Extended Recovery
In the mainframe context, **Extended Recovery** refers to advanced mechanisms and protocols, primarily within subsystems like CICS, IMS, or DB2, that ensure data integrity and consistency across multiple resource managers or distributed systems, even in the event of system failures. It typically involves sophisticated logging, restart, and two-phase commit (2PC) capabilities to coordinate updates and guarantee atomicity for a logical unit of work (LUW).
Key Characteristics
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- Two-Phase Commit (2PC): Often the cornerstone, ensuring that all participants in a distributed transaction either commit their changes permanently or roll them back completely, maintaining data integrity.
- Persistent Logging: Utilizes robust logging mechanisms (e.g., CICS journals, IMS logs, DB2 logs) to record transaction states and data changes, enabling forward recovery or backout during restarts.
- Resource Manager Coordination: Facilitates the coordination of updates across different resource managers (e.g., CICS updating DB2 and VSAM, or IMS updating DB2) within a single logical unit of work.
- Restart and Recovery: Provides the ability to restart failed transactions or subsystems from a consistent point, using log data to either complete pending commits or back out incomplete work.
- Heuristic Decisions: In extreme failure scenarios (e.g., communication loss during phase 2 of 2PC), allows for manual or automated intervention to resolve "in-doubt" transactions, though this is a last resort.
- System-Managed: Largely managed by the z/OS operating system and its subsystems, abstracting much of the complexity from application developers.
Use Cases
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- Distributed Transaction Processing: Ensuring data consistency when a CICS transaction updates both a local DB2 database and a remote IMS database, or a remote CICS region.
- Batch Backout and Restart: Providing the ability to restart a failed batch job from the last commit point, or back out all changes if the job cannot be completed.
- Data Sharing Environments: Maintaining data integrity across multiple DB2 data sharing members or IMS data sharing groups, where multiple systems access and update the same data.
- System Crash Recovery: Automatically recovering subsystems (CICS, IMS, DB2) after an abnormal termination, bringing them back to a consistent state by processing their respective logs.
- Application Program Interface (API) for LUW: Applications can use APIs (e.g.,
EXEC CICS SYNCPOINT,COMMIT WORKin SQL) to define logical units of work, which are then managed by the extended recovery mechanisms.
Related Concepts
Extended Recovery is intrinsically linked to the concept of a Logical Unit of Work (LUW), which defines a set of operations that must either all succeed or all fail together. It relies heavily on logging mechanisms (e.g., CICS journals, IMS logs, DB2 logs) to record changes and transaction states. It is a fundamental component of transaction managers like CICS Transaction Server and IMS Transaction Manager, enabling them to guarantee ACID properties (Atomicity, Consistency, Isolation, Durability) for transactions, especially in complex, multi-resource environments.
- Define Clear LUWs: Design applications with well-defined and appropriately sized logical units of work to minimize the scope of recovery and potential rollback.
- Monitor Transaction Status: Regularly monitor the status of in-doubt transactions, especially in distributed environments, to prevent resource contention or data inconsistencies.
- Implement Robust Logging: Ensure that logging facilities (e.g., log streams, journal files) are adequately sized, properly configured, and frequently backed up to support efficient recovery.
- Test Recovery Procedures: Periodically test system and application recovery procedures, including scenarios involving system crashes and distributed transaction failures, to validate their effectiveness.
- Understand Heuristic Outcomes: Be aware of the implications of heuristic decisions and have clear operational procedures for resolving them, as they can lead to data inconsistencies if not managed carefully.