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A job entry subsystem (JES) is the z/OS component that accepts jobs, queues and schedules them for processing, and manages their output. JES prepares work before a program runs and handles its output afterward; the z/OS base control program manages the job while it is executing.
What does a job entry subsystem do?
IBM describes JES as responsible for receiving jobs into z/OS, scheduling them for processing, and controlling output processing. A job can arrive through ISPF’s SUBMIT command, a network, a card reader, or a running program using the JES internal reader. JES places work in queues and coordinates with initiators, which process JCL and run batch jobs in an address space.
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- Accepts work: JES brings submitted jobs into the operating system.
- Schedules and queues work: It manages job queues and coordinates when work is made available for processing.
- Controls output: It manages job output, which can be queued for a printer or retained on spool for retrieval by an output manager.
IBM’s “What is JES?” overview summarizes these functions; the z/OS 3.2 JES2 introduction explains how JES relates to the system’s execution management.
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JES is not the component that runs the application program. It manages work around execution: it accepts and prepares the job, queues it, and handles output. The z/OS base control program manages the job during processing. Initiators process its JCL and run the batch job in an address space.
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- JES receives the job through a submission method such as ISPF, a network, or an internal reader.
- The system makes it available to run: JES manages the queue and coordinates with initiators.
- The system executes the job: the base control program manages processing while the program runs.
- JES handles the output: output is queued for a device or kept on spool for later retrieval.
This division is useful when tracing a batch job: JES handles intake and output, while the operating system manages execution. IBM describes the flow in “Processing work on z/OS: How the system starts and manages batch jobs.”
What is spooling in JES?
Spooling is the use of auxiliary storage to buffer input or output streams so they can be handled separately from the job’s immediate execution. JES can retain output on spool until it is ready for a printer or an output manager. Input can also be buffered for jobs. This lets producers and later device processing work at different times rather than requiring a job’s output to be printed as it is created.
IBM’s CICS interface to JES documentation describes spool files as buffers between the job that produces output and subsequent device processing.
What is the difference between JES2 and JES3?
JES2 and JES3 are the two JES types IBM identifies for z/OS. Both cover the core role of job intake, scheduling and queue management, and output handling. IBM says their functions are often similar and notes that most installations use JES2, but its overview does not give an adoption percentage or establish a universal reason to choose one over the other.
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| Comparison | JES2 | JES3 |
|---|---|---|
| Type | A z/OS job entry subsystem | A z/OS job entry subsystem |
| Core role | Receives and schedules work and controls output | Receives and schedules work and controls output |
| Use in installations | IBM’s overview says most installations use JES2; no percentage is stated. | The overview identifies JES3 but does not state a comparable adoption figure. |
The documented comparison supports understanding their shared purpose, not a current feature-by-feature selection or migration recommendation. A site’s existing z/OS environment is relevant when discussing which subsystem it runs.
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