Backout
In the mainframe context, **backout** refers to the process of reversing all uncommitted changes made by a transaction or program unit of work to restore data to its state prior to the transaction's initiation. This ensures data integrity and consistency, especially when a transaction fails or is explicitly canceled before reaching a commit point.
Key Characteristics
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- Atomicity Enforcement: Guarantees that a transaction is treated as a single, indivisible operation; either all its changes are applied (
commit) or none are (backout). - Data Integrity: Prevents partially completed updates from corrupting databases or files by restoring them to a consistent state.
- Log-Based Operation: Relies heavily on transaction logs (e.g., DB2 logs, CICS journals, IMS logs) to record before-images or undo information, which is then used to reverse the changes.
- Automatic or Manual: Can be triggered automatically by the system (e.g., due to an
abend, deadlock) or explicitly by an application program or operator command. - Scope of Reversal: Reverses changes made to recoverable resources such as DB2 tables, IMS databases, CICS files (VSAM RLS), and other resources managed by a transaction manager.
- Concurrency Management: Plays a crucial role in resolving deadlocks, where one transaction might be chosen to back out to allow others to proceed.
- Atomicity Enforcement: Guarantees that a transaction is treated as a single, indivisible operation; either all its changes are applied (
Use Cases
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- Abended Transaction: When a COBOL program
abends (abnormally ends) during a CICS transaction or DB2 update, the transaction manager automatically initiates a backout to undo any partial database changes. - Application Error Handling: An application program might explicitly request a backout (e.g., using
ROLLBACKin SQL,SYNCPOINT ROLLBACKin CICS) if it detects an error condition that makes further processing invalid. - System Failure: If the z/OS system or a subsystem (like DB2 or CICS) fails before a transaction commits, recovery procedures will automatically back out any in-flight, uncommitted transactions upon restart.
- Deadlock Resolution: In a multi-user environment, if two transactions are in a deadlock, the database manager (e.g., DB2) might choose one transaction as the "victim" and automatically back out its changes to resolve the contention.
- User Cancellation: A user might cancel a long-running transaction from their terminal, prompting the application or transaction manager to back out any changes made up to that point.
- Abended Transaction: When a COBOL program
Related Concepts
Backout is the inverse operation of commit, which makes all changes permanent. Both are fundamental components of transaction management and recovery in systems like CICS, IMS, and DB2, ensuring data integrity and atomicity. It relies on logging/journaling mechanisms, where before-images of data are recorded to facilitate the reversal of changes. Without robust logging, backout would not be possible. It's a critical part of the ACID properties (Atomicity, Consistency, Isolation, Durability) that characterize reliable transaction processing.