什么是用例场景？

Question

So, I'm learning various aspects of design and architecture, such as UML and use cases. But I'm not getting what a use case scenario is and I can't find much about it. Can anyone explain them to me?

Answer 1

A use case describes some added value a system under consideration will have. To achieve this added value there are usually a multitude of steps to be performed. A use case includes these steps in Activities. There is at least one sunny side activity which details the steps to be performed. That's also called a scenario. You will additionally have quite some scenarios (/Activities) that describe failure and exception paths or alternate ways to achieve the added value. The Activity itself contains all the Actions needed along with the controlled path they will have to take.

tl;dr

A scenario is another term for a UML Activity.

As always I recommend to read Bittner/Spence about Use Case Modeling. It's a complex thing and especially IT people have difficulties in grasping what it's all about.

Answer 2

To complement qwerty_so 's vey clear and insightful answer: The scenario isn't necessarily described as an activity diagram. Very often, the scenarios are first identified and described in plaintext, before being formalized in an activity diagram:

• A very popular format is Alistair Cockburn's template (see here with some examples). He suggests to start with a main success scenario, then provide alternative scenarios (for slight variations, extensions, as well as the unhappy case such as errors, exceptions, failures). By the way, his book "Writing effective use-cases" is a must-read in the field, even if it's more about use-cases than about UML.

• There are many other templates used and sometimes scenarios are called "flow" or "path". They can be presented in a tabular form or in a chronologicaly numbered sequence . Sometimes the tabular description has a column for the actor actions and a column for the system action.

Answer 3

An entire use case can be represented by one activity definition, comprising elements such as decision nodes and activity usages. A scenario is one particular path through such an activity definition. Imagine printing an activity definition and then using a highlighter to trace through decision diamonds. That's a scenario.