Well SilverStripe has a versioning system which you can see when people republish pages and what changed - under Page Version History on the left which will show you each revision. If thats not enough and you need to log every event I would say use a 'onBeforeWrite()' method on whatever you want to log. Eg if changes on page you could do an onBeforeWrite() method which saves or emails or logs to a file etc before it saves the object