UML - Use Case.

Use case is important for any software development. Use case is a structured way of describing a scenario. In extreme coding style, use case may not be a structured way rather it can be in a form of story telling. But some complex application required a structured way of use case so that we can discuss and brainstorm alternative scenarios and exception handling. The main input of use case is scenario like "buy a product". This detailed scenario can be found from "Product requirement document". But it won't describe well with in system level and who is going to execute this requirement etc. Use case is the only way to capture all those missing points from requirement document.

The main sections of use case are

1. Actors : Actors is nothing but users who is going to interact with system in real time. User can divide into their roles. So any given system, we can identify level of user roles well before implementation.

2. Use case text: This is the extension of a scenario, starts with pre-condition, main use case flow, exception and extensions. 

3. Use case diagram : Conceptional view of entire requirement. This can give us detailed view of all requirements and what can be reused(as includes) and what can be extend(as extends) and actors interacts with system.