CMRR - Couple Member and Recovery Routine

Enhanced Definition

CMRR, or Couple Member and Recovery Routine, is a critical component within a z/OS sysplex responsible for managing the recovery of a specific Coupling Facility (CF) structure when a system connected to that structure fails or loses its connectivity. It is an exit routine provided by a structure owner (e.g., DB2, IMS, CICS, or a user application) to handle the cleanup and recovery of resources associated with the failed system within the CF structure. Its primary purpose is to ensure data integrity and continued availability of shared resources across the sysplex. CMRR (Couple Member and Recovery Routine) is a critical internal routine within the z/OS Cross-System Coupling Facility (XCF) responsible for managing connections to coupling facility (CF) structures. It orchestrates recovery actions when a system loses its connection to a CF structure or when a CF structure becomes unavailable, ensuring the integrity and availability of shared data and resources across a Parallel Sysplex.

Key Characteristics

- Structure-Specific: Each CF structure can have a unique CMRR registered by its owner, tailored to the specific data and resource management needs of that structure.
- Event-Driven Invocation: The CMRR is invoked by z/OS's XCF (Cross-System Coupling Facility) component when a system that was a member of a CF structure fails, leaves the sysplex, or loses its connection to the CF.
- Resource Cleanup: Its core function is to identify and release any locks, entries, or control blocks held by the failed system within the CF structure, preventing deadlocks or inconsistencies in shared data.
- Data Integrity Assurance: By ensuring proper cleanup and resource release, CMRR plays a vital role in maintaining the integrity and consistency of shared data managed within the CF structure across all active sysplex members.
- High Availability Enabler: It significantly contributes to the high availability of sysplex applications by allowing other active systems to continue processing without being stalled by resources held by a failed system.
- Programmatic Implementation: CMRRs are typically implemented as assembler routines and registered with XCF using specific macros like IXCJOIN or IXCQUERY by the structure owner.

Use Cases

- System Failure in a Data Sharing Group: When a z/OS system running a DB2 or IMS data sharing member fails, its associated CMRR for the DB2 group buffer pool or IMS lock structures is invoked to release locks and clean up entries held by the failed member.
- Loss of Connectivity to a CF: If a system loses its path to the Coupling Facility where a critical structure resides, the CMRR for that structure will be invoked to manage the system's disconnection and potential recovery actions.
- Sysplex Partitioning: In a scenario where the sysplex partitions, and a system is isolated, CMRRs for structures it was connected to will be activated on the remaining active systems to manage its departure and resource cleanup.
- CICSplex Recovery: For CICSplex environments utilizing CF list structures for shared queues or global resource serialization, CMRRs ensure that resources held by a failed CICS region are released, allowing other regions to continue processing.

Related Concepts

CMRR is intrinsically linked to the Coupling Facility (CF) and Coupling Facility Structures (Cache, List, Lock), as it operates directly on behalf of structure owners to manage resources within these shared memory areas. It is managed and invoked by XCF (Cross-System Coupling Facility), which monitors sysplex member status and CF connectivity. CMRRs are fundamental to the robust operation of data sharing environments like DB2 Data Sharing and IMS Data Sharing, where multiple z/OS systems access and update shared data via CF structures. It's a key mechanism that enables sysplex-wide data integrity and high availability by facilitating the recovery of shared resources after a system