External

Enhanced Definition

In mainframe computing, "external" generally refers to resources, components, or code that reside outside the immediate scope or address space of a specific program, job, or system element, yet are accessed, referenced, or managed by it. This separation promotes modularity, reusability, and centralized control over critical system assets.

Key Characteristics

- Separation of Concerns: External entities are developed, compiled, and managed independently from the consuming component, promoting a modular and maintainable design.
- Dynamic Linking/Loading: Many external resources, such as subroutines or load modules, are linked or loaded dynamically at execution time, rather than being statically embedded within a program.
- Shared Access: External resources like data sets, databases, or security profiles can be shared across multiple jobs, programs, or users, fostering resource efficiency and consistency.
- System Integration: External components often integrate with the z/OS operating system or other system software (e.g., CICS, DB2) through well-defined application programming interfaces (APIs).
- Centralized Management: External entities, such as security rules, system configurations, or shared libraries, are typically managed centrally by system administrators or specialized tools.

Use Cases

- Calling External Subroutines: A COBOL application invokes a separately compiled and linked Assembler or C routine to perform a specific, reusable function like date calculations or data encryption.
- Utilizing External Security Managers (ESMs): z/OS applications and the operating system itself rely on products like RACF, ACF2, or Top Secret to validate user identities and authorize access to resources (e.g., data sets, transactions, commands).
- Accessing External Data: A batch job reads data from a VSAM KSDS or a DB2 table, where the data resides on DASD and is managed independently of the application program's executable code.
- Directing Output to an External Writer: JCL uses DD SYSOUT=(class,writer_name) to send job output to a specific user-written program for specialized processing (e.g., custom formatting, archiving, or distribution).
- Resolving External Symbols: During program linking, the linkage editor resolves references to external symbols (e.g., entry points of other modules or common data areas) defined in the External Symbol Dictionary (ESD).

Related Concepts

The concept of "external" is fundamental to the modular and secure architecture of z/OS. It underpins program reusability by allowing common functions to be developed once and called by many programs, often residing in STEPLIB or LINKLIST libraries. It is crucial for system security, as External Security Managers provide a centralized, robust layer of access control for virtually all mainframe resources. Furthermore, it is intrinsically linked to data management, as most application data resides externally to the executing program, managed by the operating system (e.g., for VSAM files) or database managers (DB2, IMS).