rotation. Finally, <code>http://www4.example.com/</code> is
not dynamically checked at all.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="ProxyHCExpr" id="ProxyHCExpr">ProxyHCExpr</a> <a name="proxyhcexpr" id="proxyhcexpr">Directive</a> <a title="Permanent link" href="#proxyhcexpr" class="permalink">¶</a></h2>
<table class="directive">
<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>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyHCExpr <em>name</em> {<em>ap_expr expression</em>}</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_hcheck</td></tr>
</table>
<p>The <code class="directive">ProxyHCExpr</code> directive allows
for creating a named condition expression that checks the response
headers of the backend server to determine its health. This named
condition can then be assigned to balancer members via the <code>hcexpr</code>
parameter.</p>
<div class="example"><h3>ProxyHCExpr: Allow for 2xx/3xx/4xx as passing</h3><pre class="prettyprint lang-config">ProxyHCExpr ok234 {%{REQUEST_STATUS} =~ /^[234]/}
ProxyPass "/apps" "balancer://foo"
<Proxy balancer://foo>
BalancerMember http://www2.example.com/ hcmethod=HEAD hcexpr=ok234 hcinterval=10
</Proxy></pre>
</div>
<div class="note">
The <a href="../expr.html">expression</a> can use curly-parens ("{}") as
quoting deliminators in addition to normal quotes.
</div>
<p>If using a health check method (eg: <code>GET</code>) which results in a response
body, that body itself can be checked via <code>ap_expr</code> using the <code>hc()</code>
expression function, which is unique to this module.</p>
<p>In the following example, we send the backend a <code>GET</code> request
and if the response body contains the phrase <em>Under maintenance</em>,
we want to disable the backend.</p>
<div class="example"><h3>ProxyHCExpr: Checking response body</h3><pre class="prettyprint lang-config">ProxyHCExpr in_maint {hc('body') !~ /Under maintenance/}
ProxyPass "/apps" "balancer://foo"
<Proxy balancer://foo>
BalancerMember http://www.example.com/ hcexpr=in_maint hcmethod=get hcuri=/status.php
</Proxy></pre>
</div>
<p><em>NOTE:</em> Since response body can quite large, it is best if used against specific status pages.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="ProxyHCTemplate" id="ProxyHCTemplate">ProxyHCTemplate</a> <a name="proxyhctemplate" id="proxyhctemplate">Directive</a> <a title="Permanent link" href="#proxyhctemplate" class="permalink">¶</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Creates a named template for setting various health check parameters</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyHCTemplate <em>name</em> <em>parameter</em>=<em>setting</em> [...]</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>FileInfo</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_hcheck</td></tr>
</table>
<p>The <code class="directive">ProxyHCTemplate</code> directive allows
for creating a named set (template) of health check parameters
that can then be assigned to balancer members via the <code>hctemplate</code>
parameter.</p>
<div class="example"><h3>ProxyHCTemplate</h3><pre class="prettyprint lang-config">ProxyHCTemplate tcp5 hcmethod=tcp hcinterval=5
ProxyPass "/apps" "balancer://foo"
<Proxy balancer://foo>
BalancerMember http://www2.example.com/ hctemplate=tcp5
</Proxy></pre>
</div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="ProxyHCTPsize" id="ProxyHCTPsize">ProxyHCTPsize</a> <a name="proxyhctpsize" id="proxyhctpsize">Directive</a> <a title="Permanent link" href="#proxyhctpsize" class="permalink">¶</a></h2>
<table class="directive">
<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>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ProxyHCTPsize <em>size</em></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>ProxyHCTPsize 16</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_proxy_hcheck</td></tr>
</table>
<p>If Apache httpd and APR are built with thread support, the health check
module will offload the work of the actual checking to a threadpool
associated with the Watchdog process, allowing for parallel checks.
The <code class="directive">ProxyHCTPsize</code> directive
determines the size of this threadpool. If set to <code>0</code>, no threadpool
is used at all, resulting in serialized health checks.</p>
<div class="example"><h3>ProxyHCTPsize</h3><pre class="prettyprint lang-config">ProxyHCTPsize 32</pre>
</div>
</div>
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