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The Job Entry Subsystem (JES) is a component of IBM's
MVS Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. IBM developed MVS, along with OS/VS1 and SVS, as a successor to OS/360. It is unrelated ...
mainframe operating systems that is responsible for managing batch workloads. In modern times, there are two distinct implementations of the Job Entry System called JES2 and JES3. They are designed to provide efficient execution of batch jobs. Job processing is divided into several phases to provide parallelism through pipelining. These phases include input processing where jobs are read and interpreted, the execution phase where jobs run, and output processing where job output is printed or stored on DASD. Jobs that are in the same phase of execution are usually said to reside on a particular queue; for example, jobs that are currently executing are on the execution queue. To improve I/O efficiency, JES performs
spooling In computing, spooling is a specialized form of multi-programming for the purpose of copying data between different devices. In contemporary systems, it is usually used for mediating between a computer application and a slow peripheral, such as ...
, which provides multiple jobs with simultaneous access to a common storage volume. JES uses a structure called a ''checkpoint'' to backup information about currently executing jobs and their associated output. The checkpoint can be used to restore jobs and output in the event of unexpected hardware or software failures. Although JES2 and JES3 provide the same core functionality, there are certain features that may be present in one JES but not the other. Because of these differences, one JES may be favored over the other in certain customer installations. JCL is used to define jobs to both JES2 and JES3, but small changes usually need to be made to the JCL to get a job written for one JES to run on the other. A common issue was that JES3 checked that all datasets listed in the JCL existed before execution or that there was a prior step where the dataset was defined as NEW,CATLG. JES2 did not insist on this, allowing the job to run even though it would fail when the step using it failed to find it.


History


Precursors

OS/360 OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJO ...
's batch job processing had limited operational flexibility and performance, which was addressed by two field-developed packages called the ''Houston Automatic Spooling Priority'' (''HASP'') and the ''Attached Support Processor'' (''ASP'').


HASP

HASP was developed by IBM Federal Systems Division contractors at the
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in
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. It originally managed job scheduling and print and punch output for a single OS/360 computer. ''Multi Access Spool'' capability was added to let peer computers share a common job queue and print/punch output queues. With the introduction of
System/370 The IBM System/370 (S/370) is a model range of IBM mainframe computers announced on June 30, 1970, as the successors to the System/360 family. The series mostly maintains backward compatibility with the S/360, allowing an easy migration path f ...
in 1972, IBM rewrote HASP to become a standard part of the system and renamed it ''Job Entry Subsystem 2''. JES2 was introduced in OS/VS2 in Release 2, also known as
MVS Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. IBM developed MVS, along with OS/VS1 and SVS, as a successor to OS/360. It is unrelated ...
, in 1973. It was many years before the HASP labels were removed from the source code, and the messages issued by JES2 are still prefixed with $HASP. Several JES2 commands continue to support specification of either JES2 or HASP to maintain
backwards compatibility Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially ...
.


ASP

ASP initially stood for ''Attached Support Processor'',Eventually renamed to ''Asymmetric Multiprocessing System'' and was developed to provide efficient use of multiple OS/360 systems with a shared workload. It allowed one central system to distribute jobs to multiple connected systems. ASP was announced in March 1967, and that year was reported to be "running very stably". ASP evolved from the design of the 7094/ 7040 Direct Coupled System, using data channel to data channel communication. By attaching an IBM 7040 as a peripheral, processor throughput was more than doubled. In a typical ASP configuration, a small mainframe such as a 360/40 called the ''support'' system controlled one or more 360/65 or larger processors called ''main'' systems. The computers were connected through selector channels on each host attached to
channel-to-channel adapter In IBM mainframe technology, a channel-to-channel adapter (CTCA) is a device that connects two input/output channels on (usually) two separate computer systems. The adapter allows one computer system to be treated as an input/output device by anot ...
s in an early form of short distance, point-to-point computer networking. ASP required the purchase of an additional computer to manage input and output of the hosts running the job workload, which was economically justified by the high cost of standalone byte-multiplexor channels needed to drive printers and punched card reader devices; the 360/50 and smaller systems had a built-in byte multiplexor channel, whereas the faster 360/65 and larger systems required a relatively expensive standalone unit. Using ASP made it possible to avoid the cost of the byte multiplexor channel, and offloading the job scheduling, print, and card handling also offloaded those functions from the larger machines. Increased reliability was another advantage to offset the added hardware cost. One or more ''main'' systems could fail or be taken offline for maintenance without taking down the whole complex. ASP was primarily targeted at large government agencies and defense contractors that might have as many as six 360/65s all being scheduled and managed by a separate ASP machine. An uncommon variant, ''local ASP'' (''LASP''), was a single large machine with the ASP functions running on the same machine. In the 1970s, a notable installation of ASP was at Princeton University controlling an IBM 360/91 mainframe. In 1973, IBM rewrote ASP and renamed it JES3, supporting MVS only.


Alternatives

There was also a JES in
OS/VS1 Operating System/Virtual Storage 1, or OS/VS1, is a discontinued IBM mainframe computer operating system designed to be run on IBM System/370 hardware. It was the successor to the Multiprogramming with a Fixed number of Tasks (MFT) option of Sys ...
that was often referred to as JES1. In addition, the ''master'' (''MSTR'') subsystem is built into the modern z/OS operating system to start jobs that run outside of the control of JES, including the primary JES itself.


Customization

Source code was provided to IBM customers for both ASP and HASP, and many customers made substantial enhancements to these programs, some of which were incorporated into the official product. Far more installations made use of HASP than ASP, and in contemporary z/OS systems, there are many more JES2 installations than JES3. Because of their unique history, IBM continues to ship JES2 and JES3
source code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the w ...
instead of
object code In computing, object code or object module is the product of a compiler. In a general sense object code is a sequence of statements or instructions in a computer language, usually a machine code language (i.e., binary) or an intermediate langu ...
, unlike most components of the operating system. To improve maintainability and serviceability of user-written enhancements, JES provides a set of exit points that pass control from the JES to user programs at key points of processing. These extensions can provide custom functionality such as special commands, custom print page headings, and non-standard job processing.


Current development

In 2017, IBM released a statement of direction for JES2 to be the "strategic" JES, meaning that all future development efforts will be focused on JES2 rather than JES3. IBM has reassured customers that JES3 will continue to be supported until an end-of-support date is announced. In February 2019, IBM announced that z/OS 2.5 (expected to be released in 2021) will be the last version of z/OS to include JES3. In October 2019, Phoenix Software International announced that it had licensed the JES3 source code from IBM and would be taking over its maintenance and enhancement.


See also

* System Display and Search Facility (SDSF) most common utility to interact with JES2 In about 1974, I worked, installing ASP on a 360/95 (or 195) system. My memory from that time was that ASP stood for "Asynchronous Scheduling Processor".


References


External links

* * {{DEFAULTSORT:Job Entry Subsystem 2 3 IBM mainframe operating systems Job scheduling Remote job entry Assembly language software