apache-http-server/docs/manual/mod/mod_proxy_hcheck.xml

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<modulesynopsis metafile="mod_proxy_hcheck.xml.meta">

<name>mod_proxy_hcheck</name>
<description>Dynamic health check of Balancer members (workers) for
<module>mod_proxy</module></description>
<status>Extension</status>
<sourcefile>mod_proxy_hcheck.c</sourcefile>
<identifier>proxy_hcheck_module</identifier>
<compatibility>Available in Apache 2.4.21 and later</compatibility>

<summary>
    <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 <module>mod_watchdog</module>.</p>

<note><title>Parameters</title>
  <p>The health check mechanism is enabled via the use of additional
     <directive module="mod_proxy">BalancerMember</directive> parameters, which are configured
     in the standard way via <directive module="mod_proxy">ProxyPass</directive>:</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></td></tr>
        		<tr><td>TCP</td><td>Check that a socket to the backend can be created: e.g. "are you up"</td><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>CPING</td><td><strong>AJP only</strong> Do <code>CPING/CPONG</code> check</td><td></td></tr>
        		<tr><td>PROVIDER</td><td>Name of <code>provider</code> to be used to check health</td><td></td></tr>
-->
				<tr><td colspan="3"></td></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>&nbsp;</td>
        <td>Additional URI to be appended to the worker URL for the health check.</td></tr>
    <tr><td>hctemplate</td>
        <td>&nbsp;</td>
        <td>Name of template, created via <directive module="mod_proxy_hcheck">ProxyHCTemplate</directive>,
            to use for setting health check parameters for this worker</td></tr>
    <tr><td>hcexpr</td>
        <td>&nbsp;</td>
        <td>Name of expression, created via <directive module="mod_proxy_hcheck">ProxyHCExpr</directive>,
            used to check response headers for health.<br/>
            <em>If not used, 2xx thru 3xx status codes imply success</em></td></tr>
    </table>
</note>

</summary>
<seealso><module>mod_proxy</module></seealso>

<section id="examples">

	<title>Usage examples</title>
    <p>The following example shows how one might configured health checking
    	for various backend servers:</p>

	<!-- This section should probably be extended with more, useful examples -->
	<highlight language="config">
ProxyHCExpr ok234 {%{REQUEST_STATUS} =~ /^[234]/}
ProxyHCExpr gdown {%{REQUEST_STATUS} =~ /^[5]/}
ProxyHCExpr in_maint {hc('body') !~ /Under maintenance/}

&lt;Proxy balancer://foo&gt;
  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/
&lt;/Proxy&gt;

ProxyPass "/" "balancer://foo"
ProxyPassReverse "/" "balancer://foo"
</highlight>

<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>

</section>

<directivesynopsis>
<name>ProxyHCExpr</name>
<description>Creates a named condition expression to use to determine health of the backend based on its response</description>
<syntax>ProxyHCExpr <em>name</em> {<em>ap_expr expression</em>}</syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<override>FileInfo</override>

<usage>
    <p>The <directive>ProxyHCExpr</directive> 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>

    <example><title>ProxyHCExpr: Allow for 2xx/3xx/4xx as passing</title>
    <highlight language="config">
ProxyHCExpr ok234 {%{REQUEST_STATUS} =~ /^[234]/}
ProxyPass "/apps"     "balancer://foo"

&lt;Proxy balancer://foo&gt;
  BalancerMember http://www2.example.com/  hcmethod=HEAD hcexpr=ok234 hcinterval=10
&lt;/Proxy&gt;
    </highlight>
    </example>

    <note>
    The <a href="../expr.html">expression</a> can use curly-parens ("{}") as
    quoting deliminators in addition to normal quotes.
    </note>

    <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>

    <example><title>ProxyHCExpr: Checking response body</title>
    <highlight language="config">
ProxyHCExpr in_maint {hc('body') !~ /Under maintenance/}
ProxyPass "/apps"     "balancer://foo"

&lt;Proxy balancer://foo&gt;
  BalancerMember http://www.example.com/ hcexpr=in_maint hcmethod=get hcuri=/status.php
&lt;/Proxy&gt;
    </highlight>
    </example>

    <p><em>NOTE:</em> Since response body can quite large, it is best if used against specific status pages.</p>
</usage>
</directivesynopsis>


<directivesynopsis>
<name>ProxyHCTemplate</name>
<description>Creates a named template for setting various health check parameters</description>
<syntax>ProxyHCTemplate <em>name</em> <em>parameter</em>=<em>setting</em> [...]</syntax>
<contextlist><context>server config</context><context>virtual host</context>
</contextlist>
<override>FileInfo</override>

<usage>
    <p>The <directive>ProxyHCTemplate</directive> 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>

    <example><title>ProxyHCTemplate</title>
    <highlight language="config">
ProxyHCTemplate tcp5 hcmethod=tcp hcinterval=5
ProxyPass "/apps"     "balancer://foo"

&lt;Proxy balancer://foo&gt;
  BalancerMember http://www2.example.com/ hctemplate=tcp5
&lt;/Proxy&gt;
    </highlight>
    </example>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>ProxyHCTPsize</name>
<description>Sets the total server-wide size of the threadpool used for the health check workers</description>
<syntax>ProxyHCTPsize <em>size</em></syntax>
<default>ProxyHCTPsize 16</default>
<contextlist><context>server config</context>
</contextlist>

<usage>
    <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 <directive>ProxyHCTPsize</directive> 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>

    <example><title>ProxyHCTPsize</title>
    <highlight language="config">
ProxyHCTPsize 32
    </highlight>
    </example>

</usage>
</directivesynopsis>

</modulesynopsis>