The conversion methodology used by ConvTek to migrate large Enterprise-wide VSE application portfolios to zOS is built upon the three fundamental principles of:
Mass conversion is the major distinction of the VSE to zOS migration approach used by ConvTek. Unlike a progressive conversion where VSE application kernels are sequentially migrated to the zOS target environment, a mass conversion results in a single switchover of the entire VSE application portfolio to zOS operations. Because the conversion process is entirely automated, the application switchover to zOS can be accomplished within a weekend with minimum end-user service disruption and minimum end-user visibility. This approach avoids many problems typically associated with a progressive conversion:
- Avoids coexistence of VSE and zOS production material (code, JCL, and files)
- Avoids file “bridges” between VSE and zOS applications for:
- Shared files and databases
- Automated job scheduling
- Automated report management
- Avoids duplicate maintenance of programs in the process of being converted
- Avoids duplicate computer operations
ConvTek uses conversion tools that automatically convert VSE JCL, code (COBOL, Assembler, PL/1, RPG II, Easytrieve, etc.), and files to native zOS. These tools (see Appendix D. Conversion Software) are exclusively designed for VSE to zOS conversions. Most specifics of the original VSE and final zOS applications can be handled automatically by the tools, after they have been custom-modified by coding specific installation options and exit routines. Once custom modified, the tools convert the entire VSE production material to zOS, in a short timeframe without manual intervention. This process includes:
- Transfer and verification of the application inventory
- Translation of application programs and associated copybooks or subprograms
- Generation of zOS load-modules
- Conversion of VSE and POWER JCL streams including application and utility steps and associated SYSIN cards
- Generation of new zOS JCL complying with local zOS production standards
- Generation of VSE and zOS jobs to transfer the VSE production files to zOS
Besides significant workload and timeframe reductions, this methodology ensures consistent and reliable results.
The repetitive conversion is an iterative method in which the conversion is improved by refining the automated conversion process and the associated software instead of the generated zOS material. After a trial mass conversion, the generated zOS material is tested for functionality in zOS. In the event that the conversion reports of zOS tests indicate conversion errors, the conversion tools are custom-modified to perform the conversion without errors, and a new trial mass conversion is run again starting from a fresh copy of VSE source material. This procedure is repeated until all conversion errors are eliminated. The actual and final conversion and the switchover to zOS do not start until trial mass conversions are error free.
By using a fresh copy of VSE source material for each trial mass conversion, all changes and modifications applied under VSE are automatically taken into account without having to follow up the VSE maintenance and to duplicate it in zOS. Therefore, it is not required to freeze or follow up development, enhancement, or maintenance of applications during the conversion.
Some non-standard source code modifications are performed manually, either because they are too complicated to automate, or because they are in very low occurrences. In both cases, the automation would not be cost effective when compared to a manual modification. Manual modifications complement the automated conversion (before or after). They can be:
- Applied to the VSE version of the source code, which is then tested and installed in production under VSE. Known as “VSE Positioning”, this technique does not require freezing any source code. For example, VSE COBOL programs must be manually positioned to:
- Move STOP RUN statements outside INPUT (or OUTPUT) SORT PROCEDURE
- Remove any access to I/O buffers before the file is opened or after the file is closed
- Remove some undocumented features of COBOL Report Writer
- Remove any use of the DISPLAY verb to embed lines into a report written with the WRITE verb
- Applied to the zOS version after automated conversion. Known as “zOS Freeze”, a procedure is defined to identify automatically and duplicate manually the VSE maintenance to the zOS “frozen” source code throughout the project.