OnlineOrNot is an website monitoring service. In particular, it provides uptime monitoring, checking whether your site is online, or not (hence the name), and page speed monitoring.
It allows you to continuously monitor any website or API server. It notifies you instantly in the case of any problems - whether that's a timeout, 4xx error, or 5xx error.
If you have any questions at all - feel free to email me at firstname.lastname@example.org.