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 todays 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

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IBM OS/390 manual Historical Perspective, Shared Application Files and Databases, Shared Application Code

OS/390 specifications

IBM OS/390, a versatile operating system, was a cornerstone in enterprise environments and played a pivotal role in mainframe computing. Released in the mid-1990s, OS/390 combined the strengths of IBM's MVS (Multiple Virtual Storage) with new features and enhancements, targeting scalability, reliability, and performance in demanding business applications.

One of the key features of OS/390 was its robust support for multiple users and processes. The system allowed thousands of concurrent users to access applications and data, ensuring high availability and minimizing downtime—a critical requirement for many large organizations. This scalability was supported through various enhancements in memory management and processor scheduling, enabling optimal resource allocation across diverse workloads.

OS/390 was known for its superior workload management capabilities. The Workload Manager (WLM) component allowed administrators to define service policies, specifying how system resources would be allocated according to the priority of tasks. This ensured that critical business processes received the necessary resources while less critical tasks were managed more flexibly.

Another significant characteristic of OS/390 was its commitment to security. The operating system provided comprehensive security features, including user authentication, data encryption, and auditing capabilities. This focus on security was vital for organizations handling sensitive data, ensuring compliance with regulations and safeguarding against unauthorized access.

OS/390 also supported advanced technologies that facilitated integration and development. The system included features like the IBM CICS (Customer Information Control System) for transaction processing and IMS (Information Management System) for database management. These technologies allowed organizations to build robust, high-performance applications tailored to specific business needs.

The ease of network integration was another strength of OS/390. With the advent of the Internet and global connectivity, OS/390 systems could easily interface with various network protocols, enabling businesses to operate in a connected world. This inclusion paved the way for many organizations to expand their capabilities and offer new services, driving digital transformation.

In conclusion, IBM OS/390 represented a significant advancement in mainframe technology, combining scalability, security, and robust workload management. Its rich feature set and support for critical enterprise applications solidified its role as a vital component of many organizations' IT infrastructures, ensuring they could meet their operational challenges head-on while supporting future growth. As technology continues to evolve, the legacy of OS/390 remains influential in the realm of computing.