Editing Job Entry Subsystem 2/3 (section)

{{short description|Component of the z/OS operating system}}
{{More citations needed|date=January 2022}}
The '''Job Entry Subsystem (JES)''' is a component of [[IBM]]'s [[MVS]] ([[MVS/370]] through [[z/OS]]) mainframe operating systems that is responsible for managing [[batch job|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. Starting with z/OS 3.1, released in September 2023, IBM z/OS no longer includes JES3, and comes with JES2 only – JES3 sites must either migrate to JES2, or license '''JES3plus''' from Phoenix Software International, who has taken over future support and development of JES3 from IBM.<ref>{{Cite web |last=Temel |first=Meral |date=September 11, 2023 |title=z/OS 3.1 Update |url=https://www.ibm.com/support/pages/system/files/inline-files/Tech%20Byte%20zOS%203.1%20Update.pdf |access-date=2025-01-27 |website=IBM}}</ref>

Job processing is divided into several phases to provide [[Parallelism (computing)|parallelism]] through [[Pipeline (computing)|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 [[Direct-access storage device|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]], 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.  [[Job Control Language|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.