Job Entry Subsystem(JES)

Job Entry Subsystem, since it’s pretty important to the ability for z/OS to run batch workloads.  

Description

– Component of z/OS that provides the necessary functions to get jobs into, and out of, the system.

– JES2 and JES3 are available.

– Manages jobs before and after running the program; the base control program manages them during processing.

The Job Entry Subsystem does just what the name implies – manages the entry (and exit) 
of jobs into z/OS.  When a batch job (or other kinds of work, actually) needs to run on z/OS, 

JES is responsible 

    - For taking that work in, 

    - Preparing for the allocation of resources, and 

    - Then passing the job into z/OS to be dispatched and run.  

JES manages the output generated by the running programs – it SPOOLs the output to the 
JES SPOOL (pretty much like a print spool on your PC) and temporarily stores it until it can 
be printed or stored or deleted.

“JES2” and “JES3”.   IBM actually sells two different JESes.  

They perform the same basic system functions, but with somewhat different interfaces.  Both were developed in the early days of z/OS, out in the field.  Some customers prefer one, some the other.  JES2 is more widely deployed, although there is a smaller, yet enthusiastic JES3 user community also. 

JCL is the “language” of batch jobs.  A programmer or user creates a JCL file (or as some old-timers say, a “deck”) that describes how a program executes.  The JCL identifies the program, its input and output files, and parameters that influence how the program runs.  

Multiple programs can be strung together in a “job stream” where data is passed to and from the programs in the stream.  

JCL files consist of a series of records, or “cards” that begin with two forward slashes.  

So if you ever see a file that has a bunch of lines that start with //, you’ll know it’s a JCL file that controls the running of a batch job stream.