How best to monitor and alert on the absence of an event in the logs?

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http://serverfault.com – With logs getting captured in syslog-ng, I'd like to be able to automatically monitor the logs and receive an alert if NO log events appear that match a certain criteria. For instance, for a subscription-based website, if 6 hours elapse with no orders, then email or text this person or group of people. What is a good way to do that? (HowTos)