CISH - CICS Shared

Enhanced Definition

CISH, or CICS Shared, refers to a mechanism within CICS (Customer Information Control System) that allows multiple CICS regions to share access to certain resources, primarily **VSAM (Virtual Storage Access Method) files**, in a controlled and synchronized manner. It is a critical component for achieving data integrity and consistency when multiple CICS applications need concurrent read/write access to the same datasets across different CICS regions. CISH (CICS Shared) refers to an internal mechanism within CICS (Customer Information Control System) that facilitates the sharing of specific data or control blocks between different CICS regions, typically residing within the same z/OS image. It is a form of inter-region communication primarily used for internal CICS control and coordination rather than general application data sharing.

Key Characteristics

- Data Integrity: Ensures that concurrent updates from multiple CICS regions to the same shared VSAM file are properly serialized, preventing data corruption and maintaining transactional consistency.
- Resource Sharing: Facilitates the sharing of VSAM KSDS (Key-Sequenced Data Set), ESDS (Entry-Sequenced Data Set), and RRDS (Relative Record Data Set) files across multiple CICS regions, often within a CICSplex environment.
- VSAM RLS Integration: CISH works in conjunction with VSAM Record-Level Sharing (RLS), which provides the underlying serialization and locking mechanisms for shared VSAM files. CISH acts as the CICS interface to RLS.
- Recovery and Restart: Supports robust recovery mechanisms for shared files, ensuring data consistency and recoverability even after CICS region failures or system outages.
- Performance Considerations: While enabling sharing, proper design, monitoring, and tuning are crucial to manage potential contention and optimize I/O performance for shared resources.
- Configuration: Requires specific definitions in the CICS System Initialization Table (SIT), File Control Table (FCT), and the VSAM dataset definitions (e.g., SHAREOPTIONS(3 3) and RLS(ENABLED)).

Use Cases

- Inter-Region Communication: Multiple CICS regions processing different parts of a complex business transaction (e.g., order entry, inventory update, customer profile management) that all need to access and update a common customer or product database.
- Load Balancing: Distributing transaction workload across several CICS regions, all of which need to access the same critical business data files, improving overall system throughput and responsiveness.
- High Availability: Providing continuous access to shared data even if one CICS region fails, as other regions can continue processing using the shared files, contributing to business continuity.
- Application Modernization: Allowing new CICS applications in separate regions to coexist and share data with legacy applications, facilitating phased migrations and integration strategies.
- Data Warehousing/Reporting: Enabling read-only access to operational data from multiple CICS regions for reporting or data extraction purposes without impacting transactional performance.

Related Concepts

CISH is intrinsically linked to CICS itself, providing a fundamental capability for enterprise-level transaction processing and data management. It relies heavily on VSAM RLS (Record-Level Sharing), which is a z/OS component responsible for managing shared access to VSAM datasets at the record level, including locking and recovery. CISH enables CICS regions within a CICSplex to leverage RLS effectively, ensuring data consistency across the entire complex. It also interacts with DFSMS (Data Facility Storage Management Subsystem) for dataset management, storage allocation, and data integrity features.

Best Practices:

Proper RLS Configuration: Ensure VSAM RLS is correctly configured and enabled for the shared datasets, including appropriate SHAREOPTIONS and RLS attributes in the DEFINE CLUSTER JCL.
Monitor Contention: Regularly monitor for file and record contention using CICS and RLS statistics (e.g., SMF records, CICS Explorer) to identify bottlenecks and optimize application design or file organization.
Transaction Design: Design CICS transactions to minimize the duration of record locks on shared files to improve concurrency and throughput. Avoid long-running transactions that hold locks unnecessarily.
Backup and Recovery Strategy: Implement a comprehensive backup and recovery strategy that accounts for shared VSAM files, potentially using DFSMSdss or other z/OS utilities for consistent backups.
Performance Tuning: Tune CICS buffer pools (VSAM LSR/GSR), dataset placement (e.g., on faster storage), and CICS region parameters to optimize I/O performance for shared files.
Security: Implement appropriate RACF or equivalent security profiles to control access to shared VSAM files from different CICS regions and user IDs, adhering to the principle of least privilege.