Event Driven

Enhanced Definition

In the context of IBM mainframe systems and z/OS, "event -driven" describes a system or program design paradigm where the flow of execution is determined by occurrences (events) rather than a predefined, sequential control flow. These events can include user interactions, system messages, timer expirations, or data arrival, prompting specific actions or processing.

Key Characteristics

- Asynchronous Processing: Events often occur asynchronously, meaning they are not synchronized with the main program's sequential flow, requiring mechanisms to handle them without blocking.
- Event Loop/Dispatcher: Many event-driven systems utilize an event loop or dispatcher that continuously monitors for events, queues them, and dispatches them to appropriate handlers.
- Event Handlers: Specific routines or modules are designed to respond to particular types of events, encapsulating the logic for processing that event.
- Responsiveness: Essential for interactive mainframe applications (e.g., CICS transactions, TSO/ISPF dialogs) to remain responsive to user input or external system triggers.
- Loose Coupling: Event-driven architectures promote loose coupling between components, as components interact by producing and consuming events rather than direct method calls.
- State Management: Often requires careful state management, as event handlers might need to access or update shared application state based on the sequence of events.

Use Cases

- CICS Transaction Processing: CICS applications are inherently event-driven, responding to terminal input (e.g., pressing Enter, PF keys), timer events, or external requests to process transactions.
- TSO/ISPF Dialogs: Interactive ISPF applications respond to user selections from menus, input in data entry panels, or command line entries to navigate and perform functions.
- System Automation: Automation tools like SA z/OS use event-driven principles to monitor system messages, resource status changes, or time-based events and trigger automated actions or recovery procedures.
- Message Queue Processing: Applications that process messages from IBM MQ queues are event-driven, being triggered or waking up when new messages arrive for processing.
- Real-time Monitoring: Performance monitors and security monitors operate in an event-driven manner, reacting to specific system events, thresholds being crossed, or security violations.

Related Concepts

Event -driven programming is foundational to interactive mainframe environments like CICS and TSO/ISPF, where user actions are the primary events that drive application flow. It contrasts sharply with traditional batch processing, which follows a predefined, sequential job stream. It is closely related to asynchronous processing and message queuing (IBM MQ), enabling applications to react to external triggers without blocking, and is a core principle behind system automation tools that monitor and respond to system events and conditions.

Best Practices:

Clear Event Definitions: Define events precisely, including their triggers, data payload, and expected outcomes, to ensure consistent and predictable handling.
Idempotent Event Handlers: Design event handlers to be idempotent where possible, meaning processing the same event multiple times has the same effect as processing it once, for robustness and recovery.
Robust Error Handling and Logging: Implement comprehensive error handling for unexpected events or failures during event processing, and log all significant events for auditing and debugging.
Performance Optimization: Optimize event dispatching and handler execution to avoid bottlenecks, especially in high-volume transaction processing environments like CICS.
Security Context: Ensure event handlers operate within appropriate security contexts and validate event data to prevent unauthorized actions or data corruption.