Definition
An event is a significant occurrence or change in a system's state that is broadcast or communicated to other parts of the system or to external observers. These occurrences are typically immutable records of something that has happened, such as a user login, a data update, or a sensor reading. Events are fundamental to event-driven architectures, where components react to these notifications rather than relying on direct requests. Understanding events is crucial for building responsive, scalable, and decoupled systems, enabling real-time processing and complex workflow automation. For example, an e-commerce platform might emit an 'OrderPlaced' event that triggers inventory updates, payment processing, and shipping notifications. While often confused with messages, events represent past facts, whereas messages are commands or requests intended to cause an action. This concept is vital for domains like system design, cloud architecture, and security monitoring, appearing in certifications such as AWS Certified Solutions Architect and CISSP.