Connectivity Supplement

DB2 for VSE Overview

In the VSE/ESA operating environment, DB2 for VSE provides the application server function in a DRDA environment. The application requester function is not provided. The various DB2 for VSE and VSE components involved in distributed database processing are described in this section. These components enable the DB2 for VSE database management system to communicate with remote DRDA application requesters in an SNA network.

CICS(ISC): The Customer Information Control System (CICS) intersystem communication component provides the SNA LU 6.2 (APPC) functions to the DB2 for VSE application server.
CICS(SPM): The CICS syncpoint management component is integral to DB2 for VSE DRDA distributed unit of work support. It acts as a syncpoint participant and is responsible for coordinating two-phase commit activity at a VSE/ESA system.
CICS(TRUE): The CICS task-related user exit is an interface used by the AXE transaction to interface with the CICS syncpoint manager.
ACF/VTAM: CICS(ISC) uses VTAM for VSE to establish, or bind, LU-to-LU sessions with remote systems. DB2 for VSE uses LU 6.2 basic conversations over these sessions to communicate with remote DRDA application requesters.
AXE: The APPC-XPCC-Exchange transaction is a CICS transaction activated by the remote DRDA application requester. It routes the DRDA data stream between the remote application requester and the DB2 for VSE application server using the CICS LU 6.2 support and the VSE XPCC functions.
DBNAME Directory: The DBNAME (database name) directory maps an incoming request for conversation allocation to a predetermined application server identified by the incoming TPN. See the SQL/DS System Administration Guide for VSE for more details.
XPCC: Cross Partition Communication Control is the VSE macro interface that provides data transfer between VSE partitions.

Application Server Communications Flow Example

Figure 35 shows how each component plays a role in establishing communications between the DB2 for VSE Application Server and the remote Application Requester.

Figure 35. Gaining Access to the Application Server

The application requester issues an APPC ALLOCATE verb with a specific LU name and transaction program name (TPN) to establish an LU 6.2 conversation with the application server. The LU name is used to route the ALLOCATE request through VTAM to CICS. Upon receiving the ALLOCATE verb, CICS verifies that an AXE transaction is defined with that TPN, and performs a CICS sign-on. If the conversation security level for the CICS connection is VERIFY, both user ID and password are expected from the application requester, and are used in the sign-on. The CICS sign-on table (DFHSNT) must be updated with this user ID and password so that the connection is accepted. If the security level is set to IDENTIFY, only the user ID is required, and CICS entrusts the security check to the remote system. If the security check is successful, CICS starts the AXE transaction to route requests and replies between the application requester and an application server. The TPN used by the application requester must also have an entry defined in the DB2 for VSE DBNAME directory that points to an operating DB2 for VSE server within the VSE system.

If the application requester wishes to take advantage of distributed unit of work support, it specifies a SYNCLVL of SYNCPT on the APPC ALLOCATE verb. When the AXE transaction has started, it queries CICS to determine the SYNCLVL of the conversation. If it is SYNCPT, it does the following:

If necessary, the AXE transaction enables TRUE support so that it can communicate with the CICS syncpoint manager.
It registers the logical unit of work with the CICS syncpoint manager.

Limitations

Unlike its VM counterpart, the DB2 for VSE application server accepts DRDA flows from remote application requesters. Private protocols are not supported. As a result, VM application requesters cannot access a VSE server with PROTOCOL=SQLDS.

The DB2 for VSE DRDA server cannot route requests from remote application requesters to a DB2 for VM server using VSE guest sharing. Such requests should be sent directly to the DB2 for VM DRDA server.

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