You can monitor your systems for management and performance problems in essentially two ways: reactively and proactively.
Reactive monitoring involves monitoring a database environment after a performance or management issue has arisen. For example, you start gathering performance statistics using third-party tools, Enterprise Manager, or home-grown scripts after users call to tell you that the system is slow. Obviously, this type of monitoring leaves a lot to be desired, because a problem has already arisen and the users of the system are already impacted. You can use the techniques discussed in this chapter for reactive monitoring, but they are most effective when used to perform proactive monitoring.