Task: Outline Use Cases

This task is where the actors and use cases are identified to support the requirements being implemented. Identifying the actors and use cases explicitly defines the scope of the system. Outlining a use case is used to capture the essence of the intention of using the system for the intended purpose.

Purpose

To define the scope of the system - what will be handled by the system and what will be handled outside the system.
To define who and what will interact with the system.
To outline the functionality of the system.

Relationships

Roles	Primary: Requirements Engineer	Additional: Stakeholder	Assisting:
Inputs	Mandatory: Stakeholder Needs System Context Diagram	Optional: None	External: None
Outputs	System Use Case System Use-Case Diagram

Steps

Find Actors

Each external entity which interacts with the system should be represented by an actor. For more information, see Guideline: Outlining Use Cases.

Find Use Cases

Consider what each actor (human or not) requires from the system. For more information, see Guideline: Outlining Use Cases.

Outline the flow of events

Describe the interaction between the system and its actors in a pure black-box fashion, without revealing anything about the internals of the system. For more information, see Guideline: Outlining Use Cases.

Describe how Use Cases and Actors interact

Not every actor is involved in each use case, so for each use case establish which actor will interact with it. For more information, see Guideline: Outlining Use Cases.

Package Use Cases and Actors

If the overall number of actors and use cases becomes too great, divide them into packages and use them as a base for assigning work and maintenance. For more information, see Guideline: Outlining Use Cases.

Capture the Use-Case Model in diagrams

Illustrate the relationships among use cases and actors, as well as among related use cases, in diagrams of the use-case model. For more information, see Guideline: Outlining Use Cases.

Develop a survey of the Use-Case Model

Include the following:

Typical sequences in which the use cases are employed by users.
Functionality not handled by the use-case model.

Evaluate your Results

Check if the work is on track, but keep the model at right level of detail. Involve all the stakeholders in reviewing the use cases. For more information, see Guideline: Outlining Use Cases.

Key Considerations

Some of the reasons to outline your use cases are:

Iterative development reduces risk. For the same reason we do not implement the whole system in one hit, we do not develop the detailed requirements all at once.
Avoid getting bogged down in too much detail too early.
You do not know everything at once. Outlining helps you discover what you don't know.
Outlining use cases creates a rough draft for fully specifying your use cases.
Outlining helps determine whether the use case is too small on its own or too big to be just one use case. It might also help decide whether the use case is actually more than just one use case.
Once you write out your steps, you might find that they really belong in another use case. If the outline shows that the use case does not have much to it, it may not be a use case at all.
Outlining adds value to finding all possible alternate flows.

More Information

Checklists	Actor Use Case Use-Case Model
Guidelines	Outlining Use Cases