Class

Enhanced Definition

In z/OS, a "Class" is a logical grouping mechanism used to categorize and manage various system resources, workloads, or entities. Its primary purpose is to enable the system to apply specific rules, priorities, or resource allocations to members of a group, thereby optimizing system performance, controlling access, or managing execution.

Key Characteristics

- JCL Job Class: A single-character identifier (A-Z, 0-9) specified in the JOB statement's CLASS parameter, which determines which initiator can select a job for execution and influences its priority within the job queue.
- WLM Service Class: Defines performance goals and resource allocation policies for workloads, allowing the Workload Manager (WLM) to dynamically manage system resources (CPU, I/O, memory) to achieve desired performance objectives.
- CICS Transaction Class: Groups CICS transactions to control their maximum concurrent executions, preventing resource exhaustion and ensuring system stability.
- RACF Resource Class: Categorizes types of resources (e.g., datasets, terminals, programs, commands) for which access rules are defined and enforced by RACF (Resource Access Control Facility).
- Configurable Policies: Class definitions are highly configurable by system programmers to align with specific operational requirements, business priorities, and performance targets.
- Resource Management Focus: Each type of class serves to manage a particular aspect of system resources, from CPU and I/O dispatching to memory and concurrent access.

Use Cases

- Job Scheduling and Prioritization: Assigning a CLASS parameter in JCL (e.g., //JOBNAME JOB (ACCT),'PROD JOB',CLASS=A) to control when and where a batch job runs, and its relative priority among other jobs in the JES queue.
- Workload Performance Management: Defining WLM Service Classes for critical online transactions (e.g., CICS, DB2) to ensure they meet specific response time goals, even under heavy system load, by prioritizing their resource access.
- Controlling Transaction Concurrency: Using CICS Transaction Classes to limit the number of concurrent executions of resource-intensive CICS transactions, preventing system overload and ensuring fair resource distribution.
- Security Access Control: Grouping similar resources under a RACF resource class (e.g., DATASET, TERMINAL, PROGRAM) to apply consistent security policies and simplify administration of access rules.
- Resource Isolation and Segregation: Separating development, test, and production workloads into different WLM Service Classes to ensure production systems receive priority resources and are isolated from non-production impacts.

Related Concepts

JCL (Job Control Language): The CLASS parameter in JCL is fundamental for job scheduling, interacting directly with JES (Job Entry Subsystem) to manage job queues and initiator assignment.
WLM (Workload Manager): WLM heavily relies on Service Classes to implement its goal-oriented resource management, dynamically adjusting resource allocation based on defined performance objectives and system conditions.
JES (Job Entry Subsystem): JES2 and JES3 use job classes to categorize jobs, manage their queues, and assign them to available initiators, which are configured to process specific classes.
RACF (Resource Access Control Facility): RACF uses resource classes to define the types of resources it protects, enabling granular access control and auditing, often interacting with other components like CICS and DB2.
CICS (Customer Information Control System): CICS utilizes transaction classes to manage the concurrency of transactions, which is crucial for