Using IMS Checkpoint Replacement mode

Some existing application programs already contain checkpoint/restart logic using the IMS extended checkpoint/restart facility. The application programs issue CALLs to IMS for sequential file access (via GSAM DBDs) and issue symbolic checkpoint CALLs (CHKP) to perform the functions of saving Working Storage, COMMITting sequential file activity, and COMMITting database activity. Application programs use the IMS facility for three different types of applications which all require restartability: IMS Database Applications, DB2 Applications, and non-DBMS Applications.

For non-IMS applications, it is highly desirable to remove IMS requirements from the application program as IMS is only made available for the restart function. Each application requiring restartability must have its own PSB, generally a DBA administrative function. For IMS applications, it is still desirable to use QUICKSTART in order to a procedural management of all restart activity in the same manner and to reduce the load on the IMS log files.

To address these issues, QUICKSTART provides an IMS Checkpoint Replacement Mode which enables applications using IMS restart mechanisms to instead use QUICKSTART checkpointing without incurring any application program changes. For non-IMS applications, the IMS libraries are no longer needed. In addition, the JCL changes to incorporate this mode are quite minimal. Member IMSXRST on SAMPLIB contains sample JCL for running these applications.

Applications wishing to take advantage of this feature are re-linked using a QUICKSTART provided utility (see member IMSRELNK on SAMPLIB). This utility replaces the DFSLI000 stub in the application load module with a QUICKSTART stub which allows QUICKSTART to intercept the XRST, CHKP, and GSAM IMS calls and replace them with the QUICKSTART equivalent calls.

This section contains the following topics: