Coupled

Enhanced Definition

In the z/OS environment, "coupled" primarily refers to the state where multiple z/OS systems are logically connected and operate together as a single system image within an **IBM Parallel Sysplex**. This coupling is facilitated by a specialized hardware component called the **Coupling Facility (CF)**, enabling high availability, workload balancing, and shared data access across the connected systems.

Key Characteristics

- Shared Resources: Coupled systems within a Sysplex can share access to common resources like data (e.g., DB2 data sharing groups, IMS data sharing groups), queues, and locks, all coordinated via the Coupling Facility.
- Coupling Facility (CF) Dependency: The coupling mechanism heavily relies on one or more Coupling Facilities, which are dedicated hardware processors or LPARs providing high-speed shared memory and specialized services for inter-system communication.
- High Availability: Coupling allows for continuous operation even if one z/OS system fails, as workloads and critical resources can be shifted to other coupled systems without interruption.
- Workload Balancing: The Workload Manager (WLM) can distribute work across coupled systems to optimize resource utilization and meet defined performance goals for various applications.
- Data Integrity: Mechanisms like Global Resource Serialization (GRS) and data sharing protocols ensure transactional integrity and consistency across all coupled systems accessing shared data.
- Single System Image (SSI): From an operational and management perspective, a Sysplex aims to present a single, unified view of the computing environment, simplifying administration and resource management.

Use Cases

- DB2 Data Sharing: Multiple DB2 subsystems running on different z/OS LPARs can concurrently access and update the same DB2 databases, leveraging the CF for lock management and buffer invalidation.
- IMS Data Sharing: Similar to DB2, multiple IMS systems can share access to IMS databases, enhancing availability, scalability, and enabling continuous operations for critical transaction processing.
- CICSplex/IMSplex: Creating application-level plexes where multiple CICS or IMS regions are coupled to share queues, transactions, and resources, providing high availability and scalability for critical business applications.
- Global Resource Serialization (GRS) Ring/Star: Coupling multiple z/OS systems to manage global enqueues, preventing concurrent updates to shared resources across the Sysplex and ensuring data integrity.
- Workload Manager (WLM) Goal Mode: WLM uses the coupling facility to manage and distribute workloads across the Sysplex, ensuring that performance goals are met for critical applications by dynamically adjusting resource allocation.

Related Concepts

The concept of "coupled" is fundamental to the IBM Parallel Sysplex architecture, which allows multiple z/OS systems to function as a single logical entity. It is inextricably linked to the Coupling Facility (CF), the specialized hardware that provides the high-speed communication and shared memory structures necessary for inter-system coordination. Technologies like DB2 Data Sharing, IMS Data Sharing, Global Resource Serialization (GRS), and the Workload Manager (WLM) all leverage this coupling to achieve high availability, scalability, and resource optimization