ELM - Extended Link Module

Enhanced Definition

An Extended Link Module (ELM) is a specialized program module used by CICS (Customer Information Control System) to facilitate the linking of a program in one CICS region to a program residing in a different CICS region or address space. It extends the standard CICS `LINK` and `XCTL` commands to enable inter-region program calls, often leveraging Multi-Region Operation (MRO) or InterSystem Communication (ISC).

Key Characteristics

- CICS-Specific: ELMs are integral to CICS environments, enabling distributed application architectures within z/OS.
- Inter-Region Program Link (IRPL): They provide the mechanism for a CICS program to dynamically invoke another program that is not defined in its local region.
- Dynamic Program Location: The ELM handles the resolution of the target program's location, allowing the calling program to remain unaware of the physical region where the called program resides.
- Parameter Passing: ELMs manage the transfer of data, typically via a COMMAREA or CHANNEL and CONTAINERS, between the calling and called programs across region boundaries.
- IBM-Supplied Module: CICS provides a standard ELM, often referred to as DFH$ELM or DFH@ELM, which is typically sufficient for most inter-region linking requirements.

Use Cases

- Service-Oriented Architectures (SOA): Implementing common business services or utility functions in a dedicated CICS region, which can then be invoked by various application regions.
- Workload Balancing: Distributing transaction processing across multiple CICS regions to optimize resource utilization and improve overall system throughput.
- Resource Isolation: Separating critical applications or sensitive data access logic into distinct CICS regions for enhanced security, stability, or easier management.
- Application Modernization: Allowing newer CICS applications to seamlessly interact with and leverage existing legacy applications or data access routines residing in older, separate CICS regions.
- Data Access Layer: Creating a dedicated CICS region that acts as a data access layer for DB2, IMS, or VSAM, which application regions then link to for data operations.

Related Concepts

ELMs are foundational to CICS Multi-Region Operation (MRO) and InterSystem Communication (ISC), as they enable the program linking aspect of these inter-CICS communication facilities. They extend the functionality of the standard CICS LINK and XCTL commands, allowing them to operate across region boundaries rather than being confined to a single CICS address space. The underlying mechanism for inter-region program linking, often involving DFHIRP, is leveraged by ELMs to establish the connection and transfer control.

Best Practices:

Utilize Standard ELM: Always use the IBM-supplied DFH$ELM or DFH@ELM unless there is a specific, well-justified requirement for custom logic, as it is optimized and fully supported.
Minimize Cross-Region Links: While powerful, inter-region links incur overhead. Design applications to minimize the number of cross-region calls and keep tightly coupled processing within a single region for optimal performance.
Optimize Data Transfer: Be mindful of the size of COMMAREAs or the number/size of CONTAINERS passed across regions, as large data volumes can impact performance. Use CHANNELS and CONTAINERS for larger or more complex data structures.
Robust Error Handling: Implement comprehensive error handling for LINK commands that target remote regions, checking RESP and RESP2 codes to gracefully manage situations where the target program or region is unavailable.
Security Configuration: Ensure proper security definitions (e.g., using RACF, ACF2, or Top Secret) are in place for transactions and programs involved in cross-region links to prevent unauthorized access and maintain data integrity.