Software Processes 4/muna software.pdfProgramming and debugging Translating a design into a program...
Transcript of Software Processes 4/muna software.pdfProgramming and debugging Translating a design into a program...
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 1
Software Processes
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 2
Objectives
To introduce software process models To describe three generic process models and
when they may be used To describe outline process models for
requirements engineering, software development, testing and evolution
To explain the Rational Unified Process model To introduce CASE technology to support
software process activities
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 3
Topics covered
Software process models Process iteration Process activities The Rational Unified Process Computer-aided software engineering
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 4
The software process
A structured set of activities required to develop a software system • Specification; • Design; • Validation; • Evolution.
A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 5
Generic software process models The waterfall model
• Separate and distinct phases of specification and development.
Evolutionary development • Specification, development and validation are
interleaved. Component-based software engineering
• The system is assembled from existing components.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 6
Waterfall model
Requir ementsdefinition
System andsoftware design
Implementa tionand unit testing
Integ ration andsystem testing
Oper ation andmaintenance
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 7
Waterfall model phases
Requirements analysis and definition System and software design Implementation and unit testing Integration and system testing Operation and maintenance The main drawback of the waterfall model is
the difficulty of accommodating change after the process is underway. One phase has to be complete before moving onto the next phase.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 8
Waterfall model problems
Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements.
Therefore, this model is only appropriate when the requirements are well-understood and changes will be fairly limited during the design process.
Few business systems have stable requirements.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 9
Evolutionary development
Exploratory development • Objective is to work with customers and to evolve
a final system from an initial outline specification. Should start with well-understood requirements and add new features as proposed by the customer.
Throw-away prototyping • Objective is to understand the system
requirements. Should start with poorly understood requirements to clarify what is really needed.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 10
Evolutionary development
Concurr entacti vities
Valida tionFinal
version
DevelopmentInter media te
versions
Specifica tionInitial
version
Outlinedescription
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 11
Evolutionary development
Problems • Lack of process visibility; • Systems are often poorly structured; • Special skills (e.g. in languages for rapid
prototyping) may be required. Applicability
• For small or medium-size interactive systems; • For parts of large systems (e.g. the user interface); • For short-lifetime systems.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 12
Component-based software engineering
Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems.
Process stages • Component analysis; • Requirements modification; • System design with reuse; • Development and integration.
This approach is becoming increasingly used as component standards have emerged.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 13
Reuse-oriented development
Requirementsspecification
Componentanalysis
Developmentand integ ration
System designwith reuse
Requirementsmodification
Systemvalidation
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 14
Process iteration
System requirements ALWAYS evolve in the course of a project so process iteration where earlier stages are reworked is always part of the process for large systems.
Iteration can be applied to any of the generic process models.
Two (related) approaches • Incremental delivery; • Spiral development.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 15
Incremental delivery
Rather than deliver the system as a single delivery, the development and delivery is broken down into increments with each increment delivering part of the required functionality.
User requirements are prioritised and the highest priority requirements are included in early increments.
Once the development of an increment is started, the requirements are frozen though requirements for later increments can continue to evolve.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 16
Incremental development
Valida teincrement
Develop systemincrement
Design systemarchitectur e
Integ rateincrement
Validatesystem
Define outline requirements
Assign requirements to increments
System incomplete
Finalsystem
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 17
Incremental development advantages
Customer value can be delivered with each increment so system functionality is available earlier.
Early increments act as a prototype to help elicit requirements for later increments.
Lower risk of overall project failure. The highest priority system services tend to
receive the most testing.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 18
Spiral development
Process is represented as a spiral rather than as a sequence of activities with backtracking.
Each loop in the spiral represents a phase in the process.
No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required.
Risks are explicitly assessed and resolved throughout the process.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 19
Spiral model of the software process
Riskanal ysis
Riskanal ysis
Riskanal ysis
Riskanal ysis Proto-
type 1
Prototype 2
Prototype 3Oper a-tionalpr oto ype
Concept ofOper a tion
Simula tions , models , benchmar ks
S/Wrequir ements
Requir ementvalida tion
DesignV&V
Productdesign Detailed
design
Code
Unit test
Integ ra tiontestAcceptance
testService De velop , verifyne xt-le vel pr oduct
Evalua te alterna tives,identify , resolv e risks
Deter mine objecti ves,alterna tives and
constr aints
Plan ne xt phase
Integ ra tionand test plan
De velopmentplan
Requir ements planLife-cy cle plan
REVIEW
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 20
Process activities
Software specification Software design and implementation Software validation Software evolution
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 21
Software specification
The process of establishing what services are required and the constraints on the system’s operation and development.
Requirements engineering process • Feasibility study; • Requirements elicitation and analysis; • Requirements specification; • Requirements validation.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 22
Software design and implementation
The process of converting the system specification into an executable system.
Software design • Design a software structure that realises the
specification; Implementation
• Translate this structure into an executable program;
The activities of design and implementation are closely related and may be inter-leaved.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 23
Design process activities
Architectural design Abstract specification Interface design Component design Data structure design Algorithm design
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 24
Structured methods
Systematic approaches to developing a software design.
The design is usually documented as a set of graphical models.
Possible models • Object model; • Sequence model; • State transition model; • Structural model; • Data-flow model.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 25
Programming and debugging
Translating a design into a program and removing errors from that program.
Programming is a personal activity - there is no generic programming process.
Programmers carry out some program testing to discover faults in the program and remove these faults in the debugging process.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 26
The debugging process
Locateerr or
Designerror repair
Repairerror
Re-testpr ogram
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 27
Software validation
Verification and validation (V & V) is intended to show that a system conforms to its specification and meets the requirements of the system customer.
Involves checking and review processes and system testing.
System testing involves executing the system with test cases that are derived from the specification of the real data to be processed by the system.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 28
Testing stages Component or unit testing
• Individual components are tested independently; • Components may be functions or objects or
coherent groupings of these entities. System testing
• Testing of the system as a whole. Testing of emergent properties is particularly important.
Acceptance testing • Testing with customer data to check that the
system meets the customer’s needs.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 29
Software evolution
Software is inherently flexible and can change. As requirements change through changing
business circumstances, the software that supports the business must also evolve and change.
Although there has been a demarcation between development and evolution (maintenance) this is increasingly irrelevant as fewer and fewer systems are completely new.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 30
Computer-aided software engineering
Computer-aided software engineering (CASE) is software to support software development and evolution processes.
Activity automation • Graphical editors for system model development; • Data dictionary to manage design entities; • Graphical UI builder for user interface construction; • Debuggers to support program fault finding; • Automated translators to generate new versions of a
program.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 31
Key points
Software processes are the activities involved in producing and evolving a software system.
Software process models are abstract representations of these processes.
General activities are specification, design and implementation, validation and evolution.
Generic process models describe the organisation of software processes. Examples include the waterfall model, evolutionary development and component-based software engineering.
Iterative process models describe the software process as a cycle of activities.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 4 Slide 32
Key points
Requirements engineering is the process of developing a software specification.
Design and implementation processes transform the specification to an executable program.
Validation involves checking that the system meets to its specification and user needs.
Evolution is concerned with modifying the system after it is in use.
CASE technology supports software process activities.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 1
Project management
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 2
Objectives
To explain the main tasks undertaken by project managers
To introduce software project management and to describe its distinctive characteristics
To discuss project planning and the planning process To show how graphical schedule representations are
used by project management To discuss the notion of risks and the risk
management process
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 3
Topics covered
Management activities Project planning Project scheduling Risk management
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 4
Concerned with activities involved in ensuring that software is delivered on time and on schedule and in accordance with the requirements of the organisations developing and procuring the software.
Project management is needed because software development is always subject to budget and schedule constraints that are set by the organisation developing the software.
Software project management
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 5
The product is intangible. The product is uniquely flexible. Software engineering is not recognized as an
engineering discipline with the sane status as mechanical, electrical engineering, etc.
The software development process is not standardised.
Many software projects are 'one-off' projects.
Software management distinctions
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 6
Proposal writing. Project planning and scheduling. Project costing. Project monitoring and reviews. Personnel selection and evaluation. Report writing and presentations.
Management activities
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 7
These activities are not peculiar to software management.
Many techniques of engineering project management are equally applicable to software project management.
Technically complex engineering systems tend to suffer from the same problems as software systems.
Management commonalities
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 8
Project staffing
May not be possible to appoint the ideal people to work on a project • Project budget may not allow for the use of highly-paid
staff; • Staff with the appropriate experience may not be
available; • An organisation may wish to develop employee skills
on a software project. Managers have to work within these constraints
especially when there are shortages of trained staff.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 9
Project planning
Probably the most time-consuming project management activity.
Continuous activity from initial concept through to system delivery. Plans must be regularly revised as new information becomes available.
Various different types of plan may be developed to support the main software project plan that is concerned with schedule and budget.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 10
Types of project plan
Plan Description
Quality plan Describes the quality procedures and standards that will beused in a project. See Chapter 27.
Validation plan Describes the approach, resources and schedule used forsystem validation. See Chapter 22.
Configurationmanagement plan
Describes the configuration management procedures andstructures to be used. See Chapter 29.
Maintenance plan Predicts the maintenance requirements of the system,maintenance costs and effort required. See Chapter 21.
Staff developmentplan.
Describes how the skills and experience of the project teammembers will be developed. See Chapter 25.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 11
Project planning process
Establish the project constraints Make initial assessments of the project parameters Define project milestones and deliverables while project has not been completed or cancelled loop Draw up project schedule Initiate activities according to schedule Wait ( for a while ) Review project progress Revise estimates of project parameters Update the project schedule Re-negotiate project constraints and deliverables if ( problems arise ) then Initiate technical review and possible revision end if end loop
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 12
The project plan
The project plan sets out: • The resources available to the project; • The work breakdown; • A schedule for the work.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 13
Project plan structure
Introduction. Project organisation. Risk analysis. Hardware and software resource
requirements. Work breakdown. Project schedule. Monitoring and reporting mechanisms.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 14
Activity organization
Activities in a project should be organised to produce tangible outputs for management to judge progress.
Milestones are the end-point of a process activity.
Deliverables are project results delivered to customers.
The waterfall process allows for the straightforward definition of progress milestones.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 15
Milestones in the RE process
Evalua tionrepor t
Prototypede velopment
Userrequirements
Requir ementsanal ysis
Feasibilityrepor t
Feasibilitystud y
Architectur aldesign
Designstud y
Systemrequirements
Requir ementsspecifica tion
ACTIVITIES
MILESTONES
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 16
Project scheduling
Split project into tasks and estimate time and resources required to complete each task.
Organize tasks concurrently to make optimal use of workforce.
Minimize task dependencies to avoid delays caused by one task waiting for another to complete.
Dependent on project managers intuition and experience.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 17
The project scheduling process
Estimate resourcesfor activities
Identify activitydependencies
Identifyactivities
Allocate peopleto activities
Softwarerequirements
Activity chartsand bar char ts
Create projectchar ts
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 18
Scheduling problems
Estimating the difficulty of problems and hence the cost of developing a solution is hard.
Productivity is not proportional to the number of people working on a task.
Adding people to a late project makes it later because of communication overheads.
The unexpected always happens. Always allow contingency in planning.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 19
Bar charts and activity networks
Graphical notations used to illustrate the project schedule.
Show project breakdown into tasks. Tasks should not be too small. They should take about a week or two.
Activity charts show task dependencies and the the critical path.
Bar charts show schedule against calendar time.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 20
Task durations and dependencies
Activity Duration (days) DependenciesT1 8T2 15T3 15 T1 (M1)T4 10T5 10 T2, T4 (M2)T6 5 T1, T2 (M3)T7 20 T1 (M1)T8 25 T4 (M5)T9 15 T3, T6 (M4)T10 15 T5, T7 (M7)T11 7 T9 (M6)T12 10 T11 (M8)
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 21
Activity network
star t
T2
M3T6
Finish
T10
M7T5
T7
M2T4
M5
T8
4/7 /03
8 da ys
1 4/7 /03 15 da ys
4/8/03
15 da ys
2 5/8/03
7 da ys
5/9/03
10 da ys
19/9/03
15 da ys
11/8/03
2 5 da ys
10 da ys
20 da ys
5 da ys25/7 /03
15 da ys
25/7 /03
1 8/7 /03
10 da ys
T1
M1 T3T9
M6
T11
M8
T12
M4
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 22
Activity timeline 4/7 11/7 18/7 25/7 1/8 8/8 15/8 22/8 29/8 5/9 12/9 19/9
T4
T1T2
M1
T7T3
M5T8
M3M2T6T5
M4T9
M7T10
M6T11
M8
T12
Star t
Finish
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 23
Staff allocation
4/7 11/7 18/7 25/7 1/8 8/8 15/8 2 2/8 2 9/8 5/9 1 2/9 19/9
T4
T8 T11
T12
T1
T3
T9
T2
T6 T10
T7
T5
Fred
Jane
Anne
Mary
Jim
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 24
Risk management
Risk management is concerned with identifying risks and drawing up plans to minimise their effect on a project.
A risk is a probability that some adverse circumstance will occur • Project risks affect schedule or resources; • Product risks affect the quality or performance of
the software being developed; • Business risks affect the organisation developing
or procuring the software.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 25
Software risks Risk Affects Description
Staff turnover Project Experienced staff will leave the project before it is finished.
Management change Project There will be a change of organisational management withdifferent priorities.
Hardware unavailability Project Hardware that is essential for the project will not bedelivered on schedule.
Requirements change Project andproduct
There will be a larger number of changes to therequirements than anticipated.
Specification delays Project andproduct
Specifications of essential interfaces are not available onschedule
Size underestimate Project andproduct
The size of the system has been underestimated.
CASE tool under-performance
Product CASE tools which support the project do not perform asanticipated
Technology change Business The underlying technology on which the system is built issuperseded by new technology.
Product competition Business A competitive product is marketed before the system iscompleted.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 26
The risk management process
Risk identification • Identify project, product and business risks;
Risk analysis • Assess the likelihood and consequences of these
risks; Risk planning
• Draw up plans to avoid or minimise the effects of the risk;
Risk monitoring • Monitor the risks throughout the project;
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 27
The risk management process
Risk avoidanceand contingency
plans
Risk planning
Prioritised risklist
Risk analysis
List of potentialrisks
Riskidentification
Riskassessment
Riskmonitoring
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 28
Risk identification
Technology risks. People risks. Organisational risks. Requirements risks. Estimation risks.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 29
Risks and risk types Risk type Possible risks
Technology The database used in the system cannot process as many transactions per secondas expected.Software components that should be reused contain defects that limit theirfunctionality.
People It is impossible to recruit staff with the skills required.Key staff are ill and unava ilable at critical times.Required training for staff is not available.
Organisational The organisation is restructured so that different management are responsible forthe project.Organisational financial problems force reductions in the project budget.
Tools The code generated by CASE tools is inefficient.CASE tools cannot be integrated.
Requirements Changes to requirements that require major design rework are proposed.Customers fail to understand the impact of requirements changes.
Estimation The time required to develop the software is underestimated.The rate of defect repair is underestimated.The size of the software is underestimated.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 30
Risk analysis
Assess probability and seriousness of each risk.
Probability may be very low, low, moderate, high or very high.
Risk effects might be catastrophic, serious, tolerable or insignificant.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 31
Risk analysis (i)
Risk Probability Effects
Organisational financial problems force reductions inthe project budget.
Low Catastrophic
It is impossible to recruit staff with the skills requiredfor the project.
High Catastrophic
Key staff are ill at critical times in the project. Moderate Serious
Software components that should be reused containdefects which limit their functionality.
Moderate Serious
Changes to requirements that require major designrework are proposed.
Moderate Serious
The organisation is restructured so that differentmanagement are responsible for the project.
High Serious
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 32
Risk analysis (ii)
Risk Probability Effects
The database used in the system cannot process asmany transactions per second as expec ted.
Moderate Serious
The time required to develop the software isunderestimated.
High Serious
CASE tools cannot be integrated. High Tolerable
Customers fail to understand the impact ofrequirements changes.
Moderate Tolerable
Required training for staff is not available. Moderate Tolerable
The rate of defect repair is underestimated. Moderate Tolerable
The size of the software is underestimated. High Tolerable
The code generated by CASE tools is inefficient. Moderate Insignificant
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 33
Risk planning
Consider each risk and develop a strategy to manage that risk.
Avoidance strategies • The probability that the risk will arise is reduced;
Minimisation strategies • The impact of the risk on the project or product will
be reduced; Contingency plans
• If the risk arises, contingency plans are plans to deal with that risk;
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 34
Risk management strategies (i)
Risk Strategy
Organisationalfinancial problems
Prepare a briefing document for senior managementshowing how th e project is making a very importantcontribution to the goals of the business.
Recruitmentproblems
Alert customer of potential difficulties and thepossibility of delays, investigate buying-incomponents.
Staff illness Reorganise team so that there is more overlap of workand people therefore understand each other’s jobs.
Defectivecomponents
Replace potentially defective components with bough t-in components of known reliability.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 35
Risk management strategies (ii)
Risk Strategy
Requirementschanges
Derive traceability information to assess requirementschange impact, maximise information hiding in thedesign.
Organisationalrestructuring
Prepare a briefing document for senior managementshowing how th e project is making a very importantcontribution to the goals of the business.
Databaseperformance
Investigate the possibility of buying a higher-performance database.
Underestimateddevelopment time
Investigate buying in components, investigate use of aprogram generator
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 36
Risk monitoring
Assess each identified risks regularly to decide whether or not it is becoming less or more probable.
Also assess whether the effects of the risk have changed.
Each key risk should be discussed at management progress meetings.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 37
Risk indicators
Risk type Potential indicators
Technology Late delivery of hardware or support software, many reportedtechnology problems
People Poor staff morale, poor relationships amongst team member,job availability
Organisational Organisational gossip, lack of action by senior management
Tools Reluctance by team members to use tools, complaints aboutCASE tools, demands for higher-powered workstations
Requirements Many requirements change requests, customer complaints
Estimation Failure to meet agreed schedule, failure to clear reporteddefects
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 38
Key points
Good project management is essential for project success.
The intangible nature of software causes problems for management.
Managers have diverse roles but their most significant activities are planning, estimating and scheduling.
Planning and estimating are iterative processes which continue throughout the course of a project.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 5 Slide 39
A project milestone is a predictable state where a formal report of progress is presented to management.
Project scheduling involves preparing various graphical representations showing project activities, their durations and staffing.
Risk management is concerned with identifying risks which may affect the project and planning to ensure that these risks do not develop into major threats.
Key points
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 1
Software Requirements
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 2
Objectives
To introduce the concepts of user and system requirements
To describe functional and non-functional requirements
To explain how software requirements may be organised in a requirements document
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 3
Topics covered
Functional and non-functional requirements User requirements System requirements Interface specification The software requirements document
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 4
Requirements engineering
The process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed.
The requirements themselves are the descriptions of the system services and constraints that are generated during the requirements engineering process.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 5
What is a requirement?
It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification.
This is inevitable as requirements may serve a dual function • May be the basis for a bid for a contract - therefore
must be open to interpretation; • May be the basis for the contract itself - therefore
must be defined in detail; • Both these statements may be called requirements.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 6
Requirements abstraction (Davis)
“If a company wishes to let a contract for a large software development project, itmust define its needs in a sufficiently abstract way that a solution is not pre-defined.The requirements must be written so that several contractors can bid for the contract,offering, perhaps, different ways of meeting the client organisation’s needs. Once acontract has been awarded, the contractor must write a system definition for the clientin more detail so that the client understands and can validate what the software willdo. Both o f these documents may be ca lled the requirements document for thesystem.”
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 7
Types of requirement User requirements
• Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers.
System requirements • A structured document setting out detailed
descriptions of the system’s functions, services and operational constraints. Defines what should be implemented so may be part of a contract between client and contractor.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 8
Definitions and specifications
1. The softw are m ust pr ovide a means of representing and1. accessing e xternal files cr ea ted b y other tools .
1.1 The user should be pr ovided with facilities to define the type of1.2 external files .1.2 Each e xternal file type ma y have an associa ted tool w hich ma y be1.2 applied to the file .1.3 Each e xternal file type ma y be r epr esented as a specific icon on1.2 the user’ s displa y.1.4 Facilities should be pr o vided for the icon r epresenting an1.2 external file type to be defined b y the user .1.5 When a user selects an icon r epr esenting an e xternal file , the1.2 effect of that selection is to apply the tool associated with the type of1.2 the external file to the file represented by the selected icon.
User requir ement definition
System requir ements specification
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 9
Requirements readers
Client mana gersSystem end-usersClient eng ineersContr actor mana gersSystem ar chitects
System end-usersClient eng ineersSystem ar chitectsSoftware de velopers
Client eng ineers (perha ps)System ar chitectsSoftware de velopers
Userrequir ements
Systemrequir ements
Software designspecifica tion
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 10
Functional and non-functional requirements
Functional requirements • Statements of services the system should provide, how the
system should react to particular inputs and how the system should behave in particular situations.
Non-functional requirements • constraints on the services or functions offered by the system
such as timing constraints, constraints on the development process, standards, etc.
Domain requirements • Requirements that come from the application domain of the
system and that reflect characteristics of that domain.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 11
Functional requirements
Describe functionality or system services. Depend on the type of software, expected users
and the type of system where the software is used.
Functional user requirements may be high-level statements of what the system should do but functional system requirements should describe the system services in detail.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 12
The LIBSYS system
A library system that provides a single interface to a number of databases of articles in different libraries.
Users can search for, download and print these articles for personal study.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 13
Examples of functional requirements
The user shall be able to search either all of the initial set of databases or select a subset from it.
The system shall provide appropriate viewers for the user to read documents in the document store.
Every order shall be allocated a unique identifier (ORDER_ID) which the user shall be able to copy to the account’s permanent storage area.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 14
Requirements imprecision
Problems arise when requirements are not precisely stated.
Ambiguous requirements may be interpreted in different ways by developers and users.
Consider the term ‘appropriate viewers’ • User intention - special purpose viewer for each
different document type; • Developer interpretation - Provide a text viewer that
shows the contents of the document.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 15
Requirements completeness and consistency
In principle, requirements should be both complete and consistent.
Complete • They should include descriptions of all facilities
required. Consistent
• There should be no conflicts or contradictions in the descriptions of the system facilities.
In practice, it is impossible to produce a complete and consistent requirements document.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 16
Non-functional requirements
These define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc.
Process requirements may also be specified mandating a particular CASE system, programming language or development method.
Non-functional requirements may be more critical than functional requirements. If these are not met, the system is useless.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 17
Non-functional classifications
Product requirements • Requirements which specify that the delivered product must
behave in a particular way e.g. execution speed, reliability, etc.
Organisational requirements • Requirements which are a consequence of organisational
policies and procedures e.g. process standards used, implementation requirements, etc.
External requirements • Requirements which arise from factors which are external to the
system and its development process e.g. interoperability requirements, legislative requirements, etc.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 18
Non-functional requirement types
Perfor mancerequir ements
Spacerequir ements
Usa bilityrequir ements
Efficiencyrequir ements
Relia bilityrequir ements
Portabilityrequir ements
Inter oper abilityrequir ements
Ethicalrequir ements
Leg islativerequir ements
Implementa tionrequir ements
Standar dsrequir ements
Deli veryrequir ements
Safetyrequir ements
Privacyrequir ements
Productrequir ements
Organisationalrequir ements
Externalrequir ements
Non-functionalrequir ements
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 19
Non-functional requirements examples Product requirement
8.1 The user interface for LIBSYS shall be implemented as simple HTML without frames or Java applets.
Organisational requirement 9.3.2 The system development process and deliverable documents shall
conform to the process and deliverables defined in XYZCo-SP-STAN-95.
External requirement 7.6.5 The system shall not disclose any personal information about
customers apart from their name and reference number to the operators of the system.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 20
Goals and requirements
Non-functional requirements may be very difficult to state precisely and imprecise requirements may be difficult to verify.
Goal • A general intention of the user such as ease of use.
Verifiable non-functional requirement • A statement using some measure that can be objectively
tested.
Goals are helpful to developers as they convey the intentions of the system users.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 21
Examples
A system goal • The system should be easy to use by experienced controllers
and should be organised in such a way that user errors are minimised.
A verifiable non-functional requirement • Experienced controllers shall be able to use all the system
functions after a total of two hours training. After this training, the average number of errors made by experienced users shall not exceed two per day.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 22
Requirements measures
Property Measure
Speed Processed transactions/secondUser/Event response timeScreen refresh time
Size M BytesNumber of ROM chips
Ease of use Training timeNumber of help frames
Reliability Mean time to failureProbability of unavailabilityRate of failure occurrenceAvailability
Robustness Time to restart after failurePercentage of events causing failureProbability of data corruption on failure
Portability Percentage of target dependent statementsNumber of target systems
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 23
Requirements interaction
Conflicts between different non-functional requirements are common in complex systems.
Spacecraft system • To minimise weight, the number of separate chips in
the system should be minimised. • To minimise power consumption, lower power chips
should be used. • However, using low power chips may mean that
more chips have to be used. Which is the most critical requirement?
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 24
Domain requirements
Derived from the application domain and describe system characteristics and features that reflect the domain.
Domain requirements be new functional requirements, constraints on existing requirements or define specific computations.
If domain requirements are not satisfied, the system may be unworkable.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 25
Library system domain requirements There shall be a standard user interface to all
databases which shall be based on the Z39.50 standard.
Because of copyright restrictions, some documents must be deleted immediately on arrival. Depending on the user’s requirements, these documents will either be printed locally on the system server for manually forwarding to the user or routed to a network printer.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 26
Train protection system
The deceleration of the train shall be computed as: • Dtrain = Dcontrol + Dgradient
where Dgradient is 9.81ms2 * compensated gradient/alpha and where the values of 9.81ms2
/alpha are known for different types of train.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 27
Domain requirements problems
Understandability • Requirements are expressed in the language of the
application domain; • This is often not understood by software engineers
developing the system. Implicitness
• Domain specialists understand the area so well that they do not think of making the domain requirements explicit.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 28
User requirements
Should describe functional and non-functional requirements in such a way that they are understandable by system users who don’t have detailed technical knowledge.
User requirements are defined using natural language, tables and diagrams as these can be understood by all users.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 29
Problems with natural language
Lack of clarity • Precision is difficult without making the document
difficult to read. Requirements confusion
• Functional and non-functional requirements tend to be mixed-up.
Requirements amalgamation • Several different requirements may be expressed
together.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 30
LIBSYS requirement
4..5 LIBSYS shall provide a financial accounting system that maintains records of all payments made by users of the system. System managers may configure this system so that regular users may receive discounted rates.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 31
Editor grid requirement
2.6 Grid facilities To assist in the positioning of entities on a diagram, the user may turn on a grid in either centimetres or inches, via an option on the control panel. Initially, the grid is off. The grid may be turned on and off at any time during an editing session and can be toggled between inches and centimetres at any time. A grid option will be provided on the reduce-to-fit view but the number of grid lines shown will be reduced to avoid filling the smaller diagram with grid lines.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 32
Requirement problems
Database requirements includes both conceptual and detailed information • Describes the concept of a financial accounting system that is
to be included in LIBSYS; • However, it also includes the detail that managers can
configure this system - this is unnecessary at this level. Grid requirement mixes three different kinds of
requirement • Conceptual functional requirement (the need for a grid); • Non-functional requirement (grid units); • Non-functional UI requirement (grid switching).
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 33
Structured presentation
2.6.1 Grid facilitiesThe editor shall provide a grid facility where a m atrix of horizontal andvertical lines provide a background to the editor window. This grid shall be apassive grid where the alignment of entities is the user's responsibility.Rationale: A grid helps the user to create a tidy diagram with well-spacedentities. Although an active grid, where entities 'snap-to' grid lines can be useful,the positioning is imprecise. The user is the best person to decide where entitiesshould be positioned.Specification: ECLIPSE/WS/Tools/DE/FS Section 5.6Source: Ray Wilson, Glasgow Office
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 34
Guidelines for writing requirements
Invent a standard format and use it for all requirements.
Use language in a consistent way. Use shall for mandatory requirements, should for desirable requirements.
Use text highlighting to identify key parts of the requirement.
Avoid the use of computer jargon.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 35
System requirements
More detailed specifications of system functions, services and constraints than user requirements.
They are intended to be a basis for designing the system.
They may be incorporated into the system contract.
System requirements may be defined or illustrated using system models discussed in Chapter 8.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 36
Requirements and design
In principle, requirements should state what the system should do and the design should describe how it does this.
In practice, requirements and design are inseparable • A system architecture may be designed to structure
the requirements; • The system may inter-operate with other systems
that generate design requirements; • The use of a specific design may be a domain
requirement.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 37
Problems with NL specification
Ambiguity • The readers and writers of the requirement must
interpret the same words in the same way. NL is naturally ambiguous so this is very difficult.
Over-flexibility • The same thing may be said in a number of different
ways in the specification. Lack of modularisation
• NL structures are inadequate to structure system requirements.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 38
Alternatives to NL specification
Notation Description
Structured naturallanguage
This approach depends on defining standard forms or templates to express therequirements specification.
Designdescriptionlanguages
This approach uses a language like a programming language but with more abstractfeatures to specify the requirements by defining an operational model of the system.This approach is not now widely used although it can be useful for interfacespecifications.
Graphicalnotations
A graphical language, supplemented by text annotations is used to define thefunctional requirements for the system. An early example of such a graphicallanguage was SADT. Now, use-case descriptions and sequence d iagrams arecommonly used .
Mathematicalspecifications
These are notations based on mathematical concepts such as finite-state machines orsets. These unambiguous specifications reduce the arguments between customer andcontractor about system functionality. Howeve r, most customers don’t understandformal specifications and are reluctant to accept it as a system contract.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 39
Structured language specifications
The freedom of the requirements writer is limited by a predefined template for requirements.
All requirements are written in a standard way. The terminology used in the description may be
limited. The advantage is that the most of the
expressiveness of natural language is maintained but a degree of uniformity is imposed on the specification.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 40
Form-based specifications
Definition of the function or entity. Description of inputs and where they come from. Description of outputs and where they go to. Indication of other entities required. Pre and post conditions (if appropriate). The side effects (if any) of the function.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 41
Form-based node specification
Insulin Pump/Control Software/SRS/3.3.2Function Compute insulin dose: Safe sugar levelDescription Computes the dose of insulin to be delivered when the current measured sugar level is in
the safe zone between 3 and 7 units.Inputs Current sugar reading (r2), the previous two readings (r0 and r1)Source Current sugar reading from sensor. Other readings from memory.Outputs CompDose ذ the dose in insulin to be deliveredDestination Main control loopAction: CompDose is zero if the sugar level is stable or falling or if the level is increasing but the rate of
increase is decreasing. If the level is increasing and the rate of increase is increasing, then CompDose iscomputed by dividing the difference between the current sugar level and the previous level by 4 androunding the result. If the result, is rounded to zero then CompDose is set to the minimum dose that canbe delivered.
Requires Two previous readings so that the rate of change of sugar level can be computed.Pre-condition The insulin reservoir contains at least the maximum allowed single dose of insulin..Post-condition r0 is replaced by r1 then r1 is replaced by r2Side-effects None
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 42
Tabular specification
Used to supplement natural language. Particularly useful when you have to define a
number of possible alternative courses of action.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 43
Tabular specification
Condition Action
Sugar level falling (r2 < r1) CompDose = 0
Sugar level stable (r2 = r1) CompDose = 0
Sugar level increasing and rate ofincrease decreasing ((r2-r1)<(r1-r0))
CompDose = 0
Sugar level increasing and rate ofincrease stable or increasing. ((r2-r1) (r1-r0))
CompDose = round ((r2-r1)/4)If rounded result = 0 thenCompDose = MinimumDose
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 44
Graphical models
Graphical models are most useful when you need to show how state changes or where you need to describe a sequence of actions.
Different graphical models are explained in Chapter 8.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 45
Sequence diagrams
These show the sequence of events that take place during some user interaction with a system.
You read them from top to bottom to see the order of the actions that take place.
Cash withdrawal from an ATM • Validate card; • Handle request; • Complete transaction.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 46
Sequence diagram of ATM withdrawal ATM Database
CardCard number
Card OKPIN request
PINOption menu
<<exception>>invalid card
Withdraw request
Amount request
Amount
Balance request
Balance
<<exception>>insufficient cash
Debit (amount)
Debit response
Card
Card removed
Cash
Cash removedReceipt
Validate card
Handle request
Completetransaction
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 47
Interface specification
Most systems must operate with other systems and the operating interfaces must be specified as part of the requirements.
Three types of interface may have to be defined • Procedural interfaces; • Data structures that are exchanged; • Data representations.
Formal notations are an effective technique for interface specification.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 48
PDL interface description
interface PrintServer {
// defines an abstract printer server// requires: interface Printer, interface PrintDoc// provides: initialize, print, displayPrintQueue, cancelPrintJob, switchPrinter
void initialize ( Printer p ) ;void print ( Printer p, PrintDoc d ) ;void displayPrintQueue ( Printer p ) ;void cancelPrintJob (Printer p, PrintDoc d) ;void switchPrinter (Printer p1, Printer p2, PrintDoc d) ;
} //PrintServer
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 49
The requirements document
The requirements document is the official statement of what is required of the system developers.
Should include both a definition of user requirements and a specification of the system requirements.
It is NOT a design document. As far as possible, it should set of WHAT the system should do rather than HOW it should do it
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 50
Users of a requirements document
Use the requirements todevelop validation tests for
the system
Use the requirementsdocument to plan a bid forthe system and to plan the
system development process
Use the requirements tounderstand what system is to
be developed
System testeng ineers
Managers
Systemeng ineers
Specify the requirements andread them to check that they
meet their needs. T heyspecify changes to the
requirements
Systemcustomers
Use the requirements to helpunderstand the system and
the relationships between itspar ts
Systemmaintenance
eng ineers
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 51
IEEE requirements standard
Defines a generic structure for a requirements document that must be instantiated for each specific system. • Introduction. • General description. • Specific requirements. • Appendices. • Index.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 52
Requirements document structure
Preface Introduction Glossary User requirements definition System architecture System requirements specification System models System evolution Appendices Index
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 53
Key points
Requirements set out what the system should do and define constraints on its operation and implementation.
Functional requirements set out services the system should provide.
Non-functional requirements constrain the system being developed or the development process.
User requirements are high-level statements of what the system should do. User requirements should be written using natural language, tables and diagrams.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 6 Slide 54
Key points
System requirements are intended to
communicate the functions that the system should provide.
A software requirements document is an agreed statement of the system requirements.
The IEEE standard is a useful starting point for defining more detailed specific requirements standards.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 1
System models
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 2
Objectives
To explain why the context of a system should be modelled as part of the RE process
To describe behavioural modelling, data modelling and object modelling
To introduce some of the notations used in the Unified Modeling Language (UML)
To show how CASE workbenches support system modelling
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 3
Topics covered
Context models Behavioural models Data models Object models CASE workbenches
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 4
System modelling
System modelling helps the analyst to understand the functionality of the system and models are used to communicate with customers.
Different models present the system from different perspectives • External perspective showing the system’s context or
environment; • Behavioural perspective showing the behaviour of the
system; • Structural perspective showing the system or data
architecture.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 5
Model types Data processing model showing how the data is
processed at different stages. Composition model showing how entities are
composed of other entities. Architectural model showing principal sub-systems. Classification model showing how entities have
common characteristics. Stimulus/response model showing the system’s
reaction to events.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 6
Context models
Context models are used to illustrate the operational context of a system - they show what lies outside the system boundaries.
Social and organisational concerns may affect the decision on where to position system boundaries.
Architectural models show the system and its relationship with other systems.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 7
The context of an ATM system
Auto-tellersystem
Securitysystem
Maintenancesystem
Accountda tabase
Usagedatabase
Branchaccounting
system
Branchcountersystem
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 8
Process models
Process models show the overall process and the processes that are supported by the system.
Data flow models may be used to show the processes and the flow of information from one process to another.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 9
Equipment procurement process
Get costestima tes
Acceptdeli very ofequipment
Checkdelivered
items
Valida tespecifica tion
Specifyequipment
requir ed
Choosesupplier
Placeequipment
order
Installequipment
Findsuppliers
Supplierda tabase
Acceptdeli vered
equipment
Equipmentda tabase
Equipmentspec.
Checkedspec.
Deliverynote
Deliverynote
Ordernotifica tion
Installa tioninstructions
Installa tionacceptance
Equipmentdetails
Check ed andsigned or der form
Orderdetails plusblank or der
for m
Spec. +supplier +estima te
Supplier listEquipment
spec.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 10
Behavioural models
Behavioural models are used to describe the overall behaviour of a system.
Two types of behavioural model are: • Data processing models that show how data is
processed as it moves through the system; • State machine models that show the systems
response to events. These models show different perspectives
so both of them are required to describe the system’s behaviour.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 11
Data-processing models
Data flow diagrams (DFDs) may be used to model the system’s data processing.
These show the processing steps as data flows through a system.
DFDs are an intrinsic part of many analysis methods.
Simple and intuitive notation that customers can understand.
Show end-to-end processing of data.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 12
Order processing DFD
Completeor der f orm
Orderdetails +
blankor der f orm
Valida teorder
Recor dor der
Send tosupplier
Adjustavailab lebudget
Budgetfile
Ordersfile
Completedorder f or m
Signedorder f or m
Signedorder f orm
Checked andsigned or der
+ or dernotifica tion
Orderamount
+ accountdetails
Signedorder f orm
Orderdetails
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 13
Data flow diagrams
DFDs model the system from a functional perspective.
Tracking and documenting how the data associated with a process is helpful to develop an overall understanding of the system.
Data flow diagrams may also be used in showing the data exchange between a system and other systems in its environment.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 14
Insulin pump DFD
Insulinrequir ementcomputa tion
Blood sugaranalysis
Blood sugarsensor
Insulindeli very
contr oller
Insulinpump
BloodBlood
parameters
Blood sugarlevel
InsulinPump contr ol
commands Insulinrequir ement
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 15
State machine models
These model the behaviour of the system in response to external and internal events.
They show the system’s responses to stimuli so are often used for modelling real-time systems.
State machine models show system states as nodes and events as arcs between these nodes. When an event occurs, the system moves from one state to another.
Statecharts are an integral part of the UML and are used to represent state machine models.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 16
Statecharts
Allow the decomposition of a model into sub-models (see following slide).
A brief description of the actions is included following the ‘do’ in each state.
Can be complemented by tables describing the states and the stimuli.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 17
Microwave oven model
Full po wer
Enabled
do: operateoven
Fullpow er
Halfpower
Halfpower
Fullpo wer
Number
Dooropen
Doorclosed
Doorclosed
Dooropen
Start
do: set power= 600
Half po werdo: set power
= 300
Set timedo: get number
exit: set time
Disab led
Operation
Cancel
Waiting
do: displaytime
Waiting
do: displaytime
do: display 'Ready'
do: display'Waiting'
Timer
Timer
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 18
Microwave oven state description
State Description
Waiting The oven is waiting for input. The display shows the current time.
Half power The oven power is set to 300 watts. The display shows شHalf powerص.
Full power The oven power is set to 600 watts. The display shows شFull powerص.
Set time The cooking time is s et to the userصs input value. The display shows the cooking timeselected and is updated as the time is set.
Disabled Oven operation is disabled for safety. Interior oven light is on. Display shows شNotreadyص.
Enabled Oven operation is enabled. Interior oven light is off. Display shows شReady to cookص.
Operation Oven in operation. Interior oven light is on. Display shows the timer countdown. Oncompletion of cooking, the buzzer is sounded for 5 s econds. Oven light is on. Displayshows شCooking completeص while buzzer is sounding.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 19
Microwave oven stimuli
Stimulus Description
Half power The user has pressed the half power button
Full power The user has pressed the full power button
Timer The user has pressed one of the timer buttons
Number The user has pressed a numeric key
Door open The oven door switch is not closed
Door closed The oven door switch is closed
Start The user has pressed the start button
Cancel The user has pressed the cancel button
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 20
Microwave oven operation
Cookdo: run
generator
Donedo: buzzer on
for 5 secs.
Waiting
Alarmdo: display
event
do: checkstatus
Checking
Turntablefault
Emitterfault
Disabled
OK
Timeout
Time
Door open Cancel
Operation
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 21
Semantic data models
Used to describe the logical structure of data processed by the system.
An entity-relation-attribute model sets out the entities in the system, the relationships between these entities and the entity attributes
Widely used in database design. Can readily be implemented using relational databases.
No specific notation provided in the UML but objects and associations can be used.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 22
Library semantic model
Sourcetitlepublisherissuedatepages
1
Articletitleauthorspdf filefee
has-links
1
Buyernameaddresse-mailbilling info
places
fee-payable-to
n
1
n
published-in
delivers in
m n
1
1
1
CopyrightAgencynameaddress
Country
copyright formtax rate
1
Order
order numbertotal paymentdatetax status
in
1
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 23
Data dictionaries
Data dictionaries are lists of all of the names used in the system models. Descriptions of the entities, relationships and attributes are also included.
Advantages • Support name management and avoid duplication; • Store of organisational knowledge linking analysis, design
and implementation;
Many CASE workbenches support data dictionaries.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 24
Data dictionary entries
Name Description Type Date
Article Details of the published article that may be ordered bypeople using LIBSYS. Entity 30.12.2002
authors The names of the authors of the article who may be duea share of the fee. Attribute 30.12.2002
Buyer The person or organisation that orders a co py of thearticle. Entity 30.12.2002
fee-payable-to
A 1:1 relationship between Article and the CopyrightAgency who should be paid the copyright fee. Relation 29.12.2002
Address(Buyer)
The address of the buyer. This is used to any paperbilling information that is required. Attribute 31.12.2002
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 25
Object models
Object models describe the system in terms of object classes and their associations.
An object class is an abstraction over a set of objects with common attributes and the services (operations) provided by each object.
Various object models may be produced • Inheritance models; • Aggregation models; • Interaction models.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 26
Object models
Natural ways of reflecting the real-world entities manipulated by the system
More abstract entities are more difficult to model using this approach
Object class identification is recognised as a difficult process requiring a deep understanding of the application domain
Object classes reflecting domain entities are reusable across systems
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 27
Inheritance models
Organise the domain object classes into a hierarchy. Classes at the top of the hierarchy reflect the
common features of all classes. Object classes inherit their attributes and services
from one or more super-classes. these may then be specialised as necessary.
Class hierarchy design can be a difficult process if duplication in different branches is to be avoided.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 28
Object models and the UML
The UML is a standard representation devised by the developers of widely used object-oriented analysis and design methods.
It has become an effective standard for object-oriented modelling.
Notation • Object classes are rectangles with the name at the top,
attributes in the middle section and operations in the bottom section;
• Relationships between object classes (known as associations) are shown as lines linking objects;
• Inheritance is referred to as generalisation and is shown ‘upwards’ rather than ‘downwards’ in a hierarchy.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 29
Library class hierarchy
Catalogue n umberAcquisition da teCostTypeStatusNumber of copies
Library item
Acquire ()Catalo gue ()Dispose ()Issue ()Return ()
AuthorEditionPub lication da teISBN
Book
YearIssue
Magazine
DirectorDate of releaseDistributor
Film
VersionPlatfor m
Computerprogram
TitlePub lisher
Pub lished item
TitleMedium
Recor ded item
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 30
User class hierarchy
NameAddressPhoneReg istration #
Library user
Reg ister ()De-reg ister ()
Affiliation
Reader
Items on loanMax. loans
Borrower
Depar tmentDepar tment phone
Staff
Major subjectHome ad dress
Student
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 31
Multiple inheritance
Rather than inheriting the attributes and services from a single parent class, a system which supports multiple inheritance allows object classes to inherit from several super-classes.
This can lead to semantic conflicts where attributes/services with the same name in different super-classes have different semantics.
Multiple inheritance makes class hierarchy reorganisation more complex.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 32
Multiple inheritance
# Tapes
Talking book
AuthorEditionPub lication da teISBN
Book
Speak erDurationRecor ding da te
Voice recording
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 33
Object aggregation
An aggregation model shows how classes that are collections are composed of other classes.
Aggregation models are similar to the part-of relationship in semantic data models.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 34
Object aggregation
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©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 35
Object behaviour modelling
A behavioural model shows the interactions between objects to produce some particular system behaviour that is specified as a use-case.
Sequence diagrams (or collaboration diagrams) in the UML are used to model interaction between objects.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 36
Issue of electronic items
:Library User
Ecat:Catalog
Lookup
Issue
Display
:Library ItemLib1:NetServer
Issue licence
Accept licence
Compress
Deliver
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 37
Structured methods
Structured methods incorporate system modelling as an inherent part of the method.
Methods define a set of models, a process for deriving these models and rules and guidelines that should apply to the models.
CASE tools support system modelling as part of a structured method.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 38
Method weaknesses
They do not model non-functional system requirements.
They do not usually include information about whether a method is appropriate for a given problem.
The may produce too much documentation. The system models are sometimes too
detailed and difficult for users to understand.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 39
CASE workbenches
A coherent set of tools that is designed to support related software process activities such as analysis, design or testing.
Analysis and design workbenches support system modelling during both requirements engineering and system design.
These workbenches may support a specific design method or may provide support for a creating several different types of system model.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 40
An analysis and design workbench
Centr alinfor ma tionrepository
Codegener ator
Querylangua gefacilities
Structur eddiag ramming
tools
Datadictionary
Repor tgener ation
facilities
Design, anal ysisand checking
tools
Formscr ea tion
tools
Impor t/e xpor tfacilities
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 41
Analysis workbench components
Diagram editors Model analysis and checking tools Repository and associated query language Data dictionary Report definition and generation tools Forms definition tools Import/export translators Code generation tools
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 42
Key points
A model is an abstract system view. Complementary types of model provide different system information.
Context models show the position of a system in its environment with other systems and processes.
Data flow models may be used to model the data processing in a system.
State machine models model the system’s behaviour in response to internal or external events
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 8 Slide 43
Key points
Semantic data models describe the logical structure of data which is imported to or exported by the systems.
Object models describe logical system entities, their classification and aggregation.
Sequence models show the interactions between actors and the system objects that they use.
Structured methods provide a framework for developing system models.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 1
Architectural Design
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 2
Objectives
To introduce architectural design and to discuss its importance
To explain the architectural design decisions that have to be made
To introduce three complementary architectural styles covering organisation, decomposition and control
To discuss reference architectures are used to communicate and compare architectures
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 3
Topics covered
Architectural design decisions System organisation Decomposition styles Control styles Reference architectures
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 4
Software architecture
The design process for identifying the sub-systems making up a system and the framework for sub-system control and communication is architectural design.
The output of this design process is a description of the software architecture.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 5
Architectural design
An early stage of the system design process. Represents the link between specification
and design processes. Often carried out in parallel with some
specification activities. It involves identifying major system
components and their communications.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 6
Advantages of explicit architecture
Stakeholder communication • Architecture may be used as a focus of
discussion by system stakeholders. System analysis
• Means that analysis of whether the system can meet its non-functional requirements is possible.
Large-scale reuse • The architecture may be reusable across a
range of systems.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 7
Architecture and system characteristics
Performance • Localise critical operations and minimise communications.
Use large rather than fine-grain components. Security
• Use a layered architecture with critical assets in the inner layers.
Safety • Localise safety-critical features in a small number of sub-
systems. Availability
• Include redundant components and mechanisms for fault tolerance.
Maintainability • Use fine-grain, replaceable components.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 8
Architectural conflicts
Using large-grain components improves performance but reduces maintainability.
Introducing redundant data improves availability but makes security more difficult.
Localising safety-related features usually means more communication so degraded performance.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 9
System structuring
Concerned with decomposing the system into interacting sub-systems.
The architectural design is normally expressed as a block diagram presenting an overview of the system structure.
More specific models showing how sub-systems share data, are distributed and interface with each other may also be developed.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 10
Packing robot control system
Visionsystem
Objectidentifica tion
system
Armcontr oller
Grippercontr oller
Packag ingselectionsystem
Packingsystem
Conveyorcontr oller
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 11
Box and line diagrams
Very abstract - they do not show the nature of component relationships nor the externally visible properties of the sub-systems.
However, useful for communication with stakeholders and for project planning.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 12
Architectural design decisions
Architectural design is a creative process so the process differs depending on the type of system being developed.
However, a number of common decisions span all design processes.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 13
Architectural design decisions
Is there a generic application architecture that can be used?
How will the system be distributed? What architectural styles are appropriate? What approach will be used to structure the system? How will the system be decomposed into modules? What control strategy should be used? How will the architectural design be evaluated? How should the architecture be documented?
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 14
Architecture reuse
Systems in the same domain often have similar architectures that reflect domain concepts.
Application product lines are built around a core architecture with variants that satisfy particular customer requirements.
Application architectures are covered in Chapter 13 and product lines in Chapter 18.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 15
Architectural styles
The architectural model of a system may conform to a generic architectural model or style.
An awareness of these styles can simplify the problem of defining system architectures.
However, most large systems are heterogeneous and do not follow a single architectural style.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 16
Architectural models
Used to document an architectural design. Static structural model that shows the major system
components. Dynamic process model that shows the process
structure of the system. Interface model that defines sub-system interfaces. Relationships model such as a data-flow model that
shows sub-system relationships. Distribution model that shows how sub-systems are
distributed across computers.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 17
System organisation
Reflects the basic strategy that is used to structure a system.
Three organisational styles are widely used: • A shared data repository style; • A shared services and servers style; • An abstract machine or layered style.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 18
The repository model
Sub-systems must exchange data. This may be done in two ways: • Shared data is held in a central database or
repository and may be accessed by all sub-systems;
• Each sub-system maintains its own database and passes data explicitly to other sub-systems.
When large amounts of data are to be shared, the repository model of sharing is most commonly used.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 19
CASE toolset architecture
Projectrepository
Designtransla tor
Programeditor
Designeditor
Codegener ator
Designanal yser
Repor tgener ator
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 20
Repository model characteristics
Advantages • Efficient way to share large amounts of data; • Sub-systems need not be concerned with how data is
produced Centralised management e.g. backup, security, etc.
• Sharing model is published as the repository schema. Disadvantages
• Sub-systems must agree on a repository data model. Inevitably a compromise;
• Data evolution is difficult and expensive; • No scope for specific management policies; • Difficult to distribute efficiently.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 21
Client-server model
Distributed system model which shows how data and processing is distributed across a range of components.
Set of stand-alone servers which provide specific services such as printing, data management, etc.
Set of clients which call on these services. Network which allows clients to access
servers.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 22
Film and picture library
Catalo gueserver
Librarycatalo gue
Videoserver
Film clipfiles
Pictureserver
Digitisedphoto g raphs
Web serv er
Film andphoto info.
Client 1 Client 2 Client 3 Client 4
Internet
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 23
Client-server characteristics
Advantages • Distribution of data is straightforward; • Makes effective use of networked systems. May require
cheaper hardware; • Easy to add new servers or upgrade existing servers.
Disadvantages • No shared data model so sub-systems use different data
organisation. Data interchange may be inefficient; • Redundant management in each server; • No central register of names and services - it may be hard
to find out what servers and services are available.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 24
Abstract machine (layered) model
Used to model the interfacing of sub-systems. Organises the system into a set of layers (or abstract
machines) each of which provide a set of services. Supports the incremental development of sub-
systems in different layers. When a layer interface changes, only the adjacent layer is affected.
However, often artificial to structure systems in this way.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 25
Version management system
Configuration management system layer
Database system layer
Operating system layer
Object management system layer
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 26
Modular decomposition styles
Styles of decomposing sub-systems into modules.
No rigid distinction between system organisation and modular decomposition.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 27
Sub-systems and modules
A sub-system is a system in its own right whose operation is independent of the services provided by other sub-systems.
A module is a system component that provides services to other components but would not normally be considered as a separate system.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 28
Modular decomposition
Another structural level where sub-systems are decomposed into modules.
Two modular decomposition models covered • An object model where the system is decomposed into
interacting object; • A pipeline or data-flow model where the system is
decomposed into functional modules which transform inputs to outputs.
If possible, decisions about concurrency should be delayed until modules are implemented.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 29
Object models
Structure the system into a set of loosely coupled objects with well-defined interfaces.
Object-oriented decomposition is concerned with identifying object classes, their attributes and operations.
When implemented, objects are created from these classes and some control model used to coordinate object operations.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 30
Invoice processing system
issue ()sendReminder ()acceptP ayment ()sendReceipt ()
invoice#dateamountcustomer
invoice#dateamountcustomer#
invoice#dateamountcustomer#
customer#namead dresscredit period
Customer
Payment
Invoice
Receipt
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 31
Object model advantages
Objects are loosely coupled so their implementation can be modified without affecting other objects.
The objects may reflect real-world entities. OO implementation languages are widely
used. However, object interface changes may
cause problems and complex entities may be hard to represent as objects.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 32
Function-oriented pipelining
Functional transformations process their inputs to produce outputs.
May be referred to as a pipe and filter model (as in UNIX shell).
Variants of this approach are very common. When transformations are sequential, this is a batch sequential model which is extensively used in data processing systems.
Not really suitable for interactive systems.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 33
Invoice processing system
Read issuedinvoices
Identifypayments
Issuereceipts
Findpa yments
due
Receipts
Issuepa ymentreminder
Reminders
Invoices Payments
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 34
Pipeline model advantages
Supports transformation reuse. Intuitive organisation for stakeholder
communication. Easy to add new transformations. Relatively simple to implement as either a
concurrent or sequential system. However, requires a common format for data
transfer along the pipeline and difficult to support event-based interaction.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 35
Control styles
Are concerned with the control flow between sub-systems. Distinct from the system decomposition model.
Centralised control • One sub-system has overall responsibility for
control and starts and stops other sub-systems. Event-based control
• Each sub-system can respond to externally generated events from other sub-systems or the system’s environment.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 36
Centralised control
A control sub-system takes responsibility for managing the execution of other sub-systems.
Call-return model • Top-down subroutine model where control starts at the
top of a subroutine hierarchy and moves downwards. Applicable to sequential systems.
Manager model • Applicable to concurrent systems. One system
component controls the stopping, starting and coordination of other system processes. Can be implemented in sequential systems as a case statement.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 37
Call-return model
Routine 1.2Routine 1.1 Routine 3.2Routine 3.1
Routine 2 Routine 3Routine 1
Mainprogram
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 38
Real-time system control
Systemcontr oller
Userinter face
Faulthandler
Computa tionpr ocesses
Actuatorprocesses
Sensorpr ocesses
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 39
Event-driven systems
Driven by externally generated events where the timing of the event is outwith the control of the sub-systems which process the event.
Two principal event-driven models • Broadcast models. An event is broadcast to all sub-
systems. Any sub-system which can handle the event may do so;
• Interrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing.
Other event driven models include spreadsheets and production systems.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 40
Broadcast model
Effective in integrating sub-systems on different computers in a network.
Sub-systems register an interest in specific events. When these occur, control is transferred to the sub-system which can handle the event.
Control policy is not embedded in the event and message handler. Sub-systems decide on events of interest to them.
However, sub-systems don’t know if or when an event will be handled.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 41
Selective broadcasting
Sub-system1
Event and messa ge handler
Sub-system2
Sub-system3
Sub-system4
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 42
Interrupt-driven systems
Used in real-time systems where fast response to an event is essential.
There are known interrupt types with a handler defined for each type.
Each type is associated with a memory location and a hardware switch causes transfer to its handler.
Allows fast response but complex to program and difficult to validate.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 43
Interrupt-driven control
Handler1
Handler2
Handler3
Handler4
Process1
Process2
Process3
Process4
Interrupts
Interruptvector
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 44
Reference architectures
Architectural models may be specific to some application domain.
Two types of domain-specific model • Generic models which are abstractions from a number of
real systems and which encapsulate the principal characteristics of these systems. Covered in Chapter 13.
• Reference models which are more abstract, idealised model. Provide a means of information about that class of system and of comparing different architectures.
Generic models are usually bottom-up models; Reference models are top-down models.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 45
Reference architectures
Reference models are derived from a study of the application domain rather than from existing systems.
May be used as a basis for system implementation or to compare different systems. It acts as a standard against which systems can be evaluated.
OSI model is a layered model for communication systems.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 46
OSI reference model
Presenta tion
Session
Transpor t
Network
Data link
Physical
7
6
5
4
3
2
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Comm unica tions medium
Networ k
Data link
Physical
Applica tion
Presenta tion
Session
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Data link
Physical
Applica tion
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 47
Case reference model
Data repository services • Storage and management of data items.
Data integration services • Managing groups of entities.
Task management services • Definition and enaction of process models.
Messaging services • Tool-tool and tool-environment communication.
User interface services • User interface development.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 48
The ECMA reference model
Toolslots
Messageservices
Task management servicesUser inter face services
Data repository servicesData integ ration services
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 49
Key points
The software architecture is the fundamental framework for structuring the system.
Architectural design decisions include decisions on the application architecture, the distribution and the architectural styles to be used.
Different architectural models such as a structural model, a control model and a decomposition model may be developed.
System organisational models include repository models, client-server models and abstract machine models.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 50
Key points
Modular decomposition models include object models and pipelining models.
Control models include centralised control and event-driven models.
Reference architectures may be used to communicate domain-specific architectures and to assess and compare architectural designs.
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 51
Architectural models
Different architectural models may be produced during the design process
Each model presents different perspectives on the architecture
©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 11 Slide 52
Architecture attributes
Performance • Localise operations to minimise sub-system communication
Security • Use a layered architecture with critical assets in inner layers
Safety • Isolate safety-critical components
Availability • Include redundant components in the architecture
Maintainability • Use fine-grain, self-contained components