XES - Cross-System Extended Services

Enhanced Definition

Cross-System Extended Services (XES) is a core component of z/OS that provides services for managing shared data and resources across multiple z/OS systems within an IBM Parallel Sysplex. It enables high-performance, high-availability data sharing and resource coordination by leveraging the Coupling Facility (CF).

Key Characteristics

- Sysplex-Wide Scope: XES operates across all z/OS images participating in a Parallel Sysplex, facilitating coordinated access to shared resources.
- Coupling Facility (CF) Exploitation: It is the primary software interface for applications and subsystems to utilize the hardware capabilities of the Coupling Facility, including its shared memory structures.
- Structure Services: XES provides services for three primary types of CF structures: Cache Structures (for shared buffer pools), List Structures (for shared queues or serialization), and Lock Structures (for distributed locking).
- Data Integrity and Coherency: XES ensures data consistency and integrity for shared resources across the sysplex, critical for transactional workloads.
- Application Programming Interface (API): Applications interact with XES through a set of macros (e.g., IXLCACHE, IXLLIST, IXLLOCK) to allocate, connect to, and manipulate CF structures.
- High Availability and Scalability: By centralizing shared resource management, XES contributes significantly to the scalability and availability characteristics of a Parallel Sysplex.

Use Cases

- DB2 Data Sharing: Essential for DB2 data sharing groups, where XES cache structures maintain buffer pool coherency and lock structures manage page/row locking across multiple DB2 members.
- IMS Data Sharing: Used by IMS for shared queues, shared Virtual Storage Option (VSO) buffers, and database locking to enable multiple IMS systems to access the same databases.
- Global Resource Serialization (GRS) Star Mode: GRS in star mode utilizes an XES list structure in the Coupling Facility to manage global enqueues, providing sysplex-wide serialization without ring-mode overhead.
- CICSplex Data Sharing: CICS regions in a CICSplex can use XES for shared temporary storage, shared named counters, and other shared resources.
- User-Written Applications: Custom applications requiring high-performance shared data and locking mechanisms across multiple z/OS systems can directly leverage the XES API.

Related Concepts

XES is a foundational technology for the Parallel Sysplex, acting as the software layer that enables multiple z/OS systems to function as a single system image for shared data. It is inextricably linked to the Coupling Facility (CF), as XES provides the application programming interface for exploiting the CF's hardware capabilities. Subsystems like DB2 Data Sharing, IMS Data Sharing, and Global Resource Serialization (GRS) in star mode are major consumers of XES services, relying on it for their core data sharing and serialization functions.

Best Practices:

Optimal CF Structure Sizing: Accurately size CF structures (cache, list, lock) using tools like CFSIZER to prevent performance degradation due to structure full conditions or excessive contention.
Implement CF Structure Duplexing: For critical structures, enable CF structure duplexing to provide continuous availability and disaster recovery capabilities in the event of a CF failure.
Proactive Monitoring: Continuously monitor XES activity, CF structure usage, and contention using RMF, SMF, and product-specific monitors to identify and address performance bottlenecks.
Exploit Latest XES Enhancements: Keep z/OS and XES components current to leverage performance improvements, new features, and increased resilience provided by IBM.
Understand Application Interaction: Developers and system programmers should have a clear understanding of how their applications or subsystems interact with XES and CF structures to optimize their design and troubleshoot issues effectively.