Developing Requirements

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Object-Oriented Software EngineeringPractical
Software Development using UML and Java

• Chapter 4
• Developing Requirements

Based on Presentations LLOSENG (Lethbridge,
Laganiere,2001, Williams 2001, Probert 2001)
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Requirements Engineering

• Motto"The hardest single part of building a
  software system is deciding what to build. No
  other part of the work so cripples the resulting
  system if done wrong. No other part is more
  difficult to rectify later."Brooks 87

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4.1 Domain Analysis

• The process by which a software engineer learns
  about the domain to better understand the
  problem
• The domain is the general field of business or
  technology in which the clients will use the
  software
• A domain expert is a person who has a deep
  knowledge of the domain
• Benefits of performing domain analysis
• Faster development
• Better system
• Anticipation of extensions

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Domain Analysis document

• A. Introduction
• B. Glossary
• C. General knowledge about the domain
• D. Customers and users
• E. The environment
• F. Tasks and procedures currently performed
• G. Competing software
• H. Similarities to other domains

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4.2 The Starting Point for Software Projects
green field project
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4.3 Defining the Problem and the Scope

• A problem can be expressed as
• A difficulty the users or customers are facing,
• Or as an opportunity that will result in some
  benefit such as improved productivity or sales.
• The solution to the problem normally will entail
  developing software
• A good problem statement is short and succinct

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Defining the Scope

• Narrow the scope by defining a more precise
  problem
• List all the things you might imagine the system
  doing
• Exclude some of these things if too broad
• Determine high-level goals if too narrow
• Example A university registration system

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4.4 What is a Requirement

• Requirement A statement about the proposed
  system that all stakeholders agree must be made
  true in order for the customers problem to be
  adequately solved.
• Short and concise piece of information
• Says something about the system
• All the stakeholders have agreed that it is valid
• It helps solve the customers problem
• A collection of requirements is a requirements
  document.

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4.5 Types of Requirements

• Functional requirements
• Describe what the system should do
• Non-functional requirements
• Constraints that must be adhered to during
  development

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Functional requirements

• What inputs the system should accept
• What outputs the system should produce
• What data the system should store that other
  systems might use
• What computations the system should perform
• The timing and synchronization of the above

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Example of Functional Requirements
A point of sale terminal must make it possible to
record sales and payments. It is intended to be
used in retail stores. The system includes
hardware components and a software to run the
system. The project goal is to develop the
software.
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Non-functional requirements

• All must be verifiable
• Three main types
• 1. Categories reflecting usability, efficiency,
  reliability, maintainability and reusability
• Response time
• Throughput
• Resource usage
• Reliability
• Availability
• Recovery from failure
• Allowances for maintainability and enhancement
• Allowances for reusability

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Non-functional requirements

• 2. Categories constraining the environment and
  technology of the system.
• Platform
• Technology to be used 
• 3. Categories constraining the project plan and
  development methods
• Development process (methodology) to be used
• Cost and delivery date
• Often put in contract or project plan instead

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Requirements Quality

• Requirements should be
• 1. Correct.
• 2. Coherent - without
• Internal conflicts (contradictions, too general
  terms having various possible interpretations).
• External conflicts (subsystem requirements
  conflicting with those of the systems).
• Conflicts with standards.
• Coherence can be verified - using formal approach
  by organizing requirements (in tables/dictionaries
  , keeping references)
• 3. Complete - having
• all the required functions,
• all the constraints,
• all contingencies

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Requirements Quality (1)

• Requirements should be
• 4. Realists - according to
• budget,
• the existing technology,
• the required schedule
• 5. Necessary
• shouldnt add non-necessary complications
• should be at the right level
• 6. Verifiable (testable)
• 7. Traceable (possibility to refer to
  requirements in a clear way).
• Non-functional requirements should be
• Measurable. Ex. rather than the system must be
  portable not more than 5 of the source code
  must be system dependent.
• Linked to functional requirements affected.

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4.6 Some Techniques for Gathering and Analysing
Requirements

• Observation
• Read documents and discuss requirements with
  users
• Shadowing important potential users as they do
  their work
• ask the user to explain everything he or she is
  doing
• Session videotaping
• Interviewing
• Conduct a series of interviews
• Ask about specific details
• Ask about the stakeholders vision for the future
  
• Ask if they have alternative ideas
• Ask for other sources of information
• Ask them to draw diagrams

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Gathering and Analysing Requirements...

• Brainstorming
• Appoint an experienced moderator
• Arrange the attendees around a table
• Decide on a trigger question
• Ask each participant to write an answer and pass
  the paper to its neighbour
• Joint Application Development (JAD) is a
  technique based on intensive brainstorming
  sessions

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Gathering and Analysing Requirements...

• Prototyping
• The simplest kind paper prototype.
• a set of pictures of the system that are shown to
  users in sequence to explain what would happen
• The most common a mock-up of the systems UI
• Written in a rapid prototyping language
• Does not normally perform any computations,
  access any databases or interact with any other
  systems
• May prototype a particular aspect of the system

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Gathering and Analysing Requirements...

• Informal use case analysis
• Determine the classes of users that will use the
  facilities of this system (actors)
• Determine the tasks that each actor will need to
  do with the system
• More on use cases in Chapter 7

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Example Library System

• List a minimal set of use cases for the following
  actors in library system Borrower, Checkout
  Clerk, Librarian, Accounting System
• Borrower
• Search for itens by title
• By author
• by subject
• Place a book on hold if it is checked out by
  somebody else
• Check the borrows personal information and list
  of book borrowed

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Example Library System (1)

• List a minimal set of use cases for the following
  actors in library system Borrower, Checkout
  Clerk, Librarian, Accounting System
• Checkout clerk
• All the Borrowers use cases, plus
• Check out an item for a borrower
• Check on an item that has been returns
• Renew an item
• Recorded that a fine has been paid
• Add a new borrower
• Update a borrowers personal information
  (address, telephone number etc.

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Example Library System (2)

• List a minimal set of use cases for the following
  actors in library system Borrower, Checkout
  Clerk, Librarian, Accounting System
• Librarian
• All the Borrowers and Checkout Clerk use cases,
  plus
• Add a new item to the collection
• Delete an item from the collection
• Change the information the system has recorded
  about an item
• Accounting System (acting autonomously)
• Obtain the amount of overdue fines paid by
  borrowers

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4.7 Types of Requirements Document

• Two extremes
• An informal outline of the requirements using a
  few paragraphs or simple diagrams
• requirements definition
• A long list of specifications that contain
  thousands of pages of intricate detail
• requirements specification

• Requirements documents for large systems are
  normally arranged in a hierarchy

Requirements
Requirements
Definition
Definition
xxxx
xxxx
xxxxxxx
xxxxxxx
Requirements
xxx
Requirements
xxx
Requirements
xxxxxxxxxxx
xxxxxxxxxxx
Specification
Specification
xxxxx
Specification
xxxxx
xxxx
xxxxxxxxxxxxx
xxxxxxxxxxxxx
xxxx
xxxx
xxxxxxx
xxxxxxx
xxxxxxx
xxxxxxx
xxxxxxx
xxx
xxx
xxx
xxx
xxx
xxxxxxxxxxxxxxx
xxxxxxxxxxx
xxxxxxxxxxxxxxx
xxxxxxxxxxx
xxxxxxxxxxx
xxxxx
xxxxx
xxxxx
xxxxxxxxxxxxx
xxxxxxxxxxxxx
xxxxxxxxxxxxx
xxxxxxx
xxxxxxx
xxxxxxx
xxx
xxx
xxx
xxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx
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Level of detail required in a requirements
document

• How much detail should be provided depends on
• The size of the system
• The need to interface to other systems
• The readership (target audience)
• The stage in requirements gathering
• The level of experience with the domain and the
  technology
• The cost that would be incurred if the
  requirements were faulty

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4.8 Reviewing Requirements

• Each individual requirement should
• Have benefits that outweigh the costs of
  development
• Be important for the solution of the current
  problem
• 80 of the users problem can be solved with 20
  of the work.
• Be expressed using a clear and consistent
  notation
• Example rather than The pharmacist will enter
  the patient ID number and then the patients
  medication record will be displayed, write
  When the pharmacist enters the patient ID
  number, then system displays the patients
  medication record.
• Be unambiguous
• Be logically consistent
• Lead to a system of sufficient quality
• All requirements should contribute to a system
  that is sufficiently usable, safe, efficient,
  reliable and maintainable.
• Be realistic with available resources
• Be verifiable
• Be uniquely identifiable
• Does not over-constrain the design of the system

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Requirements documents...

• The document should be
• sufficiently complete
• well organized
• clear
• agreed to by all the stakeholders
• Traceability

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Requirements document...

• A. Problem
• B. Background information
• C. Environment and system models
• D. Functional Requirements
• E. Non-functional requirements

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4.9 Managing Changing Requirements

• Requirements change because
• Business process changes
• Technology changes
• The problem becomes better understood
• Requirements analysis never stops
• Continue to interact with the clients and users
• The benefits of changes must outweigh the costs.
• Certain small changes (e.g. look and feel of the
  UI) are usually quick and easy to make at
  relatively little cost.
• Larger-scale changes have to be carefully
  assessed
• Forcing unexpected changes into a partially built
  system will probably result in a poor design and
  late delivery
• Some changes are enhancements in disguise
• Avoid making the system bigger, only make it
  better

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4.10 Detailed Example

• GPS-base Automotive Navigation Assistant (GANA)
• Problem GANA software will help drivers navigate
  by giving them directions to their destination.

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4.10 Detailed Example (1)

• GPS-base Automotive Navigation Assistant (GANA)
• Problem GANA software will help drivers navigate
  by giving them directions to their destination.
• Backgrounf information. See domain analysis
  document 1234 (not provided for the lectures)
• Environment and system models GANA software is
  to run on special GANA hardware, described
  separately in document 1234. As described in
  document 1234, the hardware provides the
  following to the software a) GPS position
  information, b) a wireless Internet connection to
  a map data base, c) position of a trackball, d) a
  color 10cm x 10cm LCD screen, and f) in put from
  the cars other systems containing data about
  speed and turning of the steering wheel. This
  requirements document describes the software only.

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4.10 Detailed Example(2)

• GPS-base Automotive Navigation Assistant (GANA)
• Functional Requirements.
• The system uses GPS information to calculate what
  map to display. The system also integrates
  information about the cars speed and history of
  turns made in order to refine its accuracy about
  the vehicles location.
• The system has two main interaction modes In
  setup mode, the user consults maps and specifies
  the destination. In navigation mode,the system
  assists the user to navigate to the destination.
• Setup mode
• 3.1 When the system is switched on, and the
  vehicle is stationary, it enters setup mode. If
  the vehicle is moving it enter navigation mode.
  For safety reasons you can not enter setup mode
  when the vehicle is moving

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4.10 Detailed Example(3)

• GPS-base Automotive Navigation Assistant (GANA)
• 3.2 In setup mode, the system displays a map.
  The default map is in125000 scale and is
  centered on the user's current position. At this
  scale, the map covers a square with 2.5 km sides
  (6.25 km2). Maps are oriented so true north is at
  the top.
• 3.3 When the user's current position is within
  the visible range pf the map. The system always
  indicates it with a red arrow. The arrow points
  in the direction the user is heading.
• 3.4 The system also displays in orange (
  computed in real time) the shortest route (in
  estimated travel time) from the current position
  to the center of the map. It will not be possible
  to display the entire route if the current
  position is not displayed.
• 3.5 When the user manipulates the trackball, the
  screen scrolls the map in the direction of
  rotation if the trackball, as if the user were
  grabbing the map.

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4.10 Detailed Example(4)

• GPS-base Automotive Navigation Assistant (GANA)
• 3.6. The LCD screen displays the labels Zoom
  Out, Zoom In, Go Current, Go destination,
  Set destination and Navigate above the six
  buttons (from left to right). The buttons work as
  follows
• Zoom In and Zoom Out display new maps. The scale
  of the map appears at the top right of the
  screen. There may be a delay retrieving a map, in
  which case the system displays a message
  Retrieving map. If the map or network is
  unavailable for any reason, the system displays
  Sorry. Map not available
• When the user presses Zoom In, the map scale is
  doubled so a smaller region is displayed, with
  more local detail. The maximum scale is 13125,
  means the map covers an area with 312.5 m sides
  (about 100,000 m2). If this scale is displayed,
  the Zoom In button is inoperative and its label
  appears in light gray.

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4.10 Detailed Example(5)

• GPS-base Automotive Navigation Assistant (GANA)
• When the user presses Zoom Out, the map scale is
  divided by 2 so a larger region is displayed,
  with less local detail. The minimum scale is
  1102,400,000, which means the map covers an area
  with sides pf approximately 10.000 km. If this
  scale is displayed, the Zoom Out button is
  inoperative and its label appears on light gray.
  Note that the scales are only approximate die to
  spherical aberration.
• When the user presses Set Destination, the
  location at the center of the screen (marked by
  the end of the orange route) is set as the
  destination. The shortest route from the current
  position to the destination is highlighted in red
  and is adjusted as the car moves.
• The shortest route to the set destination (red)
  is shown on top of the shortest route to the
  center of the screen (orange), and hence has
  precedence.
• When the user presses Go Current, the map jumps
  so it is centered over the current location.

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4.10 Detailed Example(6)

• GPS-base Automotive Navigation Assistant (GANA)
• When the user presses Go Destination, the map
  jumos so it is centerd over the destination. If
  no destination has been set, The destination
  defoults to the current location.
• When the user presses Navigate or the vehicle
  starts moving, the system enters navigation mode
  described bellow.
• 4. Navigation mode
• 4.1 A detailed map is never displayed in
  navigation mode since the user would not be able
  to concentrate on driving while looking at the
  map.
• 4.2 In no destination has been set, the system
  just displays the name of the cirrent highway or
  street and municipality in large font.
• 4.3 In addition if a destination has been serm
  the system displays the following in a very large
  font
• An arrow ponting up if the driver sgiuld do, in
  the follokwing formatTurn ltturning directiongt
  at ltturning landmarkgt and lthead heading
  directiongt on ltroad identificationgt towards
  ltnext landmarkgt.

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4.10 Detailed Example(7)

• GPS-base Automotive Navigation Assistant (GANA)
• The destination municipality (or address if
  within), the distance remaining, the expected
  tome remaining and the expected arrival time
• 4.4 The turning arrows and instructions are
  displayed as soon as possible as long as they
  cannot be interpreted ambiguously. So, for
  example a left turn would only appear when the
  driver must tale the next left turn.
• 4.5 The system displays the labels Speak Now,
  Volume Up, Volume Down, Guide On, Guide Off and
  Setup above the six buttons (from left to right).
  The buttons work as follows
• Speak Now Produces a computer-generated voice,
  reading the instructions that are on display.
  Every time the user presses the button. Any
  reading in progress is cancelled and the
  instructions are immediately read again starting
  from the beginning
• Volume Up and Volume Down adjust sound output.

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4.10 Detailed Example(8)

• GPS-base Automotive Navigation Assistant (GANA)
• Speak Now Produces a computer-generated voice,
  reading the instructions that are on display.
  Every time the user presses the button. Any
  reading in progress is cancelled and the
  instructions are immediately read again starting
  from the beginning
• Volume Up and Volume Down adjust sound output.
• Guide On causes a computer-generated voice to
  automatically read the instructions one minute in
  advance of any requited driver action, such as
  exiting the highway, being needed. Guide Off
  cancels this function the user would have to
  read the screen or press Speak Now. In situations
  where navigational action is required more
  frequently than once a minute, the voice reads
  the next instructions as soon as the system
  detects that the driver has responded to the
  previous instructions.
• Setup switches to setup mode if the car is
  stationary. If the car is not stationary, the
  Setup button is grayed out and is inactive.

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4.10 Detailed Example(9)

• GPS-base Automotive Navigation Assistant (GANA)
• 4.6 If the driver does not respond as expected
  to the instructions, and takes a different
  route, the system immediately calculates a new
  route.
• E. Non-functional requirements.
• 1. The system will be robust in the case of
  failure of the Internet connection or failure to
  receive the GPS signal, maintaining whatever
  service it can.
• 2. The system will be designed in a flexible way
  such that changes in wireless Internet or GPS
  technology can be incorporated in future
  releases.
• 3. The system will be designed anticipating
  incorporation of input from an inertial
  navigation unit that would take over in cases
  where GPS signals fail.

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4.13 Difficulties and Risks in Domain and
Requirements Analysis

• Lack of understanding of the domain or the real
  problem
• Do domain analysis and prototyping
• Requirements change rapidly
• Perform incremental development, build
  flexibility into the design, do regular reviews
• Attempting to do too much
• Document the problem boundaries at an early
  stage, carefully estimate the time
• It may be hard to reconcile conflicting sets of
  requirements
• Brainstorming, JAD sessions, competing prototypes
  
• It is hard to state requirements precisely
• Break requirements down into simple sentences and
  review them carefully, look for potential
  ambiguity, make early prototypes