CSC/ECE 517 Fall 2007/wiki2 4 2q

A Guide to Use Cases on the WEB

Topic :Use cases. There are even more pages on the Web on use cases than on MVC. If someone wants to learn about them, what should (s)he do? Look at the first few hits in Google? I expect we can do better than that. Write a review of the use-case sites on the Web. Which are best for learning about the concept? Which have the most instructive examples? Which teach advanced concepts that are not apparent in perusing most use-case sites? Read at least several dozen pages before deciding how to organize your overview.

Introduction

This page tries to review the use-case sites on the Web which can be useful to best understand the concept. Also a few instructive examples are covered paying attention to the basic use case as well as advanced concepts that are not apparent in perusing most use-case sites.

What is a Use Case?

Definition

"Use case is a methodology used in software and systems engineering to understand and identify the functional requirements of a system. Use cases describe the interaction between a primary actor, the initiator of the interaction and the system itself, represented as a sequence of simple steps."[1][2]

An actor who can more generally be described as the user of the system, is the external entity and can be a person or another system itself. A use case can be best described as what happens when the actor is using the system to achieve some goal. This goal is nothing but the end result which follows the sequence of events as described in the use cases.

Every use case gives a complete list of events which occur from the time the actor starts interacting with the system till the time the final goal is achieved. A use case also records all the possible scenarios that may occur while trying to reach the desired goal. We can think of it as existence of a main course of events that will occur in normal circumstances, to which the exceptional cases are added. Anything that is not relevant to the actor/user is not a part of the use case.

Listed below are a few links to external websites which further illustrate this concept.

• OO design process: Use cases, an introduction
• Use Case Fundamentals

UML Diagrams and Use cases

"The Use case diagram is used to identify the primary elements and processes that form the system. The primary elements are termed as 'actors' and the processes are called "use cases." The Use case diagram shows which actors interact with each use case."[3]

They are useful in providing a pictorial view of the interactions between the users (actors) and sets of events (use cases) associated with the system.They also evolve as the projects or system change with time. The standard elements of a use case diagram makes comprehension easy for anyone trying to understand the use case.

Listed below are useful links in understanding how to draw typical use case diagrams.

• An introduction to UML diagrams & Tutorial
• Elements of a Use Case Diagram

Examples using Use Cases

In this section, a few instructive examples have been outlined along with their references which illustrate different ways in which use cases can be written. The first example is a basic use-case template.

Basic use case template

Described below are some the characteristics of a typical use case.

 CHARACTERISTIC INFORMATION
 * Goal in Context:   Buyer issues request directly to our company, expects goods shipped and to be billed.
 * Scope:  Company
 * Preconditions:  We know Buyer, their address, etc.
 * Success End Condition:  Buyer has goods, we have money for the goods.
 * Failed End Condition:  We have not sent the goods, Buyer has not spent the money.
 * Primary Actor:  Buyer, any agent (or computer) acting for the customer
 * Trigger:  purchase request comes in.
 
 MAIN SUCCESS SCENARIO
 1. Buyer calls in with a purchase request.
 2. Company captures buyer’s name, address, requested goods, etc.
 3. Company gives buyer information on goods, prices, delivery dates, etc.
 4. Buyer signs for order.
 5. Company creates order, ships order to buyer.
 6. Company ships invoice to buyer.
 7. Buyers pays invoice.
 
 EXTENSIONS
 3a. Company is out of one of the ordered items: 
     3a1. Renegotiate order.
 4a. Buyer pays directly with credit card:
     4a1. Take payment by credit card (use case 44)
 7a. Buyer returns goods:
     7a. Handle returned goods (use case 105)

The basic steps that can be followed so as to ensure writing an effective use case has been described well in Writing Effective Use Cases.[5]

Restaurant

This example provides one use case but distinctly provides a step by step analysis of the events that constitute a Use Case. It mentions the actors, pre-conditions, Exceptions and Alternative scenarios.

• Restaurant

Purchase order

This example provides a typical use case specification by listing the Activity, Result, Constraints and Extensions involved with a purchase order from a Finance web page.

• Purchase order System

ATM system

This example is a little more involved in that it provides a number of use cases associated with using an ATM system including cash withdrawal, transfers, deposits, balance inquiry, etc.

• ATM System

A shopping website order Process

This is a very good example that describes the scenarios and extensions involved in writing a use case for a online shopping order. It also describes the UML diagram.

• Shopping website order

Self Service Machine

This example provides the typical Use Cases in a self service machine. Some of the aspects of using a self service machine are: buying a product, re-stocking the machine, collecting the money etc. The following link expands on how to do this and also provides the UML diagrams which depict the interaction between the users and system through the use case

• Self-Service Machine (page-3)

Advanced Concepts on Use Cases

Pluggable Use Cases

Most of the time it is found that common behavior is replicated in Use Cases. This can be handled by writing Use Cases with basic behavior which can be 'plugged-in' to write use cases which cover more behavior. This makes them reusable from one project to another.

"A pluggable use case is written in a generic form such that it may be exercised wherever needed. As the name implies a pluggable use case can be ‘plugged’ into any other use case even another pluggable use case. It contains a cohesive set of functionality that encapsulates the common behavior of a subgoal, where this subgoal exists in multiple transactions. Because a pluggable use case only constitutes a subgoal of another use case its context must be inferred from the invoking use case." [6]

The following link gives a detailed description of this concept

• Pluggable Use Cases

References

• [1] Ivar Jacobson (1992). Object-Oriented Software Engineering. Addison Wesley Professional. ISBN 0-201-54435.
• [2] Wikipidea Page Basic definition of Use Case
• [3] Use Case Diagrams
• [4] Basic Use Case Template
• [5] Writing Effective Use Cases
• [6] Pluggable Use Cases

External Links

• Use Case Fundamentals
• Elements of a Use Case Diagram
• Pluggable Use Cases