Use Case Instance (Scenario)   An use case instance is a specific instance of a use case. It represents the behavior of the subject, describing a particular instance of the interaction between the actors and the use case. A use case instance communicates with one or more instances of actors sending message instances to achieve the value represented by the use case instance. The use case instance never describes "how" the value is achieved, just "what" the value is.

Explanation

While a use case describes all the possible ways an actor and subject can interact, a use case instance describes a single path or scenario of this interaction. The use case describes many possible variants (including errors and exceptional behavior); the use case instance represents just one of these variants. In our daily lives there are often many ways to achieve a particular goal, for example to drive between our house and a friend's place we could take many different roads and often choose a particular route based on the time of day, traffic congestion, road works, accidents and a number of other factors. While a use case describes all of these possibilities focusing its attention on the more common ones, the use case instance (scenario) describes just a single path taken. The use case would represent the map of possible streets whereas a use case instance would be the "pen marks" drawn on the map by a friend tracing out a single path between our house and our friend's place. A given use case would typically have a number of use case instances and modelers will typically focus their attention on a number of the more important or common scenarios. Use case instances are often not made explicit and are represented simply by textual variants in the description of a use case.

Related Entries

Use Case  Base Type,  Graphically Similar,  Confused with An use case is related to a use case instance because it represents the classifier from which the instance is derived. While the use case is a generic description of all possible paths that may occur in providing the value described by the use case, a use case instance (or scenario as it is more commonly called) describes a particular path through the use case. A use case would typically have a number of use case instances.

Actor Instance  Used Together An actor instance is related to a use case instance because the actor instance derives value from the use case instance (scenario). They are related to each other using a link, which is an instance of an association relationship.