It uses the Dead man's switch technique: the monitored system must "check in" with Healthchecks.io at regular, configurable time intervals. When Healthchecks.io detects a missed check-in, it sends out alerts. Similarly, you can use this service too, to monitor a remote location that is not publicly accessible across the Internet because it does it's check outbound, and not inbound such as say UptimeRobot does. In this way, if the job has not checked in after a predetermined period, you'll receive a notification.
It is fully open source, so you can either self-host the monitoring service yourself, or their cloud service is free to use for up to 20 job monitors with e-mail alerts or 5 SMS / WhatsApp credits. If you are using the Dead Man's Switch service (open source clients), this is certainly an alternative to consider.
See
GitHub - healthchecks/healthchecks: A cron monitoring tool written in Python & Django#
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monitoring 
A cron monitoring tool written in Python & Django. Contribute to healthchecks/healthchecks development by creating an account on GitHub.
