CLAIM
In the context of mainframe and z/OS, a "claim" refers to the act of asserting control or requesting exclusive access over a shared system resource, such as a data set, a record within a database, a memory area, or a hardware device. Its primary purpose is to ensure data integrity, prevent concurrent update anomalies, and manage resource contention in a multi-user, multi-tasking environment.
Key Characteristics
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- Exclusivity: Often implies that once a resource is claimed, other tasks or programs are prevented from modifying or sometimes even reading it until the claim is released.
- Resource Specificity: Claims can be made at various levels of granularity, from an entire data set or database to a single record, block, or control block.
- Serialization Mechanism: Implemented through various serialization techniques, including operating system enqueues (
ENQ/DEQ), latches, locks (e.g., DB2 locks, IMS locks), and semaphores. - Contention Management: When multiple tasks attempt to claim the same resource simultaneously, the system manages contention, typically by making subsequent requesters wait.
- Temporary Duration: Claims are generally temporary, held only for the duration of the operation that requires exclusive access and released promptly thereafter.
- Scope: Can be local to an address space, system-wide within a single z/OS LPAR, or global across multiple z/OS systems in a Sysplex via Global Resource Serialization (GRS).
Use Cases
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- Database Updates: A COBOL application updating a specific row in a DB2 table will implicitly or explicitly claim (lock) that row to prevent other transactions from modifying it concurrently, ensuring data consistency.
- File Processing: A batch job processing a VSAM KSDS for updates might claim exclusive access to the entire data set or specific control intervals to prevent other jobs from interfering with its write operations.
- Critical Section Protection: An assembler routine or a C program might use
ENQmacros to claim a global resource name, thereby serializing access to a critical in-memory control block across multiple tasks or even multiple systems. - Transaction Processing: A CICS transaction processing a customer order might claim specific records in an IMS database or a VSAM file while it performs updates, releasing them upon transaction commit or rollback.
Related Concepts
The concept of claiming is fundamental to serialization in z/OS, which ensures that critical resources are accessed by only one task at a time or in a controlled manner. It is directly implemented by locks and enqueues (ENQ/DEQ), which are the mechanisms used to assert and manage ownership. Improper or prolonged claims can lead to resource contention and deadlocks, where two or more tasks are perpetually waiting for resources held by each other. Effective claiming is crucial for maintaining data integrity and system stability in high-volume transactional environments like CICS and IMS, often facilitated by Global Resource Serialization (GRS) in a Sysplex.
- Minimize Claim Duration: Hold claims for the shortest possible time required to complete the critical operation to reduce contention and improve overall system throughput.
- Granular Claiming: Claim resources at the most granular level necessary (e.g., a specific record instead of an entire file) to allow concurrent access to other parts of the resource.
- Consistent Claim Order: When claiming multiple resources, acquire them in a predefined, consistent order across all applications to help prevent deadlocks.
- Robust Error Handling: Implement logic to handle situations where a claim cannot be immediately granted (e.g., resource unavailable, timeout), allowing the application to retry, wait, or gracefully terminate.
- Monitor and Tune: Regularly monitor resource contention using tools like RMF, SMF, DB2 PM, or CICS monitoring tools to identify bottlenecks caused by excessive or prolonged claims and tune applications or system parameters accordingly.