摘自orafaq

One should implement a monitoring system to constantly monitor the following
aspects of a database. This can be achieved by writing custom scripts,
implementing Oracle's Enterprise Manager, or buying a third-party monitoring
product. If an alarm is triggered, the system should automatically notify the
DBA (e-mail, page, etc.) to take appropriate action.

Infrastructure availability:

1. Is the database up and responding to requests
2. Are the listeners up and responding to requests
3. Are the Oracle Names and LDAP Servers up and responding to requests
4. Are the Web Listeners up and responding to requests
5. Etc.

Things that can cause service outages:

1. Is the archive log destination filling up?
2. Objects getting close to their max extents
3. Tablespaces running low on free space/ Objects what would not be able to extend
4. User and process limits reached
5. Etc.