concept event in category software development
This is an excerpt from Manning's book Event Processing in Action.
An event is an occurrence within a particular system or domain; it is something that has happened, or is contemplated as having happened in that domain. The word event is also used to mean a programming entity that represents such an occurrence in a computing system.
We give two meanings to the word event. The first meaning refers to an actual occurrence (the something that has happened) in the real world or in some other system. The second meaning takes us into the realm of computerized event processing, where the word event is used to mean a programming entity that represents this occurrence. It’s easy to get caught up in a rather pedantic discussion about the difference between these two, but in practice we can safely use the word event to have either meaning as it’s usually easy to tell the meaning from the context. You should note that a single event occurrence can be represented by many event entities, and also be aware that a given event entity might capture only some of the facets of a particular event occurrence.
As this is such an important term, it’s worth commenting on three of the phrases used in the definition. The first of these is “system or domain.” In event processing we are chiefly concerned with real-world events–that is, events that occur in real life, such as an order being placed or a plane landing. But the techniques of event processing can also be applied to things that happen in artificial domains such as training simulators, virtual worlds, and similar virtual environments.
Next, the definition includes things that are “contemplated as having happened.” It is possible to have events that don’t correspond to actual occurrences. To explain what we mean, imagine a fraud detection system being used in a financial institution. Such a system monitors financial transactions and generates events when it suspects that a fraud is being conducted. These systems can generate false positives, so further investigation is usually required before you can be sure whether a fraud has actually taken place or not.
Finally, the definition contains the phrase “programming entity.” Elsewhere in this book we use the phrase event object, as this sounds more natural than event programming entity, but we need to make it clear that we aren’t necessarily talking about an object as defined in object-oriented programming. In some contexts you might encounter events represented as OO objects, but you can also find events that appear in other forms, such as records in a relational database, structures in a language like C or COBOL, or messages transmitted between systems. We have therefore chosen to use the more general expression programming entity in our definition.
The word event is sometimes used in event processing literature to refer to a type or class of events rather than to a specific event instance. In this book we either use the term event type in such cases, or we use the name of the type as an adjective, for example, Account Overdrawn event, unless it is obvious from the context that we mean the type rather than a specific instance.