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Job Entry Subsystem 2/3

From Alfawiki
Job Entry Subsystem (JES)
Company typeSoftware subsystem
IndustryMainframe operating systems
PredecessorHouston Automatic Spooling Priority (HASP), Attached Support Processor (ASP)
Founded1973
Headquarters
United States
ProductsJES2, JES3
OwnerIBM (JES2), Phoenix Software International (JES3)

The Job Entry Subsystem (JES) serves as an essential element within IBM's MVS family of mainframe operating systems (ranging from MVS/370 up to z/OS), tasked with managing batch processing workloads. Presently, there are two variants: JES2 and JES3. Beginning with z/OS 3.1 (released September 2023), IBM provides only JES2 as part of the operating system; users of JES3 must transition to or obtain a license for JES3plus from Phoenix Software International, which now oversees JES3's ongoing support and development.

JES orchestrates job handling by segmenting it into distinct phases to facilitate parallel execution through pipelining: these phases include input processing (reading and interpreting jobs), execution, and output processing (either printing or saving onto DASD). Jobs positioned in the same phase are organized into designated queues, such as the execution queue.

To enhance input/output efficiency, JES implements spooling, enabling multiple jobs to concurrently access shared storage resources. Additionally, it uses checkpoints to save progress for running jobs and their outputs, allowing recovery after hardware or software interruptions.

Although JES2 and JES3 share fundamental capabilities, differences in their features often influence customer selection. Both systems utilize JCL for job definitions; however, minor JCL modifications are typically necessary when migrating between them. For instance, JES3 performs a pre-execution check to confirm dataset existence or prior NEW,CATLG definitions, whereas JES2 does not, which may lead to jobs initiating but failing later due to absent datasets.

Early Developments

The batch processing capabilities of OS/360 initially suffered from limited flexibility and efficiency. These issues were addressed by user-developed solutions such as Houston Automatic Spooling Priority (HASP) and Attached Support Processor (ASP).

HASP

HASP originated from contractors at IBM Federal Systems Division working at the Johnson Space Center in Houston. It began as a tool for managing job scheduling alongside print and punch output on a single OS/360 system. Subsequently, it incorporated Multi Access Spool functionality, allowing multiple peer systems to share job and output queues.

With the launch of the System/370 in 1972, IBM transformed HASP into a standardized system component named Job Entry Subsystem 2. JES2 was introduced in OS/VS2 Release 2 (MVS) in 1973. Despite this evolution, HASP terminology persisted within source code and system messages prefixed with $HASP, and several JES2 commands still accept JES2 or HASP for backward compatibility.

ASP

Attached Support Processor (later renamed Asymmetric Multiprocessing System) was designed to optimize workload distribution among multiple mainframe systems. Released in March 1967, ASP allowed a central processor to oversee job management on connected processors running OS/360, SVS, and 7090 emulation on a 360/65 mainframe. However, integrated emulators were supported only on OS/360 for the 360/85 model and SVS on S/370 systems.

ASP evolved from the IBM 7094/7040 Direct Coupled System architecture by employing data channel communication that more than doubled processing throughput through an IBM 7040 attached as a peripheral device. Typical setups featured a smaller machine (such as a 360/40) acting as a support system controlling one or more larger main systems (like the 360/65), interconnected via selector channels and channel-to-channel adapters—an early example of point-to-point networking.

This approach necessitated acquiring an extra computer dedicated to I/O management tasks; this was economically justified given the high expense of standalone byte-multiplexor channels required for printers and card readers on larger mainframes. ASP offloaded job scheduling and I/O operations from primary systems, reducing hardware costs while enhancing reliability—allowing maintenance or failures on mainframes without disrupting the entire network.

ASP was primarily aimed at large government organizations and defense contractors operating multiple 360/65 systems; a variant named local ASP (LASP) implemented ASP functions within a single large machine. Princeton University notably deployed ASP to manage an IBM 360/91 mainframe.

In 1973, IBM reengineered ASP into JES3 with an exclusive focus on supporting MVS environments.

Alternatives

Within OS/VS1, there existed a JES variant commonly known as JES1. Moreover, the MVS operating system includes a built-in subsystem called the master (MSTR), which has the capability to initiate jobs independently from the main JES. This includes control over both the Master Scheduler and the primary JES itself. Initially, the Master subsystem's JCL was provided through IBM-supplied load modules; however, in modern MVS and z/OS versions, it is typically delivered as a component of the system parameter library (PARMLIB).

Customization

IBM released source code for ASP and HASP, allowing numerous clients to modify and enhance these systems, with some enhancements later integrated into official IBM distributions. HASP achieved wider usage compared to ASP, and currently, installations of JES2 greatly exceed those of JES3 on z/OS platforms. Due to their distinct development paths, IBM continues to distribute JES2 and JES3 in source code form rather than as object code, unlike the majority of other OS elements.

To facilitate easier maintenance of user modifications, JES provides exit points where custom programs can be inserted at critical processing phases. These extension points support specialized features such as custom commands, personalized print headers, and unconventional job handling procedures.

Current development

In 2017, IBM identified JES2 as its "strategic" JES product, directing future enhancements predominantly toward JES2 instead of JES3. Support for JES3 remains active until IBM announces an official end-of-support date. In February 2019, IBM declared that z/OS version 2.5—anticipated for release in 2021—would represent the final edition including JES3. Later that year, in October 2019, Phoenix Software International acquired a license from IBM for the JES3 source code and took over responsibility for its ongoing maintenance and development.

See also

External links

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