The PGA parameters control the APPC interface portion of the gateway. PGA parameters are specified using the SET gateway initialization parameter. For example:

SET pga_parm=value 

where:

• pga_parm is one of the PGA parameter names in the list that follows

• value is a character string with contents that depend on pga_parm

Table A-1 provides a list of PGA parameters and their descriptions.

When choosing a setting for the PGA_CAPABILITY parameter, take care to ensure that the correct setting is used based on what the remote transaction programs will be doing.

The READ_ONLY setting should always be used when the remote transaction programs are read-only, that is, when the remote transaction programs perform no database updates. READ_ONLY should never be used when the remote transaction programs perform database updates. For example, if the READ_ONLY setting is chosen and if a remote transaction program invoked by the gateway performs updates to a foreign database, then Oracle database does not provide any integrity protection for those updates. Furthermore, the READ_ONLY mode enables a gateway transaction to be part of a distributed transaction that might update several other databases. If the gateway invokes a remote transaction program that performs updates in this situation and if a failure occurs, then the database updated by the remote transaction program is not synchronized with the other databases.

In cases where the remote transaction programs perform updates to foreign databases, there are two options for the value of PGA_CAPABILITY:

• SINGLE_SITE

• COMMIT_CONFIRM

Each of these options provides protection against data integrity problems by allowing COMMIT and ROLLBACK requests to be forwarded to the remote transaction program and by informing Oracle database about the distributed update and recovery capabilities of the gateway. The particular option depends on the design of the remote transaction programs and on the capabilities of the OLTP (online transaction processor) where they execute.

If the OLTP has LU6.2 SYNCLEVEL 1 or 2 support, then the COMMIT_CONFIRM capability provides limited two-phase commit between the Oracle database and the OLTP, with the restriction that no other commit-confirm site (gateway or Oracle) can be part of the distributed transaction. If it is not possible to use COMMIT_CONFIRM, then the SINGLE_SITE capability provides update capability between Oracle database and the OLTP, with the restriction that only the OLTP can perform updates and no updates can occur on the Oracle side.

Each of the PGA_CAPABILITY options for update control imposes specific requirements on the remote transaction program and on the OLTP. For COMMIT_CONFIRM capability, these requirements are discussed in detail in Chapter 5, "Implementing Commit-Confirm," of the Oracle Database Gateway for APPC User's Guide. Also refer to Configuring the OLTP for Commit-Confirm in this guide. For SINGLE_SITE capability, the remote transaction program is responsible for performing the required tasks in response to COMMIT and ROLLBACK requests received from the gateway on behalf of the Oracle database. The gateway uses the APPC CONFIRM and SEND_ERR requests to implement COMMIT and ROLLBACK, respectively. On receipt of a CONFIRM, the remote transaction program must perform COMMIT processing and then respond to the gateway with an APPC CONFIRMED response. On receipt of SEND_ERR, the remote transaction program must perform ROLLBACK processing.

Because the distributed transaction capability of the Oracle database is affected by the PGA_CAPABILITY option used by the gateway, it is desirable to separate inquiry and update applications by using different gateway instances for each. One gateway can be defined with PGA_CAPABILITY set to READ_ONLY and others with PGA_CAPABILITY set to SINGLE_SITE or COMMIT_CONFIRM.

This allows read-only transaction programs to participate in distributed transactions under the control of the Oracle database. For example, data from DB2 can be retrieved through the READ_ONLY gateway by an inquiry-only remote transaction program and can then be used as input to database updates on the Oracle database, all in one Oracle transaction. A SINGLE_SITE gateway can be used only for accessing remote transaction programs which perform updates to foreign databases outside the scope of the Oracle database control. Data can be read from any databases accessible to the Oracle database, and that data can be used to perform updates through the gateway.

When it is necessary to update resources on both the Oracle side and the OLTP side, a COMMIT_CONFIRM gateway can be used, provided that the OLTP and the remote transaction programs are set up to implement commit-confirm.

All that is necessary to set up multiple gateway instances is to set up the following for each instance:

• An entry in the listener.ora file defining the sid of the gateway instance

• An entry in the tnsnames.ora file defining an alias to be used to connect to the gateway instance defined in listener.ora

• A database link in Oracle database that specifies the alias defined in the tnsnames.ora file in its USING parameter

Note that the gateway instances can share one common directory structure and use the same executables.

For example, to set up two gateways, PGAI and PGAU (for inquiry and update use, respectively), the following steps are required:

1. Define entries in listener.ora for two sids, PGAI and PGAU.

2. Define two aliases in tnsnames.ora that connect to the two new sids, PGAI and PGAU.

3. Define two database links in Oracle database, one connecting to PGAI and the other connecting to PGAU.

4. Finally, create the initialization files initPGAI.ora and initPGAU.ora.

   In initPGAI.ora, set PGA_CAPABILITY to READ_ONLY, and in initPGAU.ora, set PGA_CAPABILITY to SINGLE_SITE or COMMIT_CONFIRM. Then, use the PGAI gateway for inquiry-only transactions, and use the PGAU gateway for update transactions.

   The same steps can be used to set up additional gateway instances.

When deciding on the setting for the PGA_CONFIRM parameter, it is important to understand the effects of each setting. First, keep in mind that this parameter affects only those conversations running at SYNCLEVEL 1. The default setting, PGA_CONFIRM=REJECT, is correct for most applications. With this setting, the gateway generates an error if a CONFIRM request is received from the remote transaction program. If you have a remote transaction that uses CONFIRM to verify that data was received by the gateway, then you must use PGA_CONFIRM=ACCEPT to allow the gateway to respond to those incoming CONFIRM requests with CONFIRMED responses. You must be aware that the gateway sends CONFIRM requests to the remote transaction when the Oracle application has sent a COMMIT request. For the COMMIT processing to work correctly, the remote transaction must be written to perform its local commit processing whenever a CONFIRM request is received from the gateway, and respond to the gateway with CONFIRMED after the commit processing has successfully completed. If an error occurs during commit processing, then the remote transaction must respond to the gateway with SEND_ERR to indicate that the commit failed.

One special case for the use of PGA_CONFIRM=ACCEPT is with IMS/TM version 7. When using the "implied APPC" support that is provided by IMS/TM version 7, conversations that run at SYNCLEVEL 1 are handled differently than conversations that run at SYNCLEVEL 0. IMS/TM automatically generates CONFIRM requests after each APPC SEND when the conversation is at SYNCLEVEL 1. On the gateway side, if PGA_CONFIRM=ACCEPT is not specified, then the CONFIRM requests sent by IMS/TM result in errors generated by the gateway. Using PGA_CONFIRM=ACCEPT alleviates this problem, allowing the gateway to respond to incoming CONFIRM requests with CONFIRMED responses. The only limitation with running this way is that the implied APPC support provided by IMS does not notify the application when a CONFIRM request is received from the gateway. This means that the gateway cannot use CONFIRM to implement COMMIT, thereby disabling the use of COMMIT and ROLLBACK to control updates on the IMS side of the conversation.

LISTENER =
  (ADDRESS_LIST =
        (ADDRESS=
          (COMMUNITY= TCP.world)
          (Host = bay)
          (PROTOCOL= TCP)
          (Port= 2621)
        )
        (ADDRESS=
          (COMMUNITY= TCP.world)
          (Host = bay)
          (PROTOCOL= TCP)
          (Port= 2623)
        )
  )
 
SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = PGA)
      (ORACLE_HOME = /oracle/pga/12.2)
      (PROGRAM = pg4asrv)
    )
  )

ORA920 =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = bay.us.example.com)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = ORA920.bay)
    )
  )
 
PGA =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = bay)(PORT = 2621))
    )
    (CONNECT_DATA =
      (SID = PGA)
    )
    (HS = OK)
  )