Oracle Database Alert Engine

I am running Oracle XE . This database collects information about server statistics in my organization ( memory usage, threads running, etc). Now , I need to send alert/ notification emails if the certain thresholds are crossed by server ( ex: if number of threads increase more than 100, send an alert email).

One way to do this is to write stored procedures and schedule them to run every hour and send alert email if conditions are met.

But what I want to do is to use some sort of rule engine where I can specify the rules/conditions which have to be true for alerts to be sent. The reason I want this approach is so that every time there is a new rule, I do not have to write a new stored procedure and I can just specify the condition in the rule engine . So are there any Rule Engines out there that I can use (preferably open source and free to use)?