It is a very common scenario that computers, or the applications they offer, depend on other objects to function properly. A typical example is a database that an email or web server will depend upon. Another one is a host behind a private network depends that on an OpenVPN service to work. As a system administrator, your job is to know these relations — if you plan to reinstall a database cluster, you need to let people know there will be downtime for almost all applications. Nagios should also be aware of such relations.

In such cases, it is very useful for system monitoring software to consider these dependencies. When analyzing which hosts and services are not working properly, it is good to analyze such dependencies and discard things that are not working because of other failures. This way, it will be easier for you to focus on the real problems. Therefore, it allows you to get to the root cause of any malfunction much faster.

Nagios allows you to define how hosts...