Job Entry Subsystem - JES


JES is an MVS subsystem that manages jobs on the mainframe. JES  will receive jobs into MVS, check
the syntax, schedule jobs for processing and control job output.

Two types of JES:


1.JES2 Job processing:

There are 6 distinct phases to processing jobs with JES2 on MVS. All these phases except the
processing phase are handled by JES2, MVS handles the actual processing of a program.

1) Input Phase
During the Input Phase the JCL job stream is read from input devices such as card readers, terminals and
the Internal Reader. The Internal Reader is a special system program that is used by programs to submit
jobs, commands, and control statements to JES2 for processing.

JES2 will read the job stream and assign each job an ID, (Job Number). The job stream will then be
placed into special JES2 spool datasets ready for processing.

JES2 will select jobs from the spool datasets for processing.

2) Conversion Phase.
When a job is selected for processing, JES2 will start a converter program, which will merge any JCL
Procs into the main job stream and convert the job stream into text that JES2 and MVS can understand.
The text is stored on spool datasets.

At this stage the JCL syntax is checked for errors. If there are any errors then the job is queued for output processing (see 4. Output Processing) and no programs are run.

3) Processing Phase
In the processing phase JES2 will receive request from MVS initiators for work to process. JES2 selects a
job and sends it to be processed by MVS.

An initiator is a special MVS system program which starts a job. Jobs are assigned to an initiator based
on job classes. Initiators are assigned one or more classes, if a job has the same job class as an initiator,
then it can run on that initiator.

The initiator will build control blocks for the job and allocate the resources (files, tapes, etc) that you
specified in the JCL. The initiator will then begin executing the specified program.

4) Output Phase.
In the output phase JES2 will handle processing of the output from your job. JES2 will check the output
classes and device setup and queue the output for print or punch processing.

5) Hard Copy Phase
In this phase the output is selected for processing. The output will be sent to the printer or punch device. Once the output has been processed the job is placed on the purge (delete) queue.

6) Purge Phase
In the purge phase the space allocated on the spool datasets for your job and output is released (ie the output is deleted). Once the space is released, a message is issued to the system log to say that the output has been purged.


JES3 processing is considered to be centrally controlled. One image is designated as the focal point for the entry and distribution of jobs and for the control of resources needed by the jobs. That image, called the global processor, distributes work to itself and the other images in the configuration, known as local processors. It is from the global processor that JES3 manages jobs and resources for the entire JESplex, matching jobs with available resources. JES3 manages processors
(images), I/O devices, volumes, and data. To avoid delays that result when these resources are not available, JES3 ensures that they are available before selecting the job for processing. Using this centralized approach, JES3 can make informed and complex decisions about job scheduling and placement.


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