Alias
In z/OS, an alias is an alternative name assigned to a dataset, a member within a partitioned dataset (PDS or PDSE), or a catalog entry. It serves as a pointer or symbolic link, allowing the same resource to be referenced by different names without duplicating the actual data.
Key Characteristics
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- No Data Duplication: An alias does not create a separate copy of the data; it simply provides an additional name to access the existing, single instance of the resource.
- Member Aliases (Entry Point Aliases): Within a PDS or PDSE, an alias member points to a primary member (the alias base or target member). This is commonly used for load modules where different entry points are defined.
- Dataset Aliases: Can be defined in a VSAM catalog to point to a non-VSAM dataset, or within a user catalog to point to a base entry, aiding in dataset organization and search.
- Catalog Aliases: A user catalog can be defined as an alias for another user catalog, or a master catalog can have aliases for user catalogs, influencing dataset search order.
- Resolution by System: When an alias is referenced (e.g., in JCL or a program), the z/OS system resolves it to the actual target resource before processing.
- Management: Aliases are created and managed using system utilities such as
IDCAMSfor datasets and catalogs, or the Linkage Editor (IEWL) andIEBCOPYfor PDS/PDSE members.
Use Cases
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- Load Module Entry Points: A COBOL program compiled into a load module can have multiple
ENTRYstatements, each defining an alias to the main program name, allowing different functions within the module to be called directly. - Program Versioning and Compatibility: An alias can point to the current active version of a program or dataset (e.g.,
PROD.APP.LOADas an alias forPROD.APP.V2R1.LOAD), allowing applications to use a consistent name while the underlying resource changes. - System Library References: System libraries often use aliases to provide backward compatibility or alternative names for frequently used system routines (e.g.,
LINKmight be an alias forIEWL). - JCL Flexibility: JCL can refer to a dataset or program by its alias name, providing flexibility and allowing changes to the actual resource name without modifying the JCL.
- Cross-Catalog Referencing: An alias can be used to reference a dataset that resides in a different user catalog, simplifying access and management across catalog structures.
- Load Module Entry Points: A COBOL program compiled into a load module can have multiple
Related Concepts
Aliases are integral to Catalogs (VSAM and non-VSAM), which store the definitions and relationships of datasets, including alias entries. For PDS/PDSE members, aliases are stored in the directory of the dataset, alongside the primary member entries. They are crucial for program linking as the Linkage Editor (IEWL) creates aliases for ENTRY points within a load module, enabling modular programming and shared code. Aliases facilitate data management by providing flexible naming conventions without duplicating storage.
- Meaningful Naming: Use clear and descriptive alias names that indicate their purpose or relationship to the base resource to improve readability and maintainability.
- Document Aliases: Maintain comprehensive documentation of all aliases, their targets, and their intended use, especially in shared or critical libraries.
- Careful Management: When deleting, renaming, or moving base resources, ensure that all associated aliases are updated or removed to prevent JCL errors or program abends.
- Avoid Overuse: While flexible, excessive use of aliases can complicate system management and debugging; use them judiciously where they provide clear benefits.
- Performance Consideration (PDS/PDSE): While aliases don't duplicate data, a very large number of aliases in a PDS/PDSE directory can slightly increase the time required for directory searches, though this is rarely a significant performance factor.