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IER - Invalid ENQ Request

Enhanced Definition

An IER (Invalid ENQ Request) is an error condition in z/OS that occurs when a program attempts to acquire or release a serially reusable resource using the `ENQ` or `DEQ` macro, but the request parameters or the state of the resource make the operation invalid. This typically indicates an issue with the resource name, its scope, or an attempt to manipulate a resource that is not currently held or is in an unexpected state.

Key Characteristics

- Error Type: It's a system-level error indicating a failure in resource serialization logic within an application or system component, often leading to program termination.
- Causes: Can be triggered by incorrect resource names, attempting to DEQ a resource not currently held by the requesting task, requesting an ENQ with an invalid scope, or attempting to ENQ a resource exclusively that is already held exclusively by the same task.
- Manifestation: Often results in a program abend (e.g., S0C4 if the ENQ parameter list is corrupted, or specific ENQ-related abends if the system detects the invalid request directly) or an error return code from the ENQ/DEQ macro, potentially accompanied by system messages (e.g., IEC130I for dataset ENQ issues, or messages related to GRS).
- Impact: Leads to job or task termination, preventing the successful completion of work that requires the serial resource, and can indicate a flaw in application design or logic.
- Underlying Mechanism: Directly relates to the ENQ (Enqueue) and DEQ (Dequeue) macros, which are fundamental z/OS services used for serialization of serially reusable resources across tasks, address spaces, or even sysplexes.

Use Cases

- Application Logic Error: A COBOL program attempts to DEQ a resource that it never successfully ENQued, or tries to ENQ a resource with a misspelled or incorrect resource name, leading to an IER.
- Incorrect Parameter List: An assembler program passes an incorrectly formatted or addressed parameter list to the ENQ or DEQ macro, causing the system to interpret the request as invalid.
- Scope Mismatch: A batch job attempts to ENQ a resource with a SYSTEM scope when it previously held the same resource with a SYSTEMS (sysplex-wide) scope, or vice-versa, causing an internal conflict and an IER.
- Resource State Violation: A program tries to ENQ a resource exclusively that it already holds shared, without first DEQing the shared request, or attempts to DEQ a resource that is not currently held by the requesting task.

Related Concepts

IERs are intrinsically linked to serialization in z/OS, which is critical for maintaining data integrity and preventing concurrent updates to shared resources. They are a direct consequence of incorrect usage of the ENQ and DEQ macros, the primary mechanism for serialization. In a sysplex environment, IERs