Enterprise Tech Journal 2016: Issue 5 : Page 57

MacKinney Systems with SimpList. Unfortunately, IBM’s flagship development environment, the TSO-based Interactive System Productivity Facility , ISPF, offers at very best minimal and marginally usable support. The onboard support of member generations by ISPF presumes the user knows in advance the non-intuitive and disjointed path by which to navigate the breadth of the ISPF menu system to locate it. As an example, to access member generations within ISPF, a user must perform the following sequence of tasks: • The enhanced member list option must be selected. • Use ISPF 3.4 DSLIST to list the data set(s) with member generations. • Enter Browse, Edit or View to bring up the enhanced member list. • Enter S in the row selection column, and then enter a / in the Prompt field. • Enter the desired generation in the pop-up window. Later in this article an open source (GPL) ISPF dialog, PDSEGEN, will be discussed. PDSEGEN provides a remarkably robust and visually appealing suite of easily executed functions that extend, supersede and overcome the shortcomings of the implementation within the native ISPF Program Development Facility, PDF. What Are PDSE Member Generations? PDSE member generations is a feature of PDSE Version 2 data sets, which are now referred to as libraries . A PDSE library is allocated with a unique attribute, or setting, known as its maximum generation or MAXGEN with an assigned integral value greater than 0. As a precursor to using PDSE Version 2 data sets, the system programmer or storage administrator must enable member generations on a global, installationwide, basis. This task is accomplished by updating member IGDSMS00 in SYS1.PARMLIB, specifying an installationwide numeric value for the MAXGENS_LIMIT parameter. MAXGENS_LIMIT may be set and/or updated dynamically without the need for an IPL using the SET system command. When—and not if—an installation decides to implement member generations, the system programmer must establish the MAXGENS_ LIMIT setting with a reasonable upper bound. A typical, installationwide suggested range for the upper bound is 10-20 generations. However, IBM specifications permit an installation to define a maximum upper bound of 2,000,000,000 member generations. Once chosen, this setting is not fixed: It is solely based upon anticipated use and is dynamically reconfigurable to meet installation requirements. Member generations are analogous to the Generation Data Group (GDG) data sets available since OS/360 MFT/MVT. Each time a member within a PDSE Version 2 library is updated, the updated member and all records contained therein will be saved as a new generation and member—i.e., a single, point-in-time or backup copy of the original member. Each newly created, point-in-time generation is assigned an absolute generation number from 1 to 2,000,000,000. Note that member generations can only be referenced using ISPF, as described above, or by a product that is member generation “aware” or enabled as such. To reference a member generation, one must use the absolute generation number (e.g., 859) or a relative generation number (e.g., -5). Consequently, the generations can be considered as logical, point-in-time, backup copies or previous versions of the base generation (i.e., generation 0) of a PDSE member within a PDSE Version 2 library. To determine if PDSE Version 2 libraries are enabled on a z/OS system (and if one doesn’t have read access to SYS1.PARMLIB), run the following REXX exec to determine the current MAXGENS_LIMIT: E n te rp r i s e  T e c h  J o u rn a l  •  2 0 1 6 :  I s s u e  5  •  5 7

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