Expectation

Anticipating Occurrence

Enhanced Definition

In the mainframe context, "expectation" refers to the act of a program, system component, or operator waiting for a specific event, condition, or input to materialize before proceeding. This concept is fundamental to managing dependencies, ensuring proper sequencing, and reacting to dynamic changes within the z/OS environment.

Key Characteristics

- Event-Driven: Often triggered by specific system events, such as I/O completion, message arrival, timer expiration, or resource availability.
- Synchronous or Asynchronous: Can involve a blocking wait where processing halts until the event occurs (synchronous), or a non-blocking check that allows other tasks to proceed while monitoring for the event (asynchronous).
- Condition-Based: Expectations are frequently tied to specific conditions, like a dataset existing, a return code being zero, or a particular message appearing in the system log.
- Timeout Mechanisms: Robust systems often incorporate timeouts to prevent indefinite waits, allowing for error handling or alternative processing if the expected occurrence does not happen within a defined period.
- Resource Management: Crucial for managing shared resources, where a task might expect a resource to become available before it can acquire and use it.

Use Cases

- Batch Job Sequencing: A subsequent JCL step might EXPECT a specific return code (e.g., COND=(0,LT)) from a preceding step to determine whether it should execute, or EXPECT a dataset created by a prior step to be available.
- Online Transaction Processing (CICS/IMS): A CICS transaction EXPECTS user input from a terminal via a RECEIVE command, or an IMS transaction EXPECTS a message to arrive in its message queue for processing.
- System Automation: An automation script (e.g., REXX, SA z/OS policy) EXPECTS a specific message ID or text pattern to appear on the console log before initiating a corrective action or notifying an operator.
- Inter-program Communication: A COBOL program might EXPECT a response from a called subprogram or an external service (e.g., DB2, MQ) after issuing a request.
- Resource Contention: A program EXPECTS a specific enqueue (ENQ) or a database lock to be released by another task before it can acquire it and proceed with its critical section.

Related Concepts

This concept is deeply intertwined with JCL COND parameters for step execution control, CICS WAIT commands for synchronous I/O and event handling, and IMS message queues for asynchronous transaction processing. It is a core principle in system automation tools like SA z/OS and NetView, which constantly monitor and react to expected system events. Furthermore, it relates to resource management (e.g., GRS, ENQ/DEQ) where tasks await resource availability, and error handling where programs anticipate and prepare for specific failure conditions.

Best Practices:

Explicitly Define Expectations: Clearly specify the exact conditions, events, or inputs that are being awaited to avoid ambiguity and ensure correct system behavior.
Implement Robust Timeouts: Always include timeout mechanisms for any operation involving an expectation to prevent indefinite waits and ensure system resilience and responsiveness.
Design for Unexpected Outcomes: Develop comprehensive error handling and fallback logic for scenarios where the anticipated occurrence does not happen or an unexpected event intervenes.
Minimize Busy-Waiting: Where possible, utilize event -driven programming models (e.g., CICS WAIT, IMS message queues) rather than continuous polling to conserve CPU cycles and improve efficiency.
Document Dependencies: Thoroughly document all inter-job, inter-