Nagios, 2nd Edition

4.2 On-Demand Host Checks vs. Periodic Reachability Tests

As a matter of principle, Nagios performs service checks at regular intervals, with the exception of passive service checks. (See 13.2 Passive Service Checks in page 293.) Some slightly different rules apply for host checks, which play the main role. Nagios executes host checks when it needs them—that is, on demand—and uses them to monitor hosts where a service installed on them changes to an error state or hosts that lie in topological dependency to a failed host. A third way is via host dependencies, as described in 12.6.2 Only in exceptional cases: host dependencies in page 289. On-demand host checks are a core function of Nagios, as this is the only way the system can precisely inform ...