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Title: Requirements Elicitation


1
Requirements Elicitation
  • Chapter 4
  • Object-Oriented Software Engineering
  • Using UML, Patterns, and Java, 2nd Edition
  • By B. Bruegge and A. Dutoit
  • Prentice Hall, 2004.

2
Requirements Elicitation Activities
  • Identifying Actors
  • Identifying Scenarios
  • Identifying Use Cases
  • Refining Use Cases
  • Identifying Relationships between Actors and Use
    Cases
  • Identifying Initial Analysis Objects
  • Identifying Nonfunctional Requirements

3
Identifying Actors
  • Actors person or machine using the system in a
    particular role
  • Actors usually correspond to existing roles
    within the client organization
  • Related roles can be grouped together according
    to viewpoints
  • Guide Questions
  • Which user groups are supported by the system to
    perform their work?
  • Which user groups execute the systems main
    functions?
  • Which user groups perform secondary functions,
    such as maintenance and administration?
  • With what external hardware or software system
    will the system interact?
  • Watch out for confusion between actors and objects

4
Identifying Scenarios
  • Scenario
  • A narrative description of what people do and
    experience as they try to make use of computer
    systems and applications Carrol, Scenario-based
    Design, 1995
  • Informal description of a single feature from the
    viewpoint of a single actor
  • Types of Scenarios
  • As-is scenarios describes current situation
  • Visionary scenarios describes future system
  • Evaluation scenarios describes user tasks for
    evaluating the system (acceptance criteria)
  • Training scenarios introduces new users to the
    system

5
Heuristics for Identifying Scenarios
  • Ask yourself or the client the following
    questions
  • What are the primary tasks that the system needs
    to perform?
  • What data will the actor create, store, change,
    remove or add in the system? Who else can modify
    this data?
  • What external changes does the system need to
    know about?
  • What changes or events will the actor of the
    system need to be informed about?
  • However, dont rely on questionnaires alone.
  • Insist on task observation (ethnography) if the
    system already exists
  • Ask to speak to the end user, not just to the
    software contractor
  • Expect resistance and try to overcome it

6
Scenario Example Warehouse on Fire
  • Bob, driving down main street in his patrol car
    notices smoke coming out of a warehouse. His
    partner, Alice, reports the emergency from her
    car.
  • Alice enters the address of the building, a brief
    description of its location (i.e., north west
    corner), and an emergency level. In addition to a
    fire unit, she requests several paramedic units
    on the scene given that area appear to be
    relatively busy. She confirms her input and waits
    for an acknowledgment.
  • John, the Dispatcher, is alerted to the emergency
    by a beep of his workstation. He reviews the
    information submitted by Alice and acknowledges
    the report. He allocates a fire unit and two
    paramedic units to the Incident site and sends
    their estimated arrival time (ETA) to Alice.
  • Alice received the acknowledgment and the ETA.

7
Observations about Warehouse on Fire Scenario
  • Concrete scenario
  • Describes a single instance of reporting a fire
    incident.
  • Does not describe all possible situations in
    which a fire can be reported.
  • Participating actors
  • Bob, Alice and John

8
Identifying Use Cases
  • Use Case
  • Specifies all possible scenarios for a given
    functionality
  • Initiated by an actor
  • Motivations for use cases
  • Generalizing related scenarios help developers
    define the scope of the system
  • The role of each user of the system is clarified
  • Use Case Descriptions
  • Entry and exit conditions
  • Flow of events
  • Quality requirements

9
Heuristics How do I find use cases?
  • Select a narrow vertical slice of the system
    (i.e. one scenario)
  • Discuss it in detail with the user to understand
    the users preferred style of interaction
  • Select a horizontal slice (i.e. many scenarios)
    to define the scope of the system.
  • Discuss the scope with the user
  • Use illustrative prototypes (mock-ups) as visual
    support
  • Find out what the user does
  • Task observation (Good)
  • Questionnaires (Bad)

10
Order of steps when formulating use cases
  • First step name the use case
  • Use case name ReportEmergency
  • Second step Find the actors
  • Generalize the concrete names (Bob) to
    participating actors (Field officer)
  • Participating Actors
  • Field Officer (Bob and Alice in the Scenario)
  • Dispatcher (John in the Scenario)
  • Third step Then concentrate on the flow of
    events
  • Use informal natural language

11
Use Case Example ReportEmergency
  • Use case name ReportEmergency
  • Participating Actors
  • Field Officer (Bob and Alice in the Scenario)
  • Dispatcher (John in the Scenario)
  • Exceptions
  • The FieldOfficer is notified immediately if the
    connection between her terminal and the central
    is lost.
  • The Dispatcher is notified immediately if the
    connection between any logged in FieldOfficer and
    the central is lost.
  • Flow of Events on next slide.
  • Special Requirements
  • The FieldOfficers report is acknowledged within
    30 seconds. The selected response arrives no
    later than 30 seconds after it is sent by the
    Dispatcher.

12
Use Case Example ReportEmergencyFlow of Events
  • The FieldOfficer activates the Report Emergency
    function of her terminal. FRIEND responds by
    presenting a form to the officer.
  • The FieldOfficer fills the form, by selecting the
    emergency level, type, location, and brief
    description of the situation. The FieldOfficer
    also describes possible responses to the
    emergency situation. Once the form is completed,
    the FieldOfficer submits the form, at which
    point, the Dispatcher is notified.
  • The Dispatcher reviews the submitted information
    and creates an Incident in the database by
    invoking the OpenIncident use case. The
    Dispatcher selects a response and acknowledges
    the emergency report.
  • The FieldOfficer receives the acknowledgment and
    the selected response.

13
Identifying Use Cases
  • Writing Guide
  • Choose proper name use verb phrases indicate
    users objective
  • Name actors with noun phrases
  • Clearly distinguish actors actions from systems
    actions
  • Use active voice to phrase steps in flow of
    events
  • The causal relationship between steps should be
    clear
  • Describe complete user transaction
  • Describe exceptions separately
  • Do not describe the user interface
  • Use cases should not exceed 2-3 pages break up
    using ltltincludegtgt and ltltextendsgtgt relationships

14
Refining Use Cases
  • Goal completeness and correctness
  • Refining use case descriptions leads to other use
    cases and clarifies system boundaries
  • Entry and exit conditions additional use cases
    are identified as entry and exit conditions are
    refined
  • Flow of events discussing flow of events
    clarifies system boundaries
  • Quality requirements elicit nonfunctional
    requirements in the context of this particular
    functionality
  • Refinements
  • Details of objects in the system
  • Low-level interactions between actors and system
  • Access rights
  • Missing exceptions
  • Common functionality among use cases

15
Refining Use Cases
  • Heuristics
  • Use scenarios to communicate with users and
    validate functionality
  • Refine a single scenario to understand users
    assumptions
  • Define many high-level scenarios to determine
    scope of the system
  • Use mock-ups or prototypes for visual support
  • Present user with a range of alternatives
  • Detail a broad vertical slice when scope of
    system and user preferences are well-understood

16
Relationships Between Actors and Use Cases
  • Relationships between actors and use cases
  • ltltinitiategtgt
  • ltltparticipategtgt
  • Determines access rights
  • Who can initiate a functionality
  • Who else is involved in this functionality
  • Relationships between use cases
  • Heuristics for making use cases shorter and
    simpler to understand
  • ltltincludegtgt
  • For factoring out common functionality
  • Explicitly invoked from the including use case
  • ltltextendgtgt
  • For specifying exceptions
  • Entry conditions of the extending use case
    determine when it is used
  • Caveat use discretion when applying these
    decompositions (a few longer use cases are
    sometimes easier to understand than many short
    ones)

17
ltltIncludegtgt Functional Decomposition
  • Problem
  • A function in the original problem statement is
    too complex to be solvable immediately
  • Solution
  • Describe the function as the aggregation of a
    set of simpler functions. The associated use case
    is decomposed into smaller use cases

18
ltltIncludegtgt Reuse of Existing Functionality
  • Problem
  • There are already existing functions. How can we
    reuse them?
  • Solution
  • The include association from a use case A to a
    use case B indicates that an instance of the use
    case A performs all the behavior described in the
    use case B (A delegates to B)
  • Example
  • The use case ViewMap describes behavior that
    can be used by the use case OpenIncident
    (ViewMap is factored out)

Base Use Case
Supplier Use Case
Note The base case cannot exist alone. It is
always called with the supplier use case
19
ltExtendgtgt Association for Use Cases
  • Problem
  • The functionality in the original problem
    statement needs to be extended.
  • Solution
  • An extend association from a use case A to a use
    case B indicates that use case B is an extension
    of use case A.
  • Example
  • The use case ReportEmergency is complete by
    itself , but can be extended by the use case
    ConnectionDown for a specific scenario in which
    the user cannot communicate with the dispatcher

Note The base use case can be executed without
the use case extension in extend associations.
20
Generalization association in use cases
  • Problem
  • You have common behavior among use cases and want
    to factor this out.
  • Solution
  • The generalization association among use cases
    factors out common behavior. The child use cases
    inherit the behavior and meaning of the parent
    use case and add or override some behavior.
  • Example
  • Consider the use case ValidateUser, responsible
    for verifying the identity of the user. The
    customer might require two realizations
    CheckPassword and CheckFingerprint

Parent Case
Child Use Case
21
Identifying Initial Analysis Objects

Le
v
el 1
Top Level Use Case
A and B are called Participating Objects
22
Use Cases can be used by more than one object

Le
v
el 1
Top Level Use Case
Level 2 Use Cases
Le
v
el 2
Le
v
el 2
Level 3 Use Cases
Le
v
el 3
Le
v
el 3
Le
v
el 3
Operations
Le
v
el 4
Le
v
el 4
A
B
Participating Objects
23
Identifying Initial Analysis Objects
  • Identify the participating objects to create the
    initial analysis object model
  • Maintaining glossary of objects minimizes
    potential confusion in terminology between users
    and developers
  • Heuristics
  • Terms the needed clarification (by developer or
    user)
  • Recurring nouns in use cases
  • Real-world entities and resources that system
    must track
  • Use cases
  • Data sources or sinks
  • Artifacts with which user interacts
  • Use application domain terms
  • Cross-check
  • Eliminate ambiguity verify that objects with the
    same name refer to the same concept
  • Maintain consistency verify that objects do not
    refer to the same concept using different names
  • Eliminate objects not involved in any use cases

24
Identifying Nonfunctional Requirements(FURPS
Classification Scheme)
  • Quality Requirements
  • Usability
  • Reliability/Dependability
  • Safety
  • Security
  • Survivability
  • Performance
  • Response Time
  • Throughput
  • Availability
  • Accuracy
  • Supportability
  • Adaptability
  • Maintainability
  • Portability
  • Pseudo Requirements
  • Implementation
  • Interface
  • Operations
  • Packaging
  • Legal

25
Identifying Nonfunctional Requirements
  • Heuristics
  • Use a taxonomy (e.g., FURPS) to generate
    checklists
  • Give different checklists to users in appropriate
    roles
  • Checklists vary depending on application domain

26
Nonfunctional Requirements Trigger Questions
  • User interface and human factors
  • What type of user will be using the system?
  • Will more than one type of user be using the
    system?
  • What sort of training will be required for each
    type of user?
  • Is it particularly important that the system be
    easy to learn?
  • Is it particularly important that users be
    protected from making errors?
  • What sort of input/output devices for the human
    interface are available, and what are their
    characteristics?
  • Documentation
  • What kind of documentation is required?
  • What audience is to be addressed by each
    document?
  • Hardware considerations
  • What hardware is the proposed system to be used
    on?
  • What are the characteristics of the target
    hardware, including memory size and auxiliary
    storage space?

27
Nonfunctional Requirements, ctd
  • Performance characteristics
  • Are there any speed, throughput, or response time
    constraints on the system?
  • Are there size or capacity constraints on the
    data to be processed by the system?
  • Error handling and extreme conditions
  • How should the system respond to input errors?
  • How should the system respond to extreme
    conditions?
  • System interfacing
  • Is input coming from systems outside the proposed
    system?
  • Is output going to systems outside the proposed
    system?
  • Are there restrictions on the format or medium
    that must be used for input or output?

28
Nonfunctional Requirements, ctd
  • Quality issues
  • What are the requirements for reliability?
  • Must the system trap faults?
  • What is the maximum time for restarting the
    system after a failure?
  • What is the acceptable system downtime per
    24-hour period?
  • Is it important that the system be portable (able
    to move to different hardware or operating system
    environments)?
  • System Modifications
  • What parts of the system are likely candidates
    for later modification?
  • What sorts of modifications are expected?
  • Physical Environment
  • Where will the target equipment operate?
  • Will the target equipment be in one or several
    locations?
  • Will the environmental conditions in any way be
    out of the ordinary (for example, unusual
    temperatures, vibrations, magnetic fields, ...)?

29
Nonfunctional Requirements, ctd
  • Security Issues
  • Must access to any data or the system itself be
    controlled?
  • Is physical security an issue?
  • Resources and Management Issues
  • How often will the system be backed up?
  • Who will be responsible for the back up?
  • Who is responsible for system installation?
  • Who will be responsible for system maintenance?

30
Constraints (Pseudo Requirements)
  • Constraint
  • Any client restriction on the solution domain
  • Examples
  • The target platform must be an IBM/360
  • The implementation language must be COBOL
  • The documentation standard X must be used
  • A dataglove must be used
  • ActiveX must be used
  • The system must interface to a papertape reader

31
How to Specify a Use Case (Summary)
  • Name of Use Case
  • Actors
  • Description of Actors involved in use case)
  • Entry condition
  • This use case starts when
  • Flow of Events
  • Free form, informal natural language
  • Exit condition
  • This use cases terminates when
  • Exceptions
  • Describe what happens if things go wrong
  • Special Requirements
  • Nonfunctional Requirements, Constraints

32
Additional slides
33
Managing Requirements Elicitation
  • Negotiating specifications
  • Maintaining traceability
  • Tool support
  • Requirements validation

34
Negotiating Specifications (JAD)
  • Use case modeling is useful in requirements
    elicitation, but it is not the only activity
  • Requirements have to be identified and negotiated
    between different stakeholders
  • JAD Joint Application Design
  • A moderated meeting with all stakeholders
    participating
  • Users, clients, developers trained facilitator
  • Leverages group dynamics of face-to-face meetings
  • Developers get to understand application domain
  • Users get to understand potential solution domain
    tradeoffs

35
JAD Activities
  • Project definition
  • Facilitator determines objectives and scope of
    project through interviews with project manager
    and client
  • Research
  • Facilitator interviews present and future users
  • Facilitator gathers information about application
    domain
  • Facilitator creates initial high-level use cases
  • Facilitator creates initial list of problems
  • Preparation
  • Facilitator creates Working Document, agenda and
    presentation materials
  • Facilitator forms team with adequate
    representation from all stakeholders

36
JAD Activities
  • Session
  • Facilitator guides team in creating the
    requirements specification
  • Discover new requirements
  • Classify and organize requirements
  • Prioritize requirements
  • Validate requirements
  • Derive use cases
  • Activities are repeated until closure is achieved
  • Final document preparation
  • Facilitator prepares Final Document
  • Team reviews and approves Final Document

37
JAD Facilitator
  • Qualifications of JAD facilitator is crucial
  • Must keep the discussion within the scope of the
    project
  • Discern wants from needs
  • Keep the discussion within the application domain
    to avoid prescribing requirements that restrict
    the solution space unnecessarily (pushing
    specific technology, methodology or language)
  • Mediate disputes before they get out of hand
  • Watch out for political influences and hidden
    agendas

38
Maintaining Traceability
  • Traceability the ability to follow the life of a
    requirement as it is translated into design and
    then implementation and test cases
  • The system is complete when all requirements can
    be traced to implementation
  • Traceability also enables developers to uncover
    the rationale behand certain requirements and
    design decisions
  • Traceability is harder for nonfunctional
    requirements
  • Traceability is difficult to maintain manually
  • Need to maintain cross-references between
    different artifacts (requirements, design
    documents, code, test plan, user documentation)
  • Need tool support

39
Tool Support
  • Requirements for Managing Requirements
  • Store requirements in a shared repository
  • Provide multi-user access
  • Automatically create a system specification
    document from the repository
  • Allow change management
  • Provide traceability throughout the project
    lifecycle

40
Requirements Validation
  • Activity involving the client and user
  • Requirements validation is a critical step in the
    development process, usually after requirements
    engineering or requirements analysis. Also at
    delivery (client acceptance test).

41
Requirements Validation Criteria
  • Completeness
  • All possible scenarios through the system are
    described, including exceptions
  • Consistency
  • There are no contradicting requirements
  • Clarity/Unambiguity
  • The specification can only be interpreted one way
  • Correctness
  • Requirements represent accurately the system the
    client needs
  • Realism
  • The system can be implemented within constraints
  • Verifiability
  • Tests can be designed to demonstrate the system
    fulfills its requirements
  • Traceability
  • Requirements can be traced to system functions
  • System functions can be traced to requirements
  • Dependencies among requirements, system
    functions, and everything else in between can be
    tracked.

42
Summary
  • The requirements process consists of requirements
    elicitation and analysis.
  • The requirements elicitation activity is
    different for
  • Greenfield Engineering, Reengineering, Interface
    Engineering
  • Scenarios
  • Great way to establish communication with client
  • Different types of scenarios As-Is, visionary,
    evaluation and training
  • Use cases Abstraction of scenarios
  • Pure functional decomposition is bad
  • Leads to unmaintainable code
  • Pure object identification is bad
  • May lead to wrong objects, wrong attributes,
    wrong methods
  • The key to successful analysis
  • Start with use cases and then find the
    participating objects
  • If somebody asks What is this?, do not answer
    right away. Return the question or observe the
    end user What is it used for?
  • Use case modeling is a part of the requirements
    elicitation process, not the entire process
    itself.
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