IMS High Performance Unload API overview

An application programming interface (API) in IMS High Performance Unload enables an IMS DL/I batch application program, which reads a database sequentially, to use HSSR Engine without being recompiled or relink-edited. Through this API, the application program can retrieve a large number of segments more efficiently than the native IMS DL/I and the elapsed time and CPI time can be reduced.

This API supports IMS DL/I calls and IMS EXEC DLI commands. GN and GNP calls with an unqualified segment search argument (SSA) are optimized. If the calls other than GU, GN, or GNP are included, do not run the application program with IMS High Performance Unload.

An overview of the system flow of an application program is shown in IMS High Performance Unload system structure.

An application program that uses HSSR Engine for processing DL/I database calls is called an HSSR application program. To run an HSSR application in the DLI or the DBB region, a PSB is needed. In the PSB, you have to select one or more DBPCBs by specifying them in the HSSRPCB or the HSSRDBD control statement in the HSSROPT data set. The selected DBPCB is called an HSSR PCB. A DL/I call or an EXEC DLI command to the HSSR PCB is called an HSSR call. The HSSR calls are processed by HSSR Engine and the calls to the other DBPCBs are processed by native IMS DL/I.

Requirement: An application program must have been assembled, compiled, and link-edited in the same way as IMS DL/I batch application programs. For details, see IMS Application Programming.
  • The specification of HSSR PCBs through the KEYLEN keyword of PCB statement is supported for the compatibility with DBT HSSR. See Method for specifying an HSSR PCB through KEYLEN.
  • If the ULU region is used, you do not need to code the HSSRDBD or the HSSRPCB control statement. This region type is supported for the compatibility with DBT HSSR.

For the restrictions that apply to HSSR application program jobs, see Restrictions for IMS High Performance Unload.