When an instance starts, a listener process establishes a communication pathway to
Oracle. When a user process makes a connection request, the listener determines
whether it should use a shared server dispatcher process or a dedicated server process
and establishes an appropriate connection.

The listener also establishes a communication pathway between databases. When
multiple databases or instances run on one computer, as in Real Application Clusters,
service names enable instances to register automatically with other listeners on the
same computer. A service name can identify multiple instances, and an instance can
belong to multiple services. Clients connecting to a service do not have to specify
which instance they require.

监听器
1. 当一个实例启动后，监听进程建立与Oracle通信的通道
2. 多个实例能使用同一个服务名 , 实例可以属于多个服务