编写有效用例：需求分析的关键

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xiv

Exercise 26 on page 102 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

Exercise 27 on page 102 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

Exercise 29“Fix faulty ’Login’” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

Use Case 38: Use the order processing system . . . . . . . . . . . . . . . . . . . . . . . 239

Exercise 30 on page 109 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239

Exercise 34 on page 113 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

Use Case 39: Buy stocks over the web . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

Exercise 37 on page 128: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Use Case 40: Perform clean spark plugs service . . . . . . . . . . . . . . . . . . . . . . . 241

Chapter 25 Appendix C: Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . 242

Main terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

Types of use cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

Chapter 26 Appendix D: Reading . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Books referenced in the text. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Articles referenced in the text. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Online resources useful to your quest.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

1. I

NTRODUCTION

ASES

What do use cases look like?

Why would different project teams need different writing styles?

Where do they fit into the requirements gathering work?

How do we warm up for writing use cases?

It will be useful to have some thoughts on these questions in place before getting into the details

of use cases themselves. Feel free to bounce between this introduction and Use Case Body Parts,

picking up background information as you need.

1.1 What is a Use Case (more or less)?

A use case captures a contract between the stakeholders of a system about its behavior. The use

case describes the system’s behavior under various conditions as it responds to a request from one

of the stakeholders, called the primary actor. The primary actor initiates an interaction with the

system to accomplish some goal. The system responds, protecting the interests of all the stake-

holders. Different sequences of behavior, or scenarios, can unfold, depending on the particular

requests made and conditions surrounding the requests. The use case collects together those

different scenarios.

Use cases are fundamentally a text form, although they can be written using flow charts,

sequence charts, Petri nets, or programming languages. Under normal circumstances, they serve to

communicate from one person to another, often to people with no special training. Simple text is,

therefore, usually the best choice.

The use case, as a form of writing, can be put into service to stimulate discussion within a team

about an upcoming system. They might later use that the use case form to document the actual

requirements. Another team might later document the final design with the same use case form.

They might do this for a system as large as an entire company, or as small as a piece of a software

application program. What is interesting is that the same basic rules of writing apply to all these

different situations, even though the people will write with different amounts of rigor, at different

levels of technical detail.

When the use cases document an organization’s business processes, the system under discussion

is the organization itself. The stakeholders are the company shareholders, customers, vendors, and

Chapter 1. Introduction to Use Cases

What is a Use Case (more or less)? - Page 16

government regulatory agencies. The primary actors will include the company’s customers and

perhaps their suppliers.

When the use cases record behavioral requirements for a piece of software, the system under

discussion is the computer program. The stakeholders are the people who use the program, the

company owning it, government regulatory agencies, and other computer programs. The primary

actor will be the user sitting at the computer screen or another computer system.

I show several examples of use cases below. The parts of a use case are described in the next

chapter ("Use Case Body Parts"). For now, just note that the primary actor is the one with the goal

that the use case addresses. Scope identifies the system that we are discussing, the preconditions

and guarantees say what must be true before and after the use case runs. The main success

scenario is a case in which nothing goes wrong. The extensions section describes what can happen

differently during that scenario. The numbers in the extensions refer to the step numbers in the

main success scenario at which each different situation gets detected (for instance, steps 4a and 4b

indicate two different conditions that could show up at step 4). When a use case references another

use case, the second use case is written in italics or underlined

The first use case describes a person about to buy some stocks. over the web To signify that we

are dealing with a goal to be achieved in a single sitting, I mark the use case as being at the "user

goal" level, and tag the use case with the "sea-level" symbol . The second use case describes a

person trying to get paid for a car accident, a goal that takes longer than a single sitting. To show

this, I mark the level as "summary", and tag the use case with the "above sea level" kite symbol

. These symbols are all explained in more detail later.

The first use case describes the person’s interactions with a program (the "PAF" program)

running on a workstation connected to the web. I indicate that the system being discussed is a

computer system with the symbol of a black box, . The second use case describes a person’s

interaction with a company. I indicate that with the symbol of a building, . The use of symbols

are completely optional. Labeling the scope and level are not.

Here are the first two use cases.

Chapter 1. Introduction to Use Cases

What is a Use Case (more or less)? - Page 18

ASE

2: G

PAID

FOR

CAR

ACCIDENT

Primary Actor: The Claimant

Scope

: The insurance company ("MyInsCo")

Level

: Summary

Stakeholders and Interests:

the claimant - to get paid the most possible

MyInsCo - to pay the smallest appropriate amount

the dept. of insurance - to see that all guidelines are followed.

Precondition

: none

Minimal guarantees

: MyInsCo logs the claim and all activities.

Success guarantees

: Claimant and MyInsCo agree on amount to be paid, claimant gets paid

that.

Trigger:

Claimant submits a claim

Main success scenario

1. Claimant submits claim with substantiating data.

2. Insurance company verifies claimant owns a valid policy

3. Insurance company assigns agent to examine case

4. Insurance company verifies all details are within policy guidelines

5. Insurance company pays claimant and closes file.

Main success scenario

1a. Submitted data is incomplete:

1a1. Insurance company requests missing information

1a2. Claimant supplies missing information

2a. Claimant does not own a valid policy:

2a1. Insurance company declines claim, notifies claimant, records all this, terminates pro-

ceedings.

3a. No agents are available at this time

3a1. (What does the insurance company do here?)

4a. Accident violates basic policy guidelines:

4a1. Insurance company declines claim, notifies claimant, records all this, terminates pro-

ceedings.

4b. Accident violates some minor policy guidelines:

4b1. Insurance company begins negotiation with claimant as to degree of payment to be

made.

________________________________________

Most of the use cases for this book come from live projects, and I have been careful not to touch

them up (except to add the scope and level tags if they weren’t there). I want you to see samples of

what works in practice, not just what is pretty in the classroom. People rarely have time to make

the use cases formal, complete, and pretty. They usually only have time to make them "sufficient".

Sufficient is fine. It is all that is necessary. I show these real samples because you will rarely be

able to generate perfect use cases yourself, despite whatever coaching I offer in the book. I can’t

even write perfect use cases most of the time.

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