ISC - Inter-System Communication

Enhanced Definition

Inter-System Communication (ISC) is a core CICS facility that enables applications in one CICS region to communicate with and access resources or invoke transactions in another CICS region, or even with an IMS system. It facilitates distributed transaction processing and resource sharing across different z/OS images or LPARs.

Key Characteristics

- Protocol Support: Primarily utilizes SNA (Systems Network Architecture) LU6.1 or, more commonly in modern environments, LU6.2 protocols for communication between systems.
- Synchronous and Asynchronous: Supports both synchronous communication (e.g., EXEC CICS LINK or START with RETRIEVE) where the calling program waits for a response, and asynchronous communication (e.g., EXEC CICS START without RETRIEVE) for fire-and-forget scenarios.
- Resource Sharing: Allows CICS regions to share access to resources such as files (e.g., VSAM), temporary storage queues, transient data queues, and program execution.
- Configuration: Requires explicit definition of CONNECTION and SESSION resources in CICS RDO (Resource Definition Online) for each participating region to establish communication paths.
- Transparency: Aims to provide a degree of location transparency, allowing application developers to invoke remote resources using similar commands as local ones, with CICS handling the underlying communication details.
- Security Integration: Integrates with z/OS security managers like RACF to control authorization for remote access to transactions and resources.

Use Cases

- Distributed Transaction Processing: A CICS application in one region needs to update a database or perform a business logic function managed by a transaction in a different, specialized CICS region.
- Resource Offloading/Specialization: A CICS region dedicated to online user interaction offloads data access (e.g., VSAM file I/O) to another CICS region optimized for data management.
- CICS-IMS Interoperability: A CICS transaction needs to access an IMS database or invoke an IMS transaction, leveraging ISC for the communication link.
- Workload Balancing: Distributing transaction processing across multiple CICS regions on different z/OS images to balance workload, improve scalability, and enhance availability.
- Application Modularity: Breaking down large, monolithic applications into smaller, interconnected CICS regions, each responsible for specific business domains or services.

Related Concepts

ISC is a broader concept than MRO (Multi-Region Operation); while MRO is a form of ISC used for communication within the same z/OS image via cross -memory services, ISC typically refers to communication across different z/OS images using SNA. It relies on VTAM (Virtual Telecommunications Access Method) for the underlying network services to establish and manage the SNA connections. ISC enables CICS applications to interact with other CICS regions or IMS systems, forming part of a larger distributed enterprise computing environment.

Best Practices:

Define Connections Precisely: Carefully configure CONNECTION and SESSION definitions, including parameters like RECEIVECOUNT, SENDCOUNT, and MAXACTIVE, to match expected workload and optimize performance.
Implement Robust Error Handling: Design applications to gracefully handle potential communication failures, network issues, or remote system unavailability using RESP and RESP2 codes on EXEC CICS commands.
Enforce Strong Security: Utilize RACF or equivalent security profiles to control which users or CICS regions are authorized to access specific remote transactions or resources.
Monitor and Tune Performance: Regularly monitor ISC traffic, session utilization, and response times. Tune SESSIONS and CONNECTION parameters as needed, and optimize application logic to minimize unnecessary inter-region calls.
Ensure Transactional Integrity: For distributed transactions, especially with LU6.2, ensure correct implementation of SYNCPOINT processing to guarantee two-phase commit and maintain data integrity across all participating systems.