The Oracle Net Listener listens for incoming requests from the Oracle database. For the Oracle Net Listener to listen for the gateway, information about the gateway must be added to the Oracle Net Listener configuration file, listener.ora. This file by default is located in $ORACLE_HOME/network/admin, where $ORACLE_HOME is the directory under which the gateway is installed.

The following entries must be added to the listener.ora file:

• A list of Oracle Net addresses on which the Oracle Net Listener listens

• The executable name of the gateway that the Oracle Net Listener starts in response to incoming connection requests

A sample of the listener.ora entry (listener.ora.sample) is available in the $ORACLE_HOME/dg4db2/admin directory where $ORACLE_HOME is the directory under which the gateway is installed.

You must stop and restart the Oracle Net Listener to initiate the new settings, as follows:

.
.
.
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=204.179.99.15)(PORT=1551)))
Services Summary...
Service "dg4db2" has 1 instance(s).
  Instance "dg4db2", status UNKNOWN, has 1 handler(s) for this service...
The command completed successfully

In this example, the service name is dg4db2, which is the default SID value assigned during installation.

Edit the tnsnames.ora file to add a connect descriptor for the gateway. The following is the syntax of the Oracle Net entry using the TCP/IP protocol:

connect_descriptor=
   (DESCRIPTION=
      (ADDRESS=
         (PROTOCOL=TCP)
         (HOST=host_name)
         (PORT=port_number)
      )
      (CONNECT_DATA=
         (SID=gateway_sid))
      (HS=OK))

where:

To ensure higher availability, you can specify multiple listeners within the connect descriptor.

 connect_descriptor=
    (DESCRIPTION=
       (ADDRESS=
          (PROTOCOL=TCP)
          (HOST=host_name_1)
          (PORT=port_number_1)
       )
       (ADDRESS=
          (PROTOCOL=TCP)
          (HOST=host_name_2)
          (PORT=port_number_2)
       )
       (CONNECT_DATA=
          (SID=gateway_sid))
       (HS=OK))

This indicates that, if the listener for host_name_1 and port_number_1 is not available, then the second listener for host_name_2 and port_number_2 will take over.

Choose One System ID for Each DRDA Database

A separate instance of the gateway is needed for each DRDA database. Each instance needs its own gateway System ID (SID). For this example, the gateway SIDs are chosen for the instances that access the DRDA databases:

• dg4db22 for the gateway accessing database db2

• dg4db23 for the gateway accessing database db3

Create Two Initialization Parameter Files

Create an initialization parameter file for each instance of the gateway by copying the original initialization parameter file, $ORACLE_HOME/dg4db2/admin/initdg4db2.ora, twice, naming one with the gateway SID for db2 and the other with the gateway SID for db3:

$ cd $ORACLE_HOME/dg4db2/admin
$ cp initdg4db2.ora initdg4db22.ora
$ cp initdg4db2.ora initdg4db23.ora

Add Entries to listener.ora

Add two new entries to the Oracle Net Listener configuration file, listener.ora. You must have an entry for each gateway instance, even when multiple gateway instances access the same database.

The following example shows the entry for the original installed gateway first, followed by the new entries:

SID_LIST_LISTENER=
(SID_LIST=
   (SID_DESC=
      (SID_NAME=dg4db2)
      (ORACLE_HOME=oracle_home_directory)
      (PROGRAM=dg4db2)
   )
   (SID_DESC=
      (SID_NAME=dg4db22)
      (ORACLE_HOME=oracle_home_directory)
      (PROGRAM=dg4db2)
   )
   (SID_DESC=
      (SID_NAME=dg4db23)
      (ORACLE_HOME=oracle_home_directory)
      (PROGRAM=dg4db2)
   )
)

where, oracle_home_directory is the directory where the gateway resides.

If the listener is already running, use the lsnrctl command to stop the listener and then start it with the new settings, as follows:

$ lsnrctl stop
$ lsnrctl start

Enter the following to create a database link for the dg4db22 gateway:

SQL> CREATE PUBLIC DATABASE LINK DRDA2 CONNECT TO
  2  "user2" IDENTIFIED BY "password2" USING 'new_db2_using';

Enter the following to create a database link for the dg4db23 gateway:

SQL> CREATE PUBLIC DATABASE LINK DRDA3 CONNECT TO
  2  "user3" IDENTIFIED BY "password3" USING 'new_db3_using';

After the database links are created, you can verify the connection to the new DRDA databases, as in the following:

SQL> SELECT * FROM ALL_USERS@DRDA2;

SQL> SELECT * FROM ALL_USERS@DRDA3;