Cross System

Enhanced Definition

In the context of IBM mainframes and z/OS, "cross system" refers to operations, data sharing, or resource management that spans multiple independent z/OS images (Logical Partitions or LPARs) or even multiple physical mainframe systems. It involves the coordinated interaction and communication between these distinct systems to achieve a common goal, often related to high availability, workload balancing, or shared data access.

Key Characteristics

- Inter-LPAR Communication: Requires robust communication mechanisms, primarily facilitated by the Cross-System Coupling Facility (XCF) component of z/OS and the Coupling Facility (CF) hardware.
- Shared Resources: Often involves sharing critical resources like data (e.g., DB2 or IMS databases), enqueue locks, or system services across multiple z/OS instances.
- Data Integrity: Maintaining data consistency and integrity across systems is paramount, typically managed by components like Global Resource Serialization (GRS) or database-specific locking mechanisms.
- Complexity: Introduces additional complexity in configuration, management, monitoring, and problem determination compared to single-system operations.
- Scalability and Availability: Designed to enhance system scalability by distributing workloads and improve availability by providing failover capabilities across systems.

Use Cases

- Sysplex Data Sharing: Enabling multiple z/OS images within a Sysplex to concurrently access and update the same DB2 or IMS databases, improving performance and availability.
- Workload Balancing: Distributing application workloads (e.g., CICS transactions, batch jobs) across several LPARs to optimize resource utilization and response times.
- High Availability and Disaster Recovery: Providing continuous operation by allowing workloads to fail over to another z/OS image in the event of a system failure, or replicating data to a remote system for disaster recovery.
- Global Resource Serialization: Using GRS to manage shared resources (e.g., datasets, enqueues) across multiple systems, preventing conflicts and ensuring data integrity.
- Cross-System Automation: Implementing automation scripts or tools that manage and monitor processes running across different z/OS LPARs from a central point.

Related Concepts

The concept of "cross system" is foundational to Sysplex technology, where multiple z/OS images are tightly coupled to function as a single system image. It heavily relies on the Coupling Facility (CF) for high-speed shared memory and XCF for inter-system communication. Global Resource Serialization (GRS) is a critical component for managing shared resources across these systems, preventing data corruption. Applications like DB2 Data Sharing and IMS Data Sharing are prime examples of leveraging cross-system capabilities to provide high availability and scalability for enterprise data.

Best Practices:

Robust Sysplex Design: Design your Sysplex and Coupling Facility structures carefully, considering redundancy, performance, and future growth to support cross-system operations effectively.
Consistent Configuration: Ensure consistent IPL parameters, PARMLIB members, and security definitions across all participating z/OS images to avoid operational discrepancies.
Comprehensive Monitoring: Implement robust monitoring for XCF activity, Coupling Facility utilization, GRS contention, and cross-system application performance to proactively identify and resolve issues.
Thorough Testing: Regularly test cross-system failover, recovery, and data integrity scenarios, especially after system changes, to ensure resilience and proper functionality.
Security Considerations: Implement stringent security controls for cross-system communication and resource access, utilizing features like RACF (or equivalent ESM) to define appropriate access rules between LPARs.