IRC

Inter-Region Communication

Enhanced Definition

Inter-Region Communication (IRC) is a fundamental CICS facility that enables multiple CICS regions (address spaces) to communicate with each other and share resources. It allows for distributed processing, workload balancing, and resource isolation across CICS systems, whether they reside on the same or different z/OS images.

Key Characteristics

- Multi-Region Operation (MRO): Facilitates communication between CICS regions running on the *same* z/OS system, typically using z/OS cross-memory services or XCF for high-speed data transfer.
- InterSystem Communication (ISC): Enables communication between CICS regions running on *different* z/OS systems, or even with non-CICS systems, primarily utilizing SNA LU6.2 (APPC) or TCP/IP protocols.
- Resource Sharing: Allows transactions, programs, files (VSAM, DB2, IMS), temporary storage queues, and transient data queues to be accessed or initiated remotely across connected regions.
- Synchronous and Asynchronous: Supports both synchronous calls (e.g., EXEC CICS LINK to a remote program, EXEC CICS START with RETRIEVE) and asynchronous calls (e.g., EXEC CICS START without RETRIEVE).
- Configuration: Defined through CICS Resource Definition Online (RDO) using CONNECTION and SESSION definitions to specify the remote system and communication parameters.
- Security: Requires careful security configuration using external security managers like RACF to control which regions and users can access remote resources.

Use Cases

- Workload Balancing: Distributing high-volume transaction processing across multiple CICS regions to improve overall system throughput and response times.
- Resource Isolation: Separating critical applications or sensitive data into dedicated CICS regions while still allowing controlled access from other regions (e.g., a common database region).
- Distributed Applications: Building complex applications where different components or services reside in separate CICS regions, promoting modularity and maintainability.
- Data Sharing: Providing remote access to files (e.g., VSAM KSDS) or database systems (DB2, IMS) managed by a specific CICS region.
- Disaster Recovery/High Availability: Designing resilient architectures where transactions can be routed to alternative CICS regions in case of a failure.

Related Concepts

IRC is integral to the CICS architecture for distributed processing. It is the umbrella term for MRO and ISC, which are its specific implementations. MRO leverages z/OS cross -memory services or XCF, while ISC often relies on SNA's Advanced Program-to-Program Communication (APPC) via LU6.2 or modern TCP/IP connections for inter-system communication. The configuration of IRC connections and sessions is managed through CICS Resource Definition Online (RDO), defining how CICS regions interact with each other and what resources they can share.

Best Practices:

Minimize Cross-Region Hops: Design applications to reduce the number of remote calls and the depth of cross-region invocations to minimize communication overhead and latency.
Optimize Network Configuration: For ISC, ensure efficient network paths, sufficient buffer sizes, and appropriate MAXACTIVE and MAXPRTY settings on SESSION definitions to prevent bottlenecks.
Implement Robust Security: Use an external security manager (e.g., RACF) to define granular access controls for CONNECTION resources and remote transactions, ensuring only authorized regions and users can communicate.
Monitor Performance: Regularly monitor CICS statistics (e.g., DSNCICS for MRO, DSNCICS for ISC) and transaction response times to identify and troubleshoot IRC-related performance issues.
Handle Errors Gracefully: Implement comprehensive error handling for remote calls using RESP and RESP2 codes to manage communication failures and ensure application resilience.
Use SYSID Explicitly: Always specify the SYSID option when issuing CICS commands that target remote resources or transactions (e.g., EXEC CICS LINK PROGRAM(prog) SYSID(sysid)).