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<div class="section" title="Service and Host Freshness Checks">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="freshness"></a><a name="freshness_checks"></a>Service and Host Freshness Checks</h2></div></div></div>
<p><span class="bold"><strong>Introduction</strong></span></p>
<p>Icinga supports a feature that does "freshness" checking on the results of host and service checks. The purpose of
freshness checking is to ensure that host and service checks are being provided passively by external applications on a regular
basis.</p>
<p>Freshness checking is useful when you want to ensure that <a class="link" href="passivechecks.html" title="Passive Checks">passive checks</a> are being
received as frequently as you want. This can be very useful in <a class="link" href="distributed.html" title="Distributed Monitoring">distributed</a> and <a class="link" href="redundancy.html" title="Redundant and Failover Network Monitoring">failover</a> monitoring environments.</p>
<p><span class="inlinemediaobject"><img src="../images/freshness.png"></span></p>
<p><span class="bold"><strong>How Does Freshness Checking Work?</strong></span></p>
<p>Icinga periodically checks the freshness of the results for all hosts services that have freshness checking
enabled.</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<p>A freshness threshold is calculated for each host or service.</p>
</li>
<li class="listitem">
<p>For each host/service, the age of its last check result is compared with the freshness threshold.</p>
</li>
<li class="listitem">
<p>If the age of the last check result is greater than the freshness threshold, the check result is considered
"stale".</p>
</li>
<li class="listitem">
<p>If the check results is found to be stale, Icinga will force an <a class="link" href="activechecks.html" title="Active Checks">active
check</a> of the host or service by executing the command specified by in the host or service definition.</p>
</li>
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<p>An active check is executed even if active checks are disabled on a program-wide or host- or service-specific
basis.</p>
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<p>For example, if you have a freshness threshold of 60 for one of your services, Icinga will consider that service to
be stale if its last check result is older than 60 seconds.</p>
<p><span class="bold"><strong>Enabling Freshness Checking</strong></span></p>
<p>Here's what you need to do to enable freshness checking...</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<p>Enable freshness checking on a program-wide basis with the <a class="link" href="configmain.html#configmain-check_service_freshness">check_service_freshness</a> and <a class="link" href="configmain.html#configmain-check_host_freshness">check_host_freshness</a> directives.</p>
</li>
<li class="listitem">
<p>Use <a class="link" href="configmain.html#configmain-service_freshness_check_interval">service_freshness_check_interval</a> and <a class="link" href="configmain.html#configmain-host_freshness_check_interval">host_freshness_check_interval</a> options to tell Icinga how
often in should check the freshness of service and host results.</p>
</li>
<li class="listitem">
<p>Enable freshness checking on a host- and service-specific basis by setting the <span class="emphasis"><em>check_freshness</em></span>
option in your host and service definitions to a value of 1.</p>
</li>
<li class="listitem">
<p>Configure freshness thresholds by setting the <span class="emphasis"><em>freshness_threshold</em></span> option in your host and service
definitions.</p>
</li>
<li class="listitem">
<p>Configure the <span class="emphasis"><em>check_command</em></span> option in your host or service definitions to reflect a valid command
that should be used to actively check the host or service when it is detected as stale.</p>
</li>
<li class="listitem">
<p>The <span class="emphasis"><em>check_period</em></span> option in your host and service definitions is used when Icinga
determines when a host or service can be checked for freshness, so make sure it is set to a valid timeperiod.</p>
</li>
</ul></div>
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<p>If you do not specify a host- or service-specific <span class="emphasis"><em>freshness_threshold</em></span> value (or you set it to
zero), Icinga will automatically calculate a threshold automatically, based on a how often you monitor that particular
host or service. We would recommend that you explicitly specify a freshness threshold, rather than let Icinga pick one
for you.</p>
</td></tr>
</table></div>
<p><span class="bold"><strong>Example</strong></span></p>
<p>An example of a service that might require freshness checking might be one that reports the status of your nightly backup
jobs. Perhaps you have a external script that submit the results of the backup job to Icinga once the backup is
completed. In this case, all of the checks/results for the service are provided by an external application using passive checks.
In order to ensure that the status of the backup job gets reported every day, you may want to enable freshness checking for the
service. If the external script doesn't submit the results of the backup job, you can have Icinga fake a critical result
by doing something like this...</p>
<p>Here's what the definition for the service might look like (some required options are omitted)...</p>
<pre class="screen"> define service{
host_name backup-server
service_description ArcServe Backup Job
active_checks_enabled 0 ; active checks are NOT enabled
passive_checks_enabled 1 ; passive checks are enabled (this is how results are reported)
check_freshness 1
freshness_threshold 93600 ; 26 hour threshold, since backups may not always finish at the same time
check_command no-backup-report ; this command is run only if the service results are "stale"
...other options...
}</pre>
<p>Notice that active checks are disabled for the service. This is because the results for the service are only made by an
external application using passive checks. Freshness checking is enabled and the freshness threshold has been set to 26 hours.
This is a bit longer than 24 hours because backup jobs sometimes run late from day to day (depending on how much data there is
to backup, how much network traffic is present, etc.). The <span class="emphasis"><em>no-backup-report</em></span> command is executed only if the
results of the service are determined to be stale. The definition of the <span class="emphasis"><em>no-backup-report</em></span> command might
look like this...</p>
<pre class="screen"> define command{
command_name no-backup-report
command_line /usr/local/icinga/libexec/check_dummy 2 "CRITICAL: Results of backup job were not reported!"
}</pre>
<p>If Icinga detects that the service results are stale, it will run the <span class="emphasis"><em>no-backup-report</em></span> command
as an active service check. This causes the <span class="emphasis"><em>check_dummy</em></span> plugin to be executed, which returns a critical
state to Icinga. The service will then go into to a critical state (if it isn't already there) and someone will probably
get notified of the problem.</p>
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