Here are the top differences between JES2 and JES3 in the mainframe operating system MVS.
Job Entry Subsystems
MVS uses a JES to receive jobs into the operating system, schedule them for processing by MVS, and control their output processing.
JES2 is descended from Houston Automatic Spooling Program (HASP). HASP is defined as a computer program that provides supplementary job management, data management, and task management functions, such as scheduling, control of job flow, and spooling.
HASP remains in JES2 as the prefix of most module names and the prefix of all messages sent by JES2 to the operator.
Job Entry Subsystem 2
JES2 is a functional extension of the HASP II program that receives jobs into the system and processes all output data produced by the job.
So, JES2 is that component of MVS that provides the necessary functions to get jobs into, and output out of, the MVS system.
It is designed to provide efficient spooling, scheduling, and management facilities for the MVS operating system.
However, none of this explains why MVS needs a JES. Basically, by separating job processing into several tasks, MVS operates more efficiently.
At any point in time, the computer system resources are busy processing the tasks for individual jobs, while other tasks are waiting for those resources to become available.
In its most simple view, MVS divides the management of jobs and resources between the JES and the base control program of MVS. In this manner, JES2 manages jobs before and after running the program; the BCP manages them during processing.
JES2 compared to JES3
IBM provides two JESs from which to choose: JES2 and JES3. In an installation that has only one processor (computer), JES2 and JES3 perform similar functions.
That is, they read jobs into the system, convert them to internal machine-readable form, select them for processing, process their output, and purge them from the system.
However, for an installation that has more than one processor in a configuration, there are noticeable differences in how JES2 exercises independent control over its job processing functions.
That is, in the configuration, each JES2 processor controls its own job input, job scheduling, and job output processing. In a sysplex environment, it is possible to configure JES2 to share spool and checkpoint data sets with other JES2 systems in the same sysplex. This configuration is called Multi-Access Spool (MAS).
Job Entry Subsystem 3
In contrast, JES3 exercises centralized control over its processing functions through a single global JES3 processor. This global processor provides all job selection, scheduling, and device allocation functions for all the other JES3 systems.
The centralized control that JES3 exercises provides increased job scheduling control, deadline scheduling capabilities, and increased control by providing its own device allocation.