A free and open source application that monitors systems, networks and infrastructure by running simple commands that check their status. It alerts users when things break via email (often way more than necessary due to difficult of managing configurations). Nagios has been around forever so it's very mature, but is not ideally suited for cloud environments due to it's dependency on static configuration files and inability to scale to handle monitoring needs of large clusters with thousands of micro-services. We strongly advise against running Nagios for these reasons. Instead managed solutions like Datadog or NewRelic will provide much better insights as to the state of your infrastructure.