重點知識：
- 創建 additional listeners
- 創建Oracle Net Service aliases （別名）
- 配置connect-time failover
- 控制Oracle Net listener
- 使用tnsping測試Oracle Net 連接
- 何時使用shared server 和 dedicate server
一、Oracle Net Service
On the client computer, Oracle Net is a background component for application connections to the database.
On the database server, Oracle Net includes an active process called the Oracle Net Listener, which is responsible for coordinating connections between the database and external applications.
對於非oracle數據庫的Net連接，可以使用工具：Oracle Heterogeneous Services (such as Sybase, Informix, DB2, and SQL Server)
A single listener can service multiple database instances and thousands of client connections.
二