Single Region Data Set Sharing | IBM OS/390 specification

5.6.6.2 Single Region Data Set Sharing

Single ACB Open - Multiple String Processing

Full write integrity is provided within a single region provided the user uses a single ACB to process the data set. In high level languages an ACB equates to:

1.a SELECT statement in COBOL

2.a file DECLARE in PL/I

3.an ²F² statement in RPG II

If multiple ACBs (or high level language equivalents) are used, the protection of shareoptions must be relied upon unless DSNAME sharing is used (both COBOL and PL/I always use it). See ªIntra-Region Data Set Name Sharingº below.

Multiple positions may be maintained in the file via use of multiple strings (that is, the ACB STRNO parameter). The strings may be used for multiple requests from the main task or its subtask. VSAM will automatically provide exclusive control protection for output requests and read integrity if ²GET for UPDATE² is used.

Within the same region the data set can be updated concurrently (even with DISP=OLD) and VSAM ensures integrity because a single ACB control block structure is used.

On a GET UPDATE or PUT request, VSAM acquires exclusive control of the CI, after checking that no other string is accessing the CI. Any string which wants to make an update or add to the same CI, will get an ERROR CODE = X¢ 14¢ with R15 = X ¢08¢. The exclusive control ends when the subtask possessing it issues a GET UPDATE for a record in another CI or issues an ENDREQ or issues a PUT UPDATE for a record read previously by a GET UPDATE. The exclusive control does not impede simple READs for the other subtasks. The user requiring read integrity must specify UPD intent on all RPLs.

Both CICS/OS and IMS/VS DC use multiple string processing with a single ACB structure. They intercept the VSAM exclusive control error codes and suspend or wait the task until the requested resource is available.

Intra-Region Data Set Name Sharing

If DSNAME sharing is specified in the ACB (that is, MACRF=(DSN...)), a data set may be accessed from multiple ACBs within the same region. VSAM assures integrity because there is only one control block structure. This protection is provided because DSNAME sharing tells VSAM to tie the control block structure of the second ACB to the first if the data set name matches.

If DSNAME sharing is not specified in the ACB, the default (DDNAME sharing) applies and VSAM operates as if the data set is being shared by users in different address spaces. The OS/VS COBOL and OS PL/I compilers always use DSNAME sharing when multiple file statements are used.

For VSE users of CICS (since VSE/ESA 1.3), DSNAME sharing has been available as well, so VSE users will have at least the same support in this specific area, and for older VSE installations, OS/390 will provide a significant enhancement.

128VSE to OS/390 Migration Workbook

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.

IBM OS/390 manual Single Region Data Set Sharing, Single ACB Open Multiple String Processing

Models: OS/390