Uptime monitoring for small business websites

Design an uptime monitoring service for small businesses. A customer adds a URL, the service checks it every minute, and it alerts them by email and text message when the site goes down and again when it recovers. There are 15,000 customers monitoring 40,000 URLs. False alarms annoy customers more than a one-minute delay, and an alert the service fails to send is the worst outcome of all. Describe how checks are scheduled and run, how the service decides a site is really down, and how you know the monitoring itself is working.