Basic Flow , Alternate Flow in Use Case Secifications

BASIC / POSITIVE FLOW
The main flow of events describes a single path through the system. It specifies the interactions between the actor and the system for an ideal condition. It represents the most common way that the use case plays out successfully and contains the most common sequence of user-system interactions.
ALTERNATE / OPTIONAL FLOW
An alternate flow is a series of actions other than the basic flow that results in a user completing his or her goal. Often ,It is considered to be an optional flow. It means that the user has chosen to take an alternative path through the system.
EXCEPTIONAL FLOW
An exceptional flow is any action that will cause the Actor to not to complete or achieve the desired result. Exception flows represent an undesirable path to the user. However, even though the exception flow has occurred the system should still react in a way that recovers the flow and provides some useful information to the user.
USE CASE SECIFICATION
A use case specification provides details for a use case in a textual format . This explains the actors, conditions – pre & post, flow of events ,alternative flows , exceptional flow and other important information of a use case.
We can use the below sample outline to create our own use case specification documents :
1.USE CASE ID : States a unique ID for each use case.
2. USE CASE NAME : States the use case name. The name expresses the objective or result of the use case
3. BREIF DESCRIPTION : Describes the role and purpose of the use case
4. ACTORS : specifies a role played by a user or any other system that interacts with the subject.
5. PRECONDITIONS : A state of the system that must be present before a use case starts.
6. POSTCONDITIONS : A list of possible states for the system immediately after a use case is finished.
7. BASIC FLOW OF EVENTS : Describes the ideal, primary behavior of the system
8. ALTERNATIVE FLOW : Describes deviations from the basic flow,
9. EXCEPTIONAL FLOW : Explains the exceptions from the basic flow,
10. KEY SCENARIOS : Enlist the most important scenarios of the use case. There can be many scenarios possible with this use-case specification:
11. SPECIAL REQUIREMENTS : List the Nonfunctional requirements like legal and regulatory requirements; application standards; quality attributes of the system, including usability, reliability, performance, and supportability; operating systems and environments; compatibility requirements; and design constraints.