Oracle Database 11g : Connecting to Oracle - Use the Oracle Net Listener

The process is fairly simple. The listener receives a request and hands the request to a service handler, which is a server process that runs on the same platform as the Oracle database server. The service handler can be a dedicated server or a dispatcher, the latter of which works with shared servers.

The PMON background process registers the service information to the listener. During registration, PMON gives the listener information on the database services and instance information. PMON then tries to register with the listener once the listener has been started. Dynamic registration is supported with the alter system register command. If PMON has not registered with the listener, a TNS listener error will occur. View the Oracle Database 11g Error Messages reference manual for more details.

The listener will receive the database request and spawn a dedicated server process if the environment is configured for the dedicated server architecture. The listener will hand the request over to a dispatcher if running a shared server architecture. A client application can bypass the listener if it is running on the same platform as the database server. Once the listener hands off the request it will resume listening for additional network requests.

A default listener (named listener) is configured at installation with the Oracle Net Configuration Assistant making it easy to start up the default listener when a system is first built. An additional ICP protocol address is defined for external routes (EXTPROC) during installation.

The following is a sample listener.ora file:

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC0))
      )
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = TCP)(HOST = eclipse)(PORT = 1521))
      )
    )
  )

Table 1 illustrates the contents of the listener.ora file.

In the following, host defines the server name, PORT defines the port number, SERVER defines the host server name, PIPE defines the pipe name, and KEY defines a unique name for the service. It is recommended that you use the Oracle SID value for the key.

Table 2 defines the components of the protocol definition.

After installation, the Oracle Net Manager can be used to modify the listener configuration. Some of the values that can be configured for the listener include the following:

• If the default port of 1521 is not specified, the LOCAL_LISTENER initialization parameter needs to be defined through a naming method. The LOCAL_LISTENER parameter is dynamic and can be set with the alter system command.

• Be careful, because the LISTENER parameter overrides the LOCAL_LISTENER parameter. A host system can have multiple IP addresses, and a listener can be configured to listen on them.

• The I/O buffer size for send and receive operations can be defined.

• Heterogeneous services can be set to support additional services such as external routines.

• The QUEUESIZE parameter can be defined for environments that may have a large number of concurrent connection requests for a listener on a listening endpoint.

Password Authentication

In Oracle 11g, the listener administration is secure through the operating system authentication. So the administration is then restricted to the account that started the listener. Another option is to set a password for the listener. Also for remote administration of the listener a password is required. The change_password command can be used to change a password or set a new password. If a password is not set, someone can accidentally impact the availability of the database—for example, accidentally shutting down the listener. If you don’t have a listener, new sessions cannot be established. It is important that a DBA protect access to listener management.

Using the listener utility, lsnrctl, listener configurations can be managed. The following example sets the listener password:

> lsnrctl
lsnrctl> change_password
Old password: <enter>
New password: newpassword
Reenter new password: newpassword
lsnrctl> save_config

Multiple Listeners

Multiple listeners can be defined for a service and can offer a number of advantages for more complex environments. These advantages include the following:

• Failover

• Transparent application failover

• Load balancing

  The following is a sample connect descriptor for a listener:

(DESCRIPTION =
 (ADDRESS_LIST=
   (ADDRESS=(PROTOCOL=tcp)(HOST=eclipse)(PORT=1521))
  (CONNECT_DATA=
     (SERVICE_NAME=customer.us.beginner.com)))

Connection Pooling

A shared server architecture is used to improve user scalability. So, it is assumed that if this architecture is being used, there is a potential for a large number of users. As mentioned previously, at any point in time there can be a large percentage of idle processes. Connection pooling allows the database server to time out sessions that are idle and then use the connection to support an active session. These sessions remain open but in an idle state. When they become active again, a connection is reestablished.