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XCF - Cross-System Coupling Facility

Enhanced Definition

XCF (Cross-System Coupling Facility) is a z/OS component that provides services for authorized programs to communicate and share data between multiple z/OS systems within a `sysplex`. It enables systems to cooperate as a single image, facilitating high availability, workload balancing, and data sharing across the enterprise. XCF is fundamental to the operation of a `Parallel Sysplex`.

Key Characteristics

- Sysplex Communication: Provides a robust, high-performance communication infrastructure for z/OS systems within a sysplex, allowing them to exchange messages and synchronize activities.
- Group Services: Applications register as XCF members within XCF groups. XCF manages membership, delivers messages to all members of a group, and notifies members of changes in group status (e.g., a system joining or leaving).
- Coupling Facility (CF) Integration: Leverages the Coupling Facility (CF) hardware to provide shared data structures (e.g., lists, caches, locks) that can be accessed by all systems in the sysplex for high-speed data sharing and serialization.
- Signaling Services: Supports direct system-to-system communication via XCF signalling paths (either CF links or CTC links), allowing for message delivery without necessarily using CF structures.
- Reliability and Recovery: Designed for high availability, XCF automatically detects system failures, removes failed systems from XCF groups, and notifies remaining members, enabling applications to recover gracefully.
- Scalability: Allows for the dynamic addition or removal of z/OS systems from a sysplex without disrupting ongoing operations, supporting growth and flexibility.

Use Cases

- DB2 Data Sharing: Enables multiple DB2 subsystems on different z/OS systems to access and update the same set of databases concurrently, using CF lock structures and cache structures for data integrity and performance.
- CICSplex Transaction Routing: Facilitates CICSplex environments where transactions can be routed to any available CICS region across the sysplex, leveraging XCF for inter-region communication and workload balancing.
- IMS Data Sharing: Allows multiple IMS subsystems to share IMS databases and message queues, using XCF services for resource serialization and communication.
- Global Resource Serialization (GRS): In a GRSplex, XCF is used by GRS to manage and serialize access to global resources (e.g., datasets, enqueues) across all systems, preventing conflicts.
- JES2 Multi-Access Spool (MAS): Enables multiple JES2 subsystems to share a common spool, job queue, and output queue, with XCF providing the communication backbone for synchronization.

Related Concepts

XCF is the bedrock of the Parallel Sysplex architecture, providing the essential communication and synchronization services that allow multiple z/OS systems to function as a single, highly available computing environment. It relies heavily on the Coupling Facility (CF) for shared memory structures and high-speed inter-system communication. Applications like DB2 Data Sharing, CICSplex, and IMS Data Sharing are built upon XCF's capabilities to achieve their data sharing and workload balancing goals. XCF also integrates with WLM (Workload Manager) to ensure that work is distributed efficiently across the sysplex based on defined policies.

Best Practices:

Redundant CFs and Links: Always configure at least two Coupling Facilities and multiple CF links to each z/OS system to ensure high availability and prevent a single point of failure.
Proper CF Structure Sizing: Accurately size CF structures (e.g., DB2 group buffer pools, lock structures) based on application requirements to avoid performance degradation or outages due to structure full conditions.
Monitor XCF Activity: Regularly monitor XCF group activity, CF usage, and XCF signalling path performance using tools like RMF or SMF to identify potential bottlenecks or issues.
Optimize XCF Signalling Paths: Ensure that XCF signalling paths are configured efficiently, prioritizing CF links for high-volume communication and using CTC links as backups or for specific scenarios.
Application Design for XCF: When developing or configuring applications that use XCF, design them to handle XCF group membership changes and system failures gracefully, leveraging XCF's notification services for recovery.