Monitoring is essential, especially for web applications, and there are many tools available to help with it. A popular open source infrastructure-monitoring system is Nagios (http://nagios.org/). Another more modern open source alerting and metrics tool is Prometheus (https://prometheus.io/).
If you use a cloud platform, then there will be monitoring built in, for example, AWS CloudWatch or Azure Diagnostics. There are also cloud services to directly monitor your website, such as Pingdom (https://www.pingdom.com/), Uptime Robot (https://uptimerobot.com/), Datadog (https://www.datadoghq.com/), and PagerDuty (https://www.pagerduty.com/).
You probably already have a system in place to measure availability, but you can also use the ...
