JES2 compared to JES3

IBM® provides two kinds of job entry subsystems: JES2 and JES3. In many cases, JES2 and JES3 perform similar functions, but most installations use JES2.

Both JES2 and JES3 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.

Some principle differences between the two JES systems include:

In a mainframe installation that has only one processor, JES3 provides tape setup, dependent job control, and deadline scheduling for users of the system, while JES2 in the same system would require its users to manage these activities through other means. In an installation with a multiprocessor configuration, there are noticeable differences between the two, mainly in how JES2 exercises independent control over its job processing functions. That is, within the configuration, each JES2 processor controls its own job input, job scheduling, and job output processing.
In cases where multiple z/OS® systems are clustered (a sysplex), 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). 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 of the other JES3 systems.
With JES3, installations may decide whether the global JES3 or z/OS base control program will handle device allocation. With JES2, only the z/OS base control program handles device allocation.