DFLT - Default

Enhanced Definition

In mainframe systems, "default" refers to a pre-defined or automatically assumed value, setting, or behavior that is used when no explicit alternative is specified by the user, program, or system configuration. These defaults are crucial for system operation, program execution, and resource management, ensuring that processes can proceed even without complete user input. In the context of IBM mainframe systems and z/OS, a "default" refers to a pre-defined or system-assigned value, setting, or behavior that is automatically assumed or used when no explicit specification is provided by the user, program, or JCL. It serves to simplify operations and ensure consistent system behavior in the absence of explicit instructions.

Key Characteristics

- System-defined or Application-defined: Defaults can be set by the z/OS operating system itself (e.g., dataset allocation parameters), by specific subsystems (e.g., CICS transaction attributes, DB2 bind options), or by application programs (e.g., initial variable values in COBOL).
- Overridable: Most defaults are designed to be overridden by explicit user input, JCL parameters, program statements, or system commands, allowing for customization and specific requirements.
- Context-sensitive: The specific default value often depends on the context in which it is applied, such as the type of resource being allocated, the subsystem being used, or the program being executed.
- Efficiency and Simplification: Defaults simplify operations by reducing the amount of explicit information required from users or developers, making common tasks quicker and less error-prone.
- Configuration-driven: Many system-wide or subsystem-wide defaults are established through configuration files, initialization parameters (e.g., PARMLIB members, CICS SIT), or installation options.

Use Cases

- JCL Dataset Allocation: When allocating a new dataset via JCL, if parameters like SPACE=(CYL,(1,1)) or DCB attributes are omitted, z/OS will use system-defined defaults for space, record format, record length, and block size.
- COBOL Program Variables: A VALUE clause in COBOL can set an initial default value for a data item, or if omitted, numeric fields might default to zero and alphanumeric fields to spaces, depending on compiler options and WORKING-STORAGE initialization.
- CICS Transaction Definitions: When defining a CICS transaction, many attributes (e.g., TASKDATALOC, RESTART) have default values that are applied if not explicitly specified in the CEDA DEFINE TRANSACTION command or resource definition.
- DB2 Bind Options: During the BIND process for a DB2 application package, options like ISOLATION LEVEL or RELEASE have default values that are used if not explicitly provided in the BIND command.
- System Initialization Parameters: PARMLIB members in z/OS contain numerous system-wide defaults for various components, such as SMFPRMxx for SMF recording or IEASYSxx for overall system configuration.

Related Concepts

Defaults are fundamental to the operation of z/OS and its subsystems, working closely with JCL (Job Control Language), PARMLIB, and system configuration. They provide a baseline for resource allocation and program execution, ensuring that processes can run even with minimal explicit instruction. Defaults complement explicit specifications by providing fallback values, thereby reducing the verbosity of JCL, program code, and system commands while maintaining operational integrity. They are often defined and managed through system programmers and subsystem administrators who configure the environment.

Best Practices:

Understand System Defaults: Always be aware of the default values for critical parameters in JCL, COBOL, CICS, DB2, and other subsystems to avoid unexpected behavior.
Explicitly Override When Necessary: For production environments or critical processes, explicitly specify parameters rather than relying solely on defaults, especially when specific performance, security, or resource requirements exist.
Document Custom Defaults: If your organization modifies system or application defaults, ensure these changes are thoroughly documented and communicated to all relevant teams.
Test Default Behavior: When developing new applications or scripts, test how they behave when relying on defaults versus when parameters are explicitly provided, to understand potential impacts.
Review Subsystem Defaults: Periodically review and adjust subsystem-specific defaults (e.g., CICS SIT parameters, DB2 DSNZPARM) to align with current performance, security, and operational requirements.