Concatenation

Enhanced Definition

In z/OS, concatenation refers to the process of logically linking multiple sequential datasets or partitioned dataset (PDS/PDSE) members together to form a single, continuous stream of data for an application or system program. This allows a program to read data from several sources as if it were one, without physically merging the datasets.

Key Characteristics

- Logical Linking: Datasets are not physically combined; their contents are presented sequentially to the reading program or system component.
- Read-Only Access: Concatenated datasets are primarily used for input (reading). Writing to a concatenated DDNAME is generally not supported or leads to errors, as the system cannot determine which specific dataset to update.
- Order Matters: The sequence in which datasets are concatenated is critical, as the system processes them in that exact order from top to bottom.
- JCL Implementation: Achieved in JCL by specifying multiple DD statements with the same DDNAME, where only the first DD statement has a DSN and subsequent DD statements either omit DSN or specify additional datasets.
- Dataset Types: Most commonly used with sequential datasets (DSORG=PS) and partitioned datasets (DSORG=PO or PO,EXTENDED) for libraries.
- System Search: When a program opens a concatenated DDNAME and requests a member (e.g., a load module or copybook), the system searches each dataset in the concatenation order until the member is found.

Use Cases

- Program Library Search Paths: Defining STEPLIB or JOBLIB concatenations to allow a program to search multiple PDS/PDSE libraries for required load modules.
- COBOL Copybook Libraries: Concatenating several PDS/PDSEs under a SYSLIB or user-defined DDNAME so the COBOL compiler can find all necessary COPY members.
- JCL Procedure Libraries: Specifying JCLLIB or concatenating PROCLIB datasets to enable the JCL interpreter to locate called procedures (PROC) across various libraries.
- Input Data Streams: Providing a program with input that spans multiple sequential files, such as historical transaction logs or report segments, as a single logical input source.
- ISPF Application Libraries: Concatenating libraries for ISPF applications to find panels, messages, skeletons, or table definitions.

Related Concepts

Concatenation is a foundational concept in z/OS, intrinsically linked to JCL (Job Control Language), which is the primary mechanism for its implementation. It directly impacts the dataset search order for critical system components and user applications, particularly for program execution (via STEPLIB/JOBLIB) and compilation (via SYSLIB). It enables flexible dataset organization by allowing related but distinct libraries (e.g., test vs. production versions of code) to be accessed as a unified resource.

Best Practices:

Optimize Order for Performance: For library concatenations (e.g., STEPLIB, SYSLIB), place the most frequently accessed or smallest libraries first to minimize search time and I/O operations.
Avoid Excessive Concatenation: While powerful, concatenating too many datasets can degrade performance due to increased search overhead. Consolidate libraries where appropriate and feasible.
Ensure Dataset Compatibility: When concatenating sequential datasets for input, ensure they have compatible record formats (RECFM) and logical record lengths (LRECL) to prevent program abends.
Document Concatenation Logic: Clearly document the purpose and order of datasets within critical concatenations to facilitate maintenance, troubleshooting, and understanding by other team members.
Use DSORG=PO or PO,EXTENDED for Libraries: Always use partitioned datasets (PDS or PDSE) for libraries intended for concatenation, as this is the design paradigm for such usage.