F.2.6 Preferring passive checks
Instead of getting Nagios to actively start each check individually, you can also use external applications and forward check results as passive checks to Nagios. The external application—even if this is just a cron job—relieves strain on Nagios in several areas, including scheduling. Nagios needs only to accept the result and sort it accordingly.
In a number of cases you can even go a step further and do without a regular check, for instance if a UPS sends an SNMP trap when there is a power failure, which Nagios processes as a passive check (see 14.6 Application Example II: Processing SNMP Traps, page 312).
Passive check results are cached by Nagios. By default there are 4096 so-called Command Buffer Slots available ...
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