Upload
hollie-stone
View
218
Download
5
Tags:
Embed Size (px)
Citation preview
Chapter 3
Software Requirements
1
Requirements Engineering Process: A Basic Framework
Many variations and extensions • 3 fundamental activities:
understand, (formally) describe, attain an agreement on, the problem
2
User
ProblemDomain
Elicitation Specification Validation
Domain knowledge Domain knowledge
User reqs User feedback
Req. models
Val. result
knowledge
For more knowledge
(domain experts, laws, standards, policies, documents, etc.)
Requirements Engineering-I• Inception—ask a set of questions that establish …
– basic understanding of the problem– the people who want a solution– the nature of the solution that is desired, and – the effectiveness of preliminary communication and collaboration between the
customer and the developer
• Elicitation—elicit requirements from all stakeholders• Elaboration—create an analysis model that identifies data, function and behavioral
requirements• Negotiation—agree on a deliverable system that is realistic for developers and
customers
3
Requirements Engineering-II• Specification—can be any one (or more) of the following:
– A written document– A set of models– A formal mathematical– A collection of user scenarios (use-cases)– A prototype
• Validation—a review mechanism that looks for– errors in content or interpretation– areas where clarification may be required– missing information– inconsistencies (a major problem when large products or systems are engineered)– conflicting or unrealistic (unachievable) requirements.
• Requirements management– traceability tables: requirements and system aspects– Relate requirements to some aspects of the system
4
Requirements Analysis• Requirements analysis
– specifies software’s operational characteristics– indicates software's interface with other system elements – establishes constraints that software must meet
• Requirements analysis allows the software engineer (called an analyst or modeler in this role) to:– elaborate on basic requirements established during earlier
requirement engineering tasks– build models that depict user scenarios, functional activities,
problem classes and their relationships, system and class behavior, and the flow of data as it is transformed.
5
Requirements Modeling Strategies
• One view of requirements modeling, called structured analysis, considers data and the processes that transform the data as separate entities. – Data objects are modeled in a way that defines their attributes and
relationships. – Processes that manipulate data objects are modeled in a manner that shows
how they transform data as data objects flow through the system.
• A second approach to analysis modeled, called object-oriented analysis, focuses on – the definition of classes and– the manner in which they collaborate with one another to effect customer
requirements.
6
Domain Analysis• Define the domain to be investigated.• Collect a representative sample of applications in the domain.• Analyze each application in the sample.• Develop an analysis model for the objects. • In terms of data modeling, function/process modeling,
behavioral modeling, etc.
7
Building the Analysis Model• Elements of the analysis model
– Scenario-based elements• Functional—processing narratives for software functions• Use-case—descriptions of the interaction between an “actor” and
the system– Class-based elements
• Implied by scenarios– Behavioral elements
• State diagram– Flow-oriented elements
• Data flow diagram8
system description
analysis model
design model
A Bridge
Elements of Analysis Model
9
Data Modeling• examines data objects independently of processing• focuses attention on the data domain• creates a model at the customer’s level of abstraction• indicates how data objects relate to one another
10
What is a Data Object?• Object-something that is described by
a set of attributes (data items) and that will be manipulated within the software (system)– each instance of an object (e.g., a book)
can be identified uniquely (e.g., ISBN #) – each plays a necessary role in the system
i.e., the system could not function without access to instances of the object
– each is described by attributes that are themselves data items
11
object: automobileobject: automobileattributes:attributes: makemake modelmodel body typebody type priceprice options codeoptions code
What is a Relationship?• Data objects are connected to one another in different ways.
– A connection is established between person and car because the two objects are related.
• A person owns a car• A person is insured to drive a car
• The relationships owns and insured to drive define the relevant connections between person and car.
• Several instances of a relationship can exist• Objects can be related in many different ways
12
Entity-Relationship Diagrams• Entity-Relationship Diagram (ERD) is a detailed logical representation
of data for an organization and uses three main constructs.
13
Entity-Relationship Diagrams• Entities: Fundamental thing about which data may be maintained. Each
entity has its own identity.• Entity Type is the description of all entities to which a common definition
and common relationships and attributes apply.
14
• Consider an insurance company that offers both home and automobile insurance policies .These policies are offered to individuals and businesses.
Entity-Relationship Diagrams• Relationships: A relationship is a reason for associating two entity types.
– Binary relationships involve two entity types– A CUSTOMER is insured by a POLICY. A POLICY CLAIM is made against a POLICY.
• Relationships are represented by diamond notation in a ER diagram.
15
Entity-Relationship Diagrams
16
Entity-Relationship Diagrams• Cardinality defines “the maximum number of objects that can participate
in a relationship”[TIL93].– Two entity types A and B, connected by a relationship.
The cardinality of a relationship is the number of instances of entity B that can be associated with each instance of entity A
• Modality specifies if the relationship is optional (0) or mandatory (1).
17
Entity-Relationship Diagrams• Attributes: An attribute is a property or characteristic of an
entity that is of interest to organization.
• Each entity type has a set of attributes associated with it.
18
ERD Notation
19
(0, m) (1, 1)
object objectobjectrelationshiprelationship11 22
One common form:
(0, m)
(1, 1)
object 11 object 22relationship
Another common form:attribute
Building an ERD• Level 1—model all data objects (entities) and their
“connections” to one another• Level 2—model all entities and relationships• Level 3—model all entities, relationships, and the attributes
that provide further depth
20
The ERD: An Example
21
(1,1) (1,m)placesCustomer
requestfor service
generates(1,n)
(1,1)
workorder
worktasks
materials
consistsof
lists
(1,1)(1,w)
(1,1)
(1,i)
selectedfrom
standardtask table
(1,w)
(1,1)
Scenario-Based Modeling• Use-Cases
– “[Use-cases] are simply an aid to defining what exists outside the system (actors) and what should be performed by the system (use-cases).”
• a scenario that describes a “thread of usage” for a system• actors represent roles people or devices play as the system
functions• users can play a number of different roles for a given scenario
22
What to Write About?• Inception and elicitation—provide you with the information you’ll need to
begin writing use cases. • Requirements gathering meetings, QFD, and other requirements
engineering mechanisms are used to – identify stakeholders– define the scope of the problem– specify overall operational goals– establish priorities– outline all known functional requirements, and – describe the things (objects) that will be manipulated by the system.
• To begin developing a set of use cases, list the functions or activities performed by a specific actor.
23
Developing a Use-Case• Each scenario is described from the point-of-view of an “actor”—a person
or device that interacts with the software in some way• Each scenario answers the following questions:
– Who is the primary actor, the secondary actor (s)?– What are the actor’s goals?– What preconditions should exist before the story begins?– What main tasks or functions are performed by the actor?– What extensions might be considered as the story is described?– What variations in the actor’s interaction are possible?– What system information will the actor acquire, produce, or change?– Will the actor have to inform the system about changes in the external
environment?– What information does the actor desire from the system?– Does the actor wish to be informed about unexpected changes?
24
Use-Case Diagram
25Use-case diagram for surveillance function
Alternative Actions• Can the actor take some other action at this point?• Is it possible that the actor will encounter some error
condition at this point?• Is it possible that the actor will encounter behavior
invoked by some event outside the actor’s control?
26
Activity Diagram
27
Supplements the use case by providing a graphical representation of the flow of interaction within a specific scenario
Swimlane Diagrams
28
Allows the modeler to Allows the modeler to represent the flow of represent the flow of activities described by activities described by the use-case and at the the use-case and at the same time indicate which same time indicate which actor (if there are actor (if there are multiple actors involved multiple actors involved in a specific use-case) or in a specific use-case) or analysis class has analysis class has responsibility for the responsibility for the action described by an action described by an activity rectangleactivity rectangle
Flow-Oriented Modeling• Represents how data objects are transformed as they move through the
system
• A data flow diagram (DFD) is the diagrammatic form that is used
• Considered by many to be an ‘old school’ approach, flow-oriented modeling continues to provide a view of the system that is unique—it should be used to supplement other analysis model elements
• Every computer-based system is an information transform ....
29
computerbased
systeminput output
Flow Modeling Notation
30
external entity
process
data flow
data store
External Entity
31
A producer or consumer of data
Examples: a person, a device, a sensor
Another example: computer-basedsystem
Data must always originate somewhereand must always be sent to something
Process
32
A data transformer (changes inputto output)
Examples: compute taxes, determine area,format report, display graph
Data must always be processed in some way to achieve system function
Data Flow
33
computetriangle
area
base
height
area
Data flows through a system, beginningas input and be transformed into output.
Data Stores
34
Data is often stored for later use.
look-upsensordata
sensor #
report required
sensor #, type, location, age
sensor data
sensor number
type, location, age
Data Flow Diagramming: Guidelines
• all icons must be labeled with meaningful names• the DFD evolves through a number of levels of detail• always begin with a context level diagram (also called level 0)• always show external entities at level 0• always label data flow arrows• do not represent procedural logic
35
Constructing a DFD—I• review the data model to isolate data objects and use a
grammatical parse to determine “operations”• determine external entities (producers and consumers of
data)• create a level 0 DFD• Level 0 DFD Example
36
userprocessing
request
videosource NTSC
video signal
digitalvideo
processor
requestedvideosignall
monitor
Constructing a DFD—II• write a narrative describing the transform• parse to determine next level transforms• “balance” the flow to maintain data flow continuity• develop a level 1 DFD• use a 1:5 (approx.) expansion ratio
37
The Data Flow Hierarchy
38
Pa bx y
p1p2
p3p4 5
a
b
c
de
f
g
level 0
level 1
Example: SafeHome Software
39Level 0 DFD for SafeHome security function
40Level 1 DFD for SafeHome security function
41Level 2 DFD that refines the monitor sensors process
Flow Modeling Notes• each bubble is refined until it does just one thing• the expansion ratio decreases as the number of levels
increase• most systems require between 3 and 7 levels for an adequate
flow model• a single data flow item (arrow) may be expanded as levels
increase (data dictionary provides information)
42
Process Specification (PSPEC)
43
PSPEC
narrativepseudocode (PDL)equationstablesdiagrams and/or charts
bubble
DFDs: A Look Ahead
44
Maps intoanalysis model
design model
Control Flow Diagrams• Represents “events” and the processes that manage events• An “event” is a Boolean condition that can be ascertained by:
– listing all sensors that are "read" by the software.– listing all interrupt conditions.– listing all "switches" that are actuated by an operator.– listing all data conditions.– recalling the noun/verb parse that was applied to the processing
narrative, review all "control items" as possible CSPEC inputs/outputs.
45
The Control Model• the control flow diagram is "superimposed" on the DFD and shows events
that control the processes noted in the DFD• control flows—events and control items—are noted by dashed arrows• a vertical bar implies an input to or output from a control spec (CSPEC) —
a separate specification that describes how control is handled• a dashed arrow entering a vertical bar is an input to the CSPEC• a dashed arrow leaving a process implies a data condition• a dashed arrow entering a process implies a control input read directly by
the process• control flows do not physically activate/deactivate the processes—this is
done via the CSPEC
46
Control Flow Diagram
47
readoperator
input
createuser
displaysperformproblem diagnosis
reloadprocess
managecopying
beeper on/off
start
copies done
display panel enabled
full
problem light
jammed
empty
Control Flow Diagram
48State diagram for SafeHome security function
Flow-Oriented Modeling
49
The CSPEC can be:state diagram (sequential spec)
state transition table
decision tables
activation tables
combinatorial spec
Class-Based Modeling• Class-based modeling represents:
– objects that the system will manipulate – operations (also called methods or services) that will be applied to the
objects to effect the manipulation – relationships (some hierarchical) between the objects– collaborations that occur between the classes that are defined.
• The elements of a class-based model include classes and objects, attributes, operations, CRC models, collaboration diagrams and packages.
50
Class-Based Modeling• Identify analysis classes by examining the problem statement• Use a “grammatical parse” to isolate potential classes [Abb83] • Identify the attributes of each class• Identify operations that manipulate the attributes• Potential classes
– retained information– needed services– multiple attributes– common attributes– common operations– essential requirements
51
Analysis Classes• External entities (e.g., other systems, devices, people) that produce or consume
information to be used by a computer-based system.• Things (e.g, reports, displays, letters, signals) that are part of the information
domain for the problem.• Occurrences or events (e.g., a property transfer or the completion of a series of
robot movements) that occur within the context of system operation.• Roles (e.g., manager, engineer, salesperson) played by people who interact with
the system.• Organizational units (e.g., division, group, team) that are relevant to an
application.• Places (e.g., manufacturing floor or loading dock) that establish the context of the
problem and the overall function of the system.• Structures (e.g., sensors, four-wheeled vehicles, or computers) that define a class
of objects or related classes of objects.
52
Class Diagram
53
Class name
attributes
operations
Class diagram for the system class
Class Diagram
54Class diagram for FloorPlan
CRC Modeling• Class-responsibility-collaborator (CRC) modeling
[Wir90] provides a simple means for identifying and organizing the classes that are relevant to system or product requirements. Ambler [Amb95] describes CRC modeling in the following way:– A CRC model is really a collection of standard index
cards that represent classes. The cards are divided into three sections. Along the top of the card you write the name of the class. In the body of the card you list the class responsibilities on the left and the collaborators on the right.
55
CRC Modeling
56
A CRC model index card for FloorPlan class
Class Types• Entity classes, also called model or business classes, are extracted directly from the
statement of the problem (e.g., FloorPlan and Sensor).
• Boundary classes are used to create the interface (e.g., interactive screen or printed reports) that the user sees and interacts with as the software is used.
• Controller classes manage a “unit of work” [UML03] from start to finish. That is, controller classes can be designed to manage
– the creation or update of entity objects; – the instantiation of boundary objects as they obtain information from entity objects; – complex communication between sets of objects; – validation of data communicated between objects or between the user and the
application.
57
Responsibilities• System intelligence should be distributed across classes to
best address the needs of the problem• Each responsibility should be stated as generally as possible• Information and the behavior related to it should reside
within the same class• Information about one thing should be localized with a single
class, not distributed across multiple classes. • Responsibilities should be shared among related classes,
when appropriate.
58
Collaborations• Classes fulfill their responsibilities in one of two ways:
– A class can use its own operations to manipulate its own attributes, thereby fulfilling a particular responsibility, or
– a class can collaborate with other classes.
• Collaborations identify relationships between classes• Collaborations are identified by determining whether a class can fulfill
each responsibility itself• three different generic relationships between classes [WIR90]:
– the is-part-of relationship– the has-knowledge-of relationship– the depends-upon relationship
59
Composite Aggregate Class
60
Associations and Dependencies• Two analysis classes are often related to one another in some
fashion– In UML these relationships are called associations– Associations can be refined by indicating multiplicity (the
term cardinality is used in data modeling)• In many instances, a client-server relationship exists between
two analysis classes. – In such cases, a client-class depends on the server-class in
some way and a dependency relationship is established
61
Class Diagrams
62
Top: MultiplicityBottom: Dependencies
Analysis Packages• Various elements of the analysis model (e.g., use-cases,
analysis classes) are categorized in a manner that packages them as a grouping
• The plus sign preceding the analysis class name in each package indicates that the classes have public visibility and are therefore accessible from other packages.
• Other symbols can precede an element within a package. A minus sign indicates that an element is hidden from all other packages and a # symbol indicates that an element is accessible only to packages contained within a given package.
63
Analysis Packages
64
Behavioral Modeling• The behavioral model indicates how software will respond to
external events or stimuli. To create the model, the analyst must perform the following steps:– Evaluate all use-cases to fully understand the sequence of interaction
within the system.– Identify events that drive the interaction sequence and understand
how these events relate to specific objects.– Create a sequence for each use-case.– Build a state diagram for the system.– Review the behavioral model to verify accuracy and consistency.
65
State Representations• In the context of behavioral modeling, two different
characterizations of states must be considered: – the state of each class as the system performs its function and– the state of the system as observed from the outside as the system
performs its function
• The state of a class takes on both passive and active characteristics [CHA93]. – A passive state is simply the current status of all of an object’s
attributes.– The active state of an object indicates the current status of the object
as it undergoes a continuing transformation or process
66
State Diagram
67State diagram for the ControlPanel class
The States of a System• state—a set of observable circumstances that characterizes
the behavior of a system at a given time• state transition—the movement from one state to another• event—an occurrence that causes the system to exhibit some
predictable form of behavior• action—process that occurs as a consequence of making a
transition
68
Behavioral Modeling• make a list of the different states of a system (How
does the system behave?)• indicate how the system makes a transition from one
state to another (How does the system change state?)– indicate event– indicate action
• draw a state diagram or a sequence diagram
69
Sequence Diagram
70Sequence diagram (partial) for the SafeHome security function
Requirements Documentation• This is the way of representing requirements in a consistent
format– called Software Requirements Specifications (SRS) document
• SRS serves many purposes depending upon who is writing it.– written by customer and/or developer
• Serves as contract between customer & developer.• SRS Should
– Correctly define all requirements– not describe any design details– not impose any additional constraints
71
Requirements Documentation• Characteristics of a good SRS
– Correct : An SRS is correct if and only if every requirement stated therein is one that the software shall meet.
– Unambiguous : An SRS is unambiguous if and only if, every requirement stated therein has only one interpretation.
– Complete : An SRS is complete if and only if, it includes the following elements
• All significant requirements, whether related to functionality, performance, design constraints, attributes or external interfaces.
• Responses to both valid & invalid inputs.• Full Label and references to all figures, tables and diagrams in the SRS and
definition of all terms and units of measure.
72
Requirements Documentation• Characteristics of a good SRS (continued)
– Consistent : An SRS is consistent if and only if, no subset of individual requirements described in it conflict.
– Ranked for important and/or stability : If an identifier is attached to every requirement to indicate either the importance or stability of that particular requirement.
– Verifiable : An SRS is verifiable, if and only if, every requirement stated therein is verifiable.
73
Requirements Documentation• Characteristics of a good SRS (continued)
– Modifiable : An SRS is modifiable, if and only if, its structure and style are such that any changes to the requirements can be made easily, completely, and consistently while retaining structure and style.
– Traceable : An SRS is traceable, if the origin of each of the requirements is clear and if it facilitates the referencing of each requirement in future development or enhancement documentation.
74
Requirements Documentation• Organization of the SRS
– IEEE has published guidelines and standards to organize an SRS.
1. Introduction 1.1 Purpose 1.2 Scope 1.3 Definition, Acronyms and abbreviations 1.4 References 1.5 Overview
75
Requirements Documentation2. The Overall Description 2.1 Product Perspective 2.2 Product Functions
2.1.1 System Interfaces 2.3 User Characteristics 2.1.2 Interfaces 2.4 Constraints 2.1.3 Hardware Interfaces 2.5 Assumptions for dependencies 2.1.4 Software Interfaces 2.6 Apportioning of requirements 2.1.5 Communication Interfaces 2.1.6 Memory Constraints 2.1.7 Operations 2.1.8 Site Adaptation Requirements
76
Requirements Documentation3. Specific Requirements 3.1 External Interfaces 3.2 Functions 3.3 Performance requirements 3.4 Logical database requirements 3.5 Design Constraints 3.6 Software System attributes 3.7 Organization of specific requirements 3.8 Additional Comments.
77
Writing the Software Specification
Everyone knew exactly what had to be done until someone wrote it down!
Specification Guidelines
79
use a layered format that provides increasing detail as the "layers" deepen use consistent graphical notation and apply textual terms consistently (stay away from aliases) be sure to define all acronyms be sure to include a table of contents; ideally, include an index and/or a glossary write in a simple, unambiguous style (see "editing suggestions" on the following pages) always put yourself in the reader's position, "Would I be able to understand this if I wasn't intimately familiar with the system?"
Specification Guidelines
80
Be on the lookout for persuasive connectors, ask why? keys: certainly, therefore, clearly, obviously, it follows that ... Watch out for vague terms keys: some, sometimes, often, usually,ordinarily, most, mostly ... When lists are given, but not completed, be sure all items are understood keys: etc., and so forth, and so on, such as Be sure stated ranges don't contain unstated assumptions e.g., Valid codes range from 10 to 100. Integer? Real? Hex? Beware of vague verbs such as handled, rejected, processed, ... Beware "passive voice" statements e.g., The parameters are initialized. By what? Beware "dangling" pronouns e.g., The I/O module communicated with the data validation module and its contol flag is set. Whose control flag?
Specification Guidelines
81
When a term is explicitly defined in one place, try substituting the definition forother occurrences of the term When a structure is described in words, draw a picture When a structure is described with a picture, try to redraw the picture to emphasize different elements of the structure When symbolic equations are used, try expressing their meaning in words When a calculation is specified, work at least two examples Look for statements that imply certainty, then ask for proof keys; always, every, all, none, never Search behind certainty statements—be sure restrictions or limitations are realistic