2.4.x/docs/manual/mod/mod_heartbeat.xml

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

<name>mod_heartbeat</name>
<description>Sends messages with server status to frontend proxy</description>
<status>Experimental</status>
<sourcefile>mod_heartbeat</sourcefile>
<identifier>heartbeat_module</identifier>
<compatibility>Available in Apache 2.3 and later</compatibility>

<summary>
<p> <module>mod_heartbeat</module> sends multicast messages to a <module>mod_heartmonitor</module> listener 
that advertises the servers current connection count.  Usually, <module>mod_heartmonitor</module>
will be running on a proxy server with <module>mod_lbmethod_heartbeat</module> loaded, which allows
<directive module="mod_proxy">ProxyPass</directive> to use the "heartbeat" <em>lbmethod</em> inside
of <directive module="mod_proxy">ProxyPass</directive>.</p>
<p>
    <module>mod_heartbeat</module> itself is loaded on the origin server(s) that serve requests 
    through the proxy server(s).  
</p>

    <note type="warning">
        To use <module>mod_heartbeat</module>,
        <module>mod_status</module> and <module>mod_watchdog</module>
        must be either a static modules or, if a dynamic module, must
        be loaded before <module>mod_heartbeat</module>.
    </note>

</summary>

<section id="consuming">
    <title>Consuming mod_heartbeat Output</title>
  <p>
      Every 1 second, this module generates a single multicast UDP
      packet, containing the number of busy and idle workers.  The
      packet is a simple ASCII format, similar to GET query parameters
      in HTTP.
  </p>
  
<example><title>An Example Packet</title>
v=1&amp;ready=75&amp;busy=0
</example>

  <p>
    Consumers should handle new variables besides busy and ready,
    separated by '&amp;', being added in the future.
  </p>

</section>

<directivesynopsis>
<name>HeartbeatAddress</name>
<description>Multicast address for heartbeat packets</description>
<syntax>HeartbeatAddress <var>addr:port</var></syntax>
<default>disabled</default>
<contextlist><context>server config</context></contextlist>

<usage>
<p>The <directive>HeartbeatAddress</directive> directive specifies the
multicast address to which <module>mod_heartbeat</module> will send
status information. This address will usually correspond to a configured
 <directive module="mod_heartmonitor">HeartbeatListen</directive> on a
frontend proxy system.</p>
<highlight language="config">
HeartbeatAddress 239.0.0.1:27999
</highlight>    
</usage>
</directivesynopsis>

</modulesynopsis>