Oracle Database Administration: The Essential Refe

Oracle services

SQL*Net connections to Oracle databases are made through services, which are actually processes running on the Oracle Server host machine. Each Oracle server maintains one or more listener processes, which are used to monitor the network for incoming connection requests. When such a request is received, the listener process is responsible for determining which Oracle instance the incoming request is seeking a connection with, and establishing an appropriate server process for that connection. The server process may be established as a dedicated process, or may use the Multi-Threaded Server, depending on how the Oracle instance is configured. For more information, see Chapter 10.

In order to identify the Oracle instance to connect with, the SQL*Net connection request specifies the name of a service (sometimes called a database alias ), which in turn corresponds to a particular Oracle instance on a specific host machine. The relationships between service names and specific instances and hosts are typically maintained in the file tnsnames.ora (discussed in more detail later in this chapter). For example, the service name “test” might actually refer to an Oracle database instance with an SID of TST23, which is accessible via TCP/IP at address 123.234.210.001 using port 1526. It is certainly simpler to refer to “test”!