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; and

• 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 RTPs will be doing.

The READ_ONLY setting should always be used when the RTPs are read-only, that is, when the RTPs perform no database updates. READ_ONLY should never be used when the RTPs perform database updates. For example, if the READ_ONLY setting is chosen, and if a RTP called by the gateway performs updates to a foreign database, then the Oracle database does not provide any integrity protection for those updates. Further, READ_ONLY mode allows a gateway transaction to be part of a distributed transaction that might update several other databases. If the gateway calls a RTP that performs updates in this situation and if a failure occurs, then the database updated by the RTP is out of sync with the other databases.

In cases where the RTPs perform updates to foreign databases, there are two options for PGA_CAPABILITY:

• SINGLE_SITE

• COMMIT_CONFIRM

Each of these options provides protection against data integrity problems by permitting COMMIT and ROLLBACK requests to be forwarded to the RTP and by informing the Oracle database about the distributed update and recovery capabilities of the gateway. The particular option chosen depends on the design of the RTPs and upon the capabilities of the OLTP 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 the 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 RTP 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-ConfirmConfiguring the OLTP for Commit-Confirm in this guide. For SINGLE_SITE capability, the RTP is responsible for performing the suitable 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 command, the RTP must perform COMMIT processing and then respond to the gateway with an APPC CONFIRMED response. On receipt of a SEND_ERR command, the RTP 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 RTP, 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 RTPs which perform updates to foreign databases outside the scope of control of the Oracle database . 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 RTPs 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 the 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 the 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 upon 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 suitable 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 issued a COMMIT request. In order 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/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 = C:\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)
  )