Nagios (http://www.nagios.org) is an open source monitoring and notification utility. It enables users to monitor various resources, such as CPU, memory, disk usage, network status, reachability, HTTP status, testing of web page rendering, and various checks using Nagios-compatible sensors. There is a giant list of Nagios plugins that covers the monitoring of almost all popular services and software. The best thing with Nagios is its plugin architecture. You can write a simple plugin for custom resource monitoring. Thus, effectively, if you can measure a state, you can monitor its source in Nagios. This section will discuss, very briefly, Nagios setup and how it can be enabled to monitor system resources and Cassandra.