Chapter 9 Design Engineering

Transcription

1 Software Engineering: A Practitioner s s Approach, 6/e Chapter 9 Design Engineering copyright 1996, 2001, 2005 R.S. Pressman & Associates, Inc. For University Use Only May be reproduced ONLY for student use at the university level when used in conjunction with Software Engineering: A Practitioner's Approach. Any other reproduction or use is expressly prohibited. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Analysis Model -> Design Model sc e n a r i o- ba se d e l e me n ts f l ow- or i e n te d e l e me n ts Com ponent - Lev el Design use-cases - text use-case diagrams activity diagrams swim lane diagrams Analysis Model data flow diagrams control-flow diagrams processing narratives Int erfa c e Design c l a ss- ba se d e l e me n ts class diagrams analysis packages CRC models collaboration diagrams be h a v i or a l e l e me n ts state diagrams sequence diagrams Arc hit ec t ura l Design Da t a / Cla ss Design Design Model 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

2 Design and Quality the design must implement all of the explicit requirements contained in the analysis model, and it must accommodate all of the implicit requirements desired by the customer. the design must be a readable, understandable guide for those who generate code and for those who test and subsequently support the software. the design should provide a complete picture of the software,, addressing the data, functional, and behavioral domains from an implementation perspective. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Quality Guidelines A design should exhibit an architecture that (1) has been created using recognizable architectural styles or patterns, (2) is composed of components that exhibit good design characteristics and (3) can be implemented in an evolutionary fashion For smaller systems, design can sometimes be developed linearly. A design should be modular; ; that is, the software should be logically partitioned into elements or subsystems A design should contain distinct representations of data, architecture, interfaces, and components. A design should lead to data structures that are appropriate for the classes to be implemented and are drawn from recognizable data patterns. A design should lead to components that exhibit independent functional characteristics. A design should lead to interfaces that reduce the complexity of connections between components and with the external environment. A design should be derived using a repeatable method that is driven by information obtained during software requirements analysis. A design should be represented using a notation that effectively communicates its meaning. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

3 Design Principles The design process should not suffer from tunnel vision. The design should be traceable to the analysis model. The design should not reinvent the wheel. The design should minimize the intellectual distance [DAV95] between the software and the problem as it exists in the real world. The design should exhibit uniformity and integration. The design should be structured to accommodate change. The design should be structured to degrade gently, even when aberrant data, events, or operating conditions are encountered. Design is not coding, coding is not design. The design should be assessed for quality as it is being created, not after the fact. The design should be reviewed to minimize conceptual (semantic) errors. From Davis [DAV95] 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Fundamental Concepts abstraction data, procedure, control architecture the the overall structure of the software patterns conveys the essence of a proven design solution modularity compartmentalization of data and function hiding controlled interfaces Functional independence single-minded function and low coupling refinement elaboration elaboration of detail for all abstractions Refactoring a a reorganization technique that simplifies the design 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

4 Data Abstraction door manufacturer model number type swing direction inserts lights type number weight opening mechanism implemented as a data structure 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Procedural Abstraction open details of enter algorithm implemented with a "knowledge" of the object that is associated with enter 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

5 Architecture The overall structure of the software and the ways in which that structure provides conceptual integrity for a system. [SHA95a] Structural properties. This aspect of the architectural design representation defines the components of a system (e.g., modules, objects, filters) and the manner in which those components are packaged and interact with one another. For example, objects are packaged to encapsulate both data and the processing that manipulates the data and interact via the invocation of methods Extra-functional properties. The architectural design description should address how the design architecture achieves requirements for performance, capacity, reliability, security, adaptability, and other system characteristics. Families of related systems. The architectural design should draw upon repeatable patterns that are commonly encountered in the design of families of similar systems. In essence, the design should have the ability to reuse architectural building blocks. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Patterns Design Pattern Template Pattern name describes the essence of the pattern in a short but expressive name Intent describes the pattern and what it does Also-known-as lists lists any synonyms for the pattern Motivation provides an example of the problem Applicability notes specific design situations in which the pattern is applicable Structure describes the classes that are required to implement the pattern Participants describes the responsibilities of the classes that are required to implement the pattern Collaborations describes how the participants collaborate to carry out their responsibilities Consequences describes the design forces that affect the pattern and the potential trade-offs that must be considered when the pattern is implemented Related patterns cross-references related design patterns 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

6 Modular Design easier to build, easier to change, easier to fix... 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Modularity: Trade-offs What is the "right" number of modules for a specific software design? cost of software module development cost module integration cost optimal number of modules number of modules 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

7 Information Hiding clients module controlled interface "secret" algorithm data structure details of external interface resource allocation policy a specific design decision 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Why Information Hiding? reduces the likelihood of side effects limits the global impact of local design decisions emphasizes communication through controlled interfaces discourages the use of global data leads to encapsulation an an attribute of high quality design results in higher quality software 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

8 open Stepwise Refinement walk to door; reach for knob; open door; walk through; close door. repeat until door opens turn knob clockwise; if knob doesn't turn, then take key out; find correct key; insert in lock; endif pull/push door move out of way; end repeat 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Functional Independence COHESION - the degree to which a module performs one and only one function. COUPLING - the degree to which a module is "connected" to other modules in the system. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

9 Sizing Modules: Two Views What's inside?? How big is it?? MODULE 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Refactoring Fowler [FOW99] defines refactoring in the following manner: "Refactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code [design] yet improves its internal structure. When software is refactored, the existing design is examined for redundancy unused design elements inefficient or unnecessary algorithms poorly constructed or inappropriate data structures or any other design failure that can be corrected to yield a better design. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

10 OO Design Concepts Design classes Entity classes Boundary classes Controller classes Inheritance all all responsibilities of a superclass is immediately inherited by all subclasses Messages stimulate stimulate some behavior to occur in the receiving object Polymorphism a a characteristic that greatly reduces the effort required to extend the design 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Design Classes Analysis classes are refined during design to become entity classes Boundary classes are developed during design to create the interface (e.g., interactive screen or printed reports) that the user sees and interacts with as the software is used. Boundary classes are designed with the responsibility of managing the way entity objects are represented to users. Controller classes are designed to manage the creation or update of entity objects; the instantiation of boundary objects as they obtain information from entity objects; complex communication between sets of objects; validation of data communicated between objects or between the user and the application. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

11 Inheritance Design options: The class can be designed and built from scratch. That is, inheritance is not used. The class hierarchy can be searched to determine if a class higher in the hierarchy (a superclass)contains most of the required attributes and operations. The new class inherits from the superclass and additions may then be added, as required. The class hierarchy can be restructured so that the required attributes and operations can be inherited by the new class. Characteristics of an existing class can be overridden and different versions of attributes or operations are implemented for the new class. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Messages :SenderObject message (<parameters>) :ReceiverObject 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

12 Conventional approach Polymorphism case of graphtype: if graphtype = linegraph then DrawLineGraph (data); if graphtype = piechart then DrawPieChart (data); if graphtype = histogram then DrawHisto (data); if graphtype = kiviat then DrawKiviat (data); end case; All of the graphs become subclasses of a general class called graph. Using a concept called overloading [TAY90], each subclass defines an operation called draw.. An object can send a draw message to any one of the objects instantiated from any one of the subclasses. The object receiving the message will invoke its own draw operation to create the appropriate graph. graphtype draw 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, high a na ly sis model The Design Model class diagrams analysis packages CRC models collaboration diagrams data flow diagrams control-flow diagrams processing narratives use-cases - text use-case diagrams activity diagrams swim lane diagrams collaboration diagrams state diagrams sequence diagrams class diagrams analysis packages CRC models collaboration diagrams data flow diagrams control-flow diagrams processing narratives state diagrams sequence diagrams Requirements: constraints interoperability targets and configuration design class realizations subsystems collaboration diagrams design model refinements to: design class realizations subsystems low collaboration diagrams technical interface design Navigation design GUI design component diagrams design classes activity diagrams sequence diagrams refinements to: component diagrams design classes activity diagrams sequence diagrams design class realizations subsystems collaboration diagrams component diagrams design classes activity diagrams sequence diagrams deployment diagrams architecture elements interface elements component-level elements process d imension deployment-level elements 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

13 Design Model Elements Data elements Data model --> data structures Data model --> database architecture Architectural elements Application domain Analysis classes, their relationships, collaborations and behaviors are transformed into design realizations Patterns and styles (Chapter 10) Interface elements the user interface (UI) external interfaces to other systems, devices, networks or other producers or consumers of information internal interfaces between various design components. Component elements Deployment elements 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Interface Elements WirelessPDA MobilePhone ControlPanel LCDdisplay LEDindicators keypadcharacteristics speaker wirelessinterface KeyPad readkeystroke() decodekey() displaystatus () lightleds() sendcontrolmsg() <<interface>> KeyPad readkeystroke() decodekey() Figure 9.6 UML interface representation for Cont rolpa nel 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

14 Component Elements SensorManagement Sensor 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Deployment Elements Control Panel CPI server Security homeowneraccess Personal computer externalaccess Security Surveillance homemanagement communication 6/e and are provided with permission by R.S. Pressman Figure 9.8 & Associates, UML deployment Inc., diagram copyright for SafeHome 1996, 2001,

15 Design Patterns The best designers in any field have an uncanny ability to see patterns that characterize a problem and corresponding patterns that can be combined to create a solution A description of a design pattern may also consider a set of design forces. Design forces describe non-functional requirements (e.g., ease of maintainability, portability) associated the software for which the pattern is to be applied. The pattern characteristics (classes, responsibilities, and collaborations) indicate the attributes of the design that may be adjusted to enable the pattern to accommodate a variety of problems. 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001, Frameworks A framework is not an architectural pattern, but rather a skeleton with a collection of plug points (also called hooks and slots) ) that enable it to be adapted to a specific problem domain. Gamma et al note that: Design patterns are more abstract than frameworks. Design patterns are smaller architectural elements than frameworks Design patterns are less specialized than frameworks 6/e and are provided with permission by R.S. Pressman & Associates, Inc., copyright 1996, 2001,

16 From Analysis to Design analysis model Maps into design model 1 Analysis to Design Data Object Description Entity- Relationship Diagram Data Flow Diagram Process Specification (PSPEC) procedural design Data Dictionary interface design State-Transition Diagram Control Specification (CSPEC) architectural design data design THE ANALYSIS MODEL THE DESIGN MODEL 2

17 Structured Design Uses problem definition to guide solution Transform the problem description (DFD) into a solution description via design strategies Manages complexity with black boxes Modules which can be reasoned about solely on their input and output Uses graphic tools (structure chart) Offers strategies for deriving solution Transform and transaction analysis Offers design quality criteria 3 Simplifying a System Structured design seeks to manage the complexity of large systems in two ways: Boss partitioning divide a large system into a collection of smaller, interrelated modules hierarchical organization arrange the modules into subgroups which report to (fewer) high-level modules Input Read Process Output Write 4

18 Partitioned Black Boxes Easily constructed Compose modules based only on I/O Easily tested Narrow search for faults based on test of I/O only Easily corrected Repair modules in isolation to give correct I/O Easily understood Lesser need to understand details of other modules Easily modified Add to system functionality by adding modules 5 Structured Design Tools Structure Chart Shows Partitioning each box is a module Hierarchy Managing modules are shown above with arrow pointing downward Communication Small flags indicate control, data, and descriptive information as it is passed from one module to another Structure Charts Provides A semi-formal view of system or program structure Documentation and blue-prints for programmers and maintainers 6

19 Structure Chart module A module B module C module D data control flag module E module F module G module H 7 Design Guidelines Cohesion the degree modules are sufficient to carry out one, single, well-defined function Coupling module independence preferred to tight interdependence Module Size reasonable size Span of Control known and limited Scope of Effect/Control clear path of relationships in the hierarchy 8

20 An Architectural Design Method customer requirements "four bedrooms, three baths, lots of glass..." architectural design 9 Deriving Program Architecture Program Architecture 10

21 Partitioning the Architecture horizontal and vertical partitioning are required 11 Horizontal Partitioning define separate branches of the module hierarchy for each major function use control modules to coordinate communication between functions function 1 function 3 function 2 12

22 Vertical Partitioning: Factoring design so that decision making and work are stratified decision making modules should reside at the top of the architecture decision-makers workers 13 Flow Characteristics Transform flow Transaction flow 14

23 Transform Mapping a c b data flow model d e f g i h j x1 x2 x3 x4 b c d e f g i a h j 15 First Level Factoring main program controller input controller processing controller output controller 16

24 Second Level Mapping D C B A main control mapping from the flow boundary outward A C B D 17 Transaction Flow incoming flow action path T 18

25 Transaction Mapping a d e f b t g h i x1 l j k t m n b x2 x3 x4 a d e f g h x3.1 l m n i j k 19 Transaction Example fixture setting fixture servos operator commands process operator commands report display screen assembly record robot control robot control software 20

26 Isolate Flow Paths read command command command validate command invalid command determine type send control value robot control produce error msg error msg status read fixture status read record combined status assembly record determine setting record calc. output values raw setting values format report format setting fixture setting report start/stop 21 Map the Flow Model process operator commands command input controller determine type read command validate command produce error message fixture status controller report generation controller send control value Specify response each of the action paths must be expanded further 22

27 Refining the Structure Chart process operator commands command input controller determine type read command validate command produce error message fixture status controller report generation controller send control value read fixture status determine setting format setting read record calculate output values format report 23 Library: Transformation or Transaction? Not a sequential straight line Multiple transactions types check in check out renew etc Transaction! 24

28 Company Payroll: Transformation or Transaction? One transaction type: calculate pay Sequential straight-line computation Read all employee records Calculate pay Print checks i.e., batch processing Transformation! 25 Integrated Case I-CASE Projectmanagement Prototyping Strategic planning Documentation Test Central encyclopedia or repository Database-scheme generator Codegeneration Bridge Enterprise data dictionary External toolkits Existing toolkits Reportgenerator queries Analysis and design Modification Workstation 26

29 CASE Tool Categories Procedure oriented tools. Primarily based on structured analysis and structured design. Data oriented tools. Primarily based on data modeling. Object oriented tools. Supporting object-oriented analysis, -design and -programming. Mathematical tools. Supporting parallelism and temporal dependencies. 27 Procedure oriented Tools Supporting methods of the seventies developed by Yourdon/ DeMarco and Gane/Sarson and others. Modeling techniques supported: Data Flow Diagrams Entity Relationship Diagrams Structure Charts Data dictionary Checks for data completeness, accuracy, and integrity Teamwork, Software through Pictures (StP), Foundation, ProKit*Workbench. 28

30 Restrictive, Guided or Flexible Tools Restrictive approach Providing methodology support by enforcing e.g. the levelling of data flow diagrams using top-down partitioning. The analyst would be required to start with the context diagram and decompose top down. Likewise structure charts would be derived from data flow diagrams, rather than produced independently. Guided approach Providing methodology support by suggesting, but not enforcing, top-down partitioning. The analyst would be informed about what should be done next and what operations are appropriate in a given situation. Flexible approach The analyst would be permitted to choose his/her own process and to choose whether or not the specifications produced are properly structured and consistent according to the methodology-base of the tool. Vessey, I., S. I. Jarvenpaa, and N. Tractinsky, 1992, Evaluation of Vendor Products: CASE Tools as Methodology Companions: Communications of the ACM, v. 35, p CASE Tool Pros Saves time Efficient organization of complex data Network use facilitates group work Supports modeling activities Provides consistent methodological framework Provides continuity and integration of life cycle phases 30

31 CASE Tool Cons Complicated to learn, use, and maintain Expensive Ineffective if used inappropriately Needs organization-wide support 31 A CASE of dashed Expectations Great Expectations Large productivity effects 25% Improvements for analysis 20% For requirements definition 30% For maintenance Rapid penetration Reality Productivity effects still inconclusive. Penetration slow, perhaps even declining. 32

32 Agile vs. SASD 33 Agile Agile and SASD SASD Business value for the customer/ user Develop as quickly and cheaply as possible Involve the user in development Iteratively elicit requirements (negotiation/emergence) Business value for the customer/ user Use models and work products to ensure quality User is involved early on, but not during development With req ts established, sequentially follow process model Priority on eliminating unneeded req ts Priority on traceability and work products 34

33 extreme Programming Practices The Planning Game Quickly determine the scope of the next release. Priorities Small releases Release new versions in very short cycles Metaphor Find a simple metaphor describing how the system works Simple design Make the design as simple as possible Testing Test the code continuously. Write the tests before the production code. Refactoring Restructure the code to remove duplications, improve communication, simplify, or add flexibility Pair programming Two programmers at one machine Collective ownership Everyone owns and can change any code anywhere in the system Continuous integration Integrate and build the system many times a day Sustainable pace Don t work more than 40 hours a week Whole team Include a real user in the team Coding standards Use a coding standard to improve communication 35 Supporting each other Whole team Metaphor Sustainable pace The Planning Game Refactoring Simple design Small releases Pair programming Testing Collective ownership Coding standards Continuous integration 36