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109 lines
5.0 KiB
109 lines
5.0 KiB
<h1>Monitoring Cron Jobs</h1>
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<p>SITE_NAME is perfectly suited for monitoring cron jobs. All you have to do is
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update your cron job command to send an HTTP request to SITE_NAME
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after completing the job.</p>
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<p>Let's look at an example:</p>
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<div class="bash highlight"><pre><span></span><code>$ crontab -l
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<span class="c1"># m h dom mon dow command</span>
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<span class="m">8</span> <span class="m">6</span> * * * /home/user/backup.sh
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</code></pre></div>
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<p>The above job runs <code>/home/user/backup.sh</code> every day at 6:08. The backup
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script is presumably a headless, background process. Even if it works
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correctly currently, it can start silently failing in the future without
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anyone noticing.</p>
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<p>You can set up SITE_NAME to notify you whenever the backup script does not
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run on time, or it does not complete successfully. Here are the steps to do that.</p>
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<ol>
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<li>
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<p>If you have not already, sign up for a free SITE_NAME account.</p>
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</li>
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<li>
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<p>In your SITE_NAME account, <strong>add a new check</strong>.</p>
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</li>
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<li>
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<p>Give the check <strong>a meaningful name</strong>. Good naming will become
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increasingly important as you add more checks to your account.</p>
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</li>
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<li>
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<p>Edit the check's <strong>schedule</strong>:</p>
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<ul>
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<li>change its type from "Simple" to "Cron"</li>
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<li>enter <code>8 6 * * *</code> in the cron expression field</li>
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<li>set the timezone to match your machine's timezone</li>
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</ul>
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</li>
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<li>
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<p>Take note of your check's unique <strong>ping URL</strong>.</p>
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</li>
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</ol>
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<p>Finally, edit your cron job definition and append a curl or wget call
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after the command:</p>
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<div class="bash highlight"><pre><span></span><code>$ crontab -e
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<span class="c1"># m h dom mon dow command</span>
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<span class="m">8</span> <span class="m">6</span> * * * /home/user/backup.sh <span class="o">&&</span> curl -fsS --retry <span class="m">5</span> -o /dev/null PING_URL
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</code></pre></div>
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<p>Now, each time your cron job runs, it will send an HTTP request to the ping URL.
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Since SITE_NAME knows your cron job's schedule, it can calculate
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the dates and times when the job should run. As soon as your cron job doesn't
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report at an expected time, SITE_NAME will send you a notification.</p>
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<p>This monitoring technique takes care of various failure scenarios that could
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potentially go unnoticed otherwise:</p>
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<ul>
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<li>The whole machine goes down (power outage, janitor stumbles on wires, VPS provider problems, etc.)</li>
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<li>the cron daemon is not running or has an invalid configuration</li>
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<li>cron does start your task, but the task exits with a non-zero exit code</li>
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</ul>
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<h2>Curl Options</h2>
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<p>The extra options in the above example tell curl to retry failed HTTP requests, and
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silence output unless there is an error. Feel free to adjust the curl options to
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suit your needs.</p>
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<dl>
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<dt><strong>&&</strong></dt>
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<dd>Run curl only if <code>/home/user/backup.sh</code> exits with an exit code 0.</dd>
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<dt><strong>-f, --fail</strong></dt>
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<dd>Makes curl treat non-200 responses as errors.</dd>
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<dt><strong>-s, --silent</strong></dt>
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<dd>Silent or quiet mode. Hides the progress meter, but also hides error messages.</dd>
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<dt><strong>-S, --show-error</strong></dt>
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<dd>Re-enables error messages when -s is used.</dd>
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<dt><strong>--retry <num></strong></dt>
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<dd>If a transient error is returned when curl tries to perform a
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transfer, it will retry this number of times before giving up.
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Setting the number to 0 makes curl do no retries (which is the default).
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Transient error is a timeout or an HTTP 5xx response code.</dd>
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<dt><strong>-o /dev/null</strong></dt>
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<dd>Redirect curl's stdout to /dev/null (error messages still go to stderr).</dd>
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</dl>
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<h2>Looking up Your Machine's Time Zone</h2>
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<p>If your cron job consistently pings SITE_NAME an hour early or an hour late,
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the likely cause is a timezone mismatch: your machine may be using a timezone
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different from what you have configured on SITE_NAME.</p>
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<p>On modern GNU/Linux systems, you can look up the time zone using the
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<code>timedatectl status</code> command and looking for "Time zone" in its output:</p>
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<div class="text highlight"><pre><span></span><code>$ timedatectl status
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Local time: C 2020-01-23 12:35:50 EET
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Universal time: C 2020-01-23 10:35:50 UTC
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RTC time: C 2020-01-23 10:35:50
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<span class="hll"> Time zone: Europe/Riga (EET, +0200)
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</span>System clock synchronized: yes
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NTP service: active
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RTC in local TZ: no
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</code></pre></div>
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<h2>Viewing Cron Logs Using <code>journalctl</code></h2>
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<p>On a systemd-based system, you can use the <code>journalctl</code> utility to see system logs,
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including logs from the cron daemon.</p>
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<p>To see live logs:</p>
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<div class="bash highlight"><pre><span></span><code>journalctl -f
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</code></pre></div>
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<p>To see the logs from e.g. the last hour, and only from the cron daemon:</p>
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<div class="bash highlight"><pre><span></span><code>journalctl --since <span class="s2">"1 hour ago"</span> -t CRON
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</code></pre></div>
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