3.3.2 Historical Perspective
The progressive conversion approach was the only solution available until the
early 80s.
More recently modern VSE operations have substantially grown in size,
complexity and integration, making it more difficult to implement a progressive
conversion.
It is also because the mass conversion approach, which was in a pioneer stage
in the early 80s, has matured to become safe and proven alternative. Hundreds
of mass conversions have been successfully completed worldwide in the past 15
years.
3.3.3 Shared Application Files and Databases
With today′s highly integrated VSE application portfolios, it becomes increasingly
difficult to define isolated application kernels for a progressive conversion. Most
applications share access to the same permanent files or databases. If some
files and databases need to be accessed at the same time by some application
kernels running in production under VSE and other application kernels running in
production under OS/390, those files and databases must be duplicated under
VSE and OS/390. The duplicate versions must then be kept in sync, which
requires developing complicated application bridges between VSE and OS/390.
The bridges must constantly be changed, as application kernels are
progressively migrated from VSE to OS/390.
3.3.4 Shared Application Code
A similar challenge exists for reusable code, such as JCL procedures,
subroutines, macros, copybooks and includes. Duplicate versions must be
maintained under VSE and OS/390 while application kernels sharing usage of
those code items run on different operating systems. Duplicate source storage
systems and change control procedures must be maintained during the overlap.
3.3.5 Operations Support Staffing
Supporting and operating dual VSE and OS/390 production environments
requires a larger staff and skill set than for a single production environment.
3.3.6 Automated Operations Tools
The complexity and sophistication of modern VSE operations shows in the
catalog of automated operations tools on which they rely. Those tools often
include a job scheduler, a report manager and a tape manager, which
complicates the organization and implementation of a progressive conversion.
It is very challenging to coordinate the overall job scheduling when two
synchronized and inter-dependent parts of the application portfolio run on two
separate operating systems under the automated control of two separate job
schedulers. Job schedulers are not designed or able to coordinate production
between two separate operating systems. In addition, as discussed above for
shared permanent data file and databases, the on-going progressive migration of
application kernels, forces to constantly change the automated job scheduling on
each side.
A similar challenge awaits progressive conversion teams with the division of
report management instructions between two report managers running on two
50 VSE to OS/390 Migration Workbook