Event (computing)

In computing, an event is a detectable occurrence or change in state that the system is designed to monitor, such as user input, hardware interrupt, system notification, or change in data or conditions. When associated with an event handler, an event triggers a response. The handler may run synchronously, where the execution thread is blocked until the event handler completes its processing, or asynchronously, where the event may be processed later. Even when synchronous handling appears to block execution, the underlying mechanism in many systems is still asynchronous, managed by the event loop.

Events can be implemented through various mechanisms such as callbacks, message objects, signals, or interrupts, and events themselves are distinct from the implementation mechanisms used. Event propagation models, such as bubbling, capturing, and pub/sub, define how events are distributed and handled within a system. Other key aspects include event loops, event queueing and prioritization, event sourcing, and complex event processing patterns. These mechanisms contribute to the flexibility and scalability of event-driven systems.

Events vs. messages

In distributed systems, events represent a fact or state change (e.g., OrderPlaced) and are typically broadcast asynchronously to multiple consumers, promoting loose coupling and scalability.