Requirements Modelling

Question 1

Breifly describe use case modelling

Why do we do it?

Answer 1

User-centric approach. Identify typical users and the tasks they typically perform to capture functional requirements incrementally.

Idea is that it’s hard to specify all system reqs top-down, but is easier to identify what single users expect to be able to do. Structure the requirements model gradually, identifying obvious event flows first, and filling gaps with alternative event flows.

Question 2

What is use case modelling good at? What is it bas at?

Answer 2

Good at capturing what a system does

Not good at expressing control logic

Question 3

What is an Actor?

Answer 3

Type of external user interacting with the system; not the individual but the role they play

Not necessarily a person, eg external computer system

Question 4

What is a Use Case?

Answer 4

sequence of related transactions between an actor and a system that yields a result of measurable value.

Use cases are restricted to a particular size
related collection of evetn flows
Accomplish a single business objective

Question 5

How do we identify use cases?

Answer 5

Interviews: identify main actors, main use cases, secondary actors, and extra use cases

Documenting: use usa case diagram to link actors to use cases

Question 6

UML Syntax: how do we draw use cases?

Answer 6

labelled ellipse

verb expressions to label each use case
within or outside the version boundary

Question 7

UML Syntax: how do we draw actors?

Answer 7

labelled stick figure

name each actor according to role played

Question 8

UML Syntax: how do we draw associations

Answer 8

Line not quite touching each end

link actors to use cases

Question 9

Why and How do we document use cases?

Answer 9

Why: to describe the funcitonality in detail.

How: want to collect scenarios incrementally. Document main success scenario first (normal flow), document extensions later (divergences from normal flow, variant flows)
Write as a sequence of events, in style of a dialogue

Question 10

3 different levels of formality for writing use cases

Answer 10

Brief: v short description; fit in spreadsheet cell

Casual: several paragraphs in natural language

Fully dressed: written according to structured template

Question 11

What are the required sections for writing a “fully dressed use case”

Answer 11

Name; purpose; enabling condition; flow of events; success condition

Question 12

What is an optional extension, when looking at use cases?

Answer 12

optional inserted behaviour that is sometimes executed; then normal flow is resumed

Question 13

What is an alternative extension, when looking at use cases?

Answer 13

A second successful path through the use case; that may resume normal flow or end flow independently

Question 14

What is a failure extension, when looking at use cases?

Answer 14

An exception that causes the use case to fail without reaching its objective, a failure condition must be stated

Question 15

What sections are required for documenting a use case extension?

Answer 15

Name, extension point, enabling condition, flow of events, success/failure condition

Question 16

Pros and Cons of an incremental approach to use cases (3, 3)

Answer 16

Pros:
Stakeholders will focus on succesful outcomes
Analyst will later ask what can go wrong at each step
Easy to add variant flows at a later date

Cons:
Not all applications wiill have a distinguished normal flow
by default, UML treats all extensions as optional insertions
resume label resembles GOTO programs which were considered very poor structure

Question 17

UML Syntax: how do we draw an include association?

Answer 17

dashed arrow with open arrow-head labelled with < include >, pointed to the included case

Question 18

When do we use an include dependency for use cases?

Answer 18

When multiple use cases share a common subsequence

Question 19

UML Syntax: how do we draw an extend association?

Answer 19

dashed arrow with open arrow-head labelled with < extend >, pointed to the use case that is being extended

Question 20

When do we use an extend dependency for use cases?

Answer 20

When we want to include an optional variant flow for a use case, and want to make it explicit in the use case diagram

Question 21

UML Syntax: how do we draw a generalisation arrow?

Answer 21

An arrow with a triangular closed arrow head with a solid line, points to more general case,

e.g. worker ———-|> employee
worker is a kind of employee

Question 22

What is the definition of a scenario?

Answer 22

One complete plath traced through a use case ending in a success or failure.

Question 23

Why are scenarios useful?

Answer 23

Form the basis for testing, useful for test driven development.