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SOEN 343 Software Design

SOEN 343 Software Design. Section H Fall 2006 Dr Greg Butler http://www.cs.concordia.ca/~gregb/home/soen343h-f06.html. Responsibilities, Principles, Patterns. RDD (Responsibility Driven Design) GRASP Principles Cohesion, Coupling Introduction to Patterns and Architecture.

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SOEN 343 Software Design

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  1. SOEN 343Software Design Section H Fall 2006 Dr Greg Butler http://www.cs.concordia.ca/~gregb/home/soen343h-f06.html

  2. Responsibilities, Principles, Patterns RDD (Responsibility Driven Design) GRASP Principles Cohesion, Coupling Introduction to Patterns and Architecture

  3. Responsibility-Driven Design (RDD) • Detailed object design is usually done from the point of view of the metaphor of: • Objects have responsibilities • Objects collaborate • Responsibilities are an abstraction. • The responsibility for persistence. • Large-grained responsibility. • The responsibility for the sales tax calculation. • More fine-grained responsibility.

  4. The 9 GRASP Principles • Creator • Expert • Controller • Low Coupling • High Cohesion • Polymorphism • Pure Fabrication • Indirection • Protected Variations

  5. Object Responsibilities • A responsibility is an obligation of an object in terms of its behavior.

  6. General Classification of Kinds of Responsibility • To know. • To do. • To decide.

  7. Responsibilities – A Boat Metaphor • What kind of responsibilities do each of the following “objects” have: … • To know. • To do. • To decide.

  8. Responsibilities – A Boat Metaphor Kind of responsibility for: • Captain • To know? • To do? • To decide?

  9. Responsibilities – A Boat Metaphor Kind of responsibility for: • Navigator. • To know? • To do? • To decide?

  10. Responsibilities – A Boat Metaphor Kind of responsibility for: • Compass. • To know? • To do? • To decide?

  11. RDD Example: Apply IE Information Expert: Give task to the object having the information to perform the task. Example: Larman 17.11 NextGEN POS application “Who should be responsible for knowing the grand total of a sale?”

  12. Fig. 9.2 NextGEN Domain Model

  13. Fig. 17.14 NextGEN Design

  14. IE Example Responsibilities • Sale: knows sale total • SalesLineItem: knows line item subtotal • ProductDescription: knows product price

  15. Fig. 17.17 NextGEN Design

  16. RDD Example: Apply Creator Larman 17.10: NextGEN example “Who should be responsible for creating a new SalesItem instance? Exercise!

  17. Design Principles Design for change

  18. GRASP General Responsibility Assignment Software Patterns. • Information Expert • Creator • Low Coupling • High Cohesion • Controller (still to come, from ch17) • Polymorphism • …

  19. Cohesion • Measure of the degree of “relatedness” that elements within a module share. • Degree to which the tasks performed by a single module are functionally related. • Brain storm: • Why put procedures/methods together within a module/class?

  20. Levels Of Cohesion

  21. Coupling • Measures the degree of dependency that exists between modules. • Brain storm: • Give examples of code that creates coupling.

  22. Coupling A uses a service/method m of B A passes on an object o returned from B.m() A provides visibility of B to C by returning a reference to B in A.getB() A.m( B b, …) A calls C.m(B b …) which expects a B object A class X in A has an attribute of type Y defined in B

  23. Coupling A.m() changes an attribute in B via B.setX() A.m() changes a (public) attribute in B directly via assignment A changes a “flag” in B (ie an attribute which controls execution decisions in B; ie behaviour of B as seen by others) A and B both refer to an object o, and A can change o …

  24. How Do I Come Up With a Design? • Given • Product requirements. • Project plan • How do I come up with a design?

  25. Design – Repeat Successes • Has a (successful) similar product been built? • Yes, then reuse domain specific: • Architectural • Style (e.g. client/server, database, process control) • Patterns. • Design Patterns (& idioms). • Use Domain model as source of inspiration.

  26. Design – New Application Area? • Has a (successful) similar product been built? • No, then choose among general: • Architectural • Style (e.g. client/server, database, process control) • Patterns. • Design Patterns (& idioms). • Use Domain model as source of inspiration.

  27. Dataflow Pipes and filters Batch sequential Data-centered Repository Blackboard Virtual Machine Interpreter Rule-based system Call and Return Main program and subroutine Object-oriented (& Data abstraction) Layered Independent Components Communicating processes Client/server Event systems Implicit invocation Explicit invocation Common Architectural Styles [FYI]

  28. Layered Architectural Style Our focus today: • Architectural style: Layered. • References • Larman, Chapter 13. • Fowler, EA. • Briefly, lets review Client-Server

  29. Client-Server (Two-tiered System) • “… most people see tier as implying a physical separation. Client-server systems are often described as two-tier systems …” [Fowler,p19]

  30. Enterprise Application Layers

  31. Enterprise Application Layers Presentation Domain Logic Data Source

  32. Layering – General Scheme Layers • Presentation / Application. • UI. • Generally “thin”. • (Term “application” can be misleading. It does not mean …) • Domain / Business Logic. • Core system functionality. • Technical Services.

  33. Domain Logic (Layer) • “… also referred to as business logic. … It involves calculations based on inputs and stored data, validation of any data that comes in from the presentation, and figuring out exactly what data source logic to dispatch …” [Fowler, p.20]

  34. Layered Style Characteristics • Each layer offers services to layers above. • Hence, layers build upon each other to provide increased functionality.

  35. Layers: Functionality Presentation Domain Functionality / services Data Source

  36. Layers: Dependencies Presentation Domain Dependencies Data Source

  37. Layer Dependencies Example

  38. Not permitted in pure style Layering – Pure Style • Pure style: components are permitted to use services of other components in • same layer. • layer immediately below.

  39. Where to Run Your Layers • [Folwer, pp. 22-24] Your software application ? ?

  40. Where to Run Your Layers EA software Technical Services

  41. EA Layers Refined Presentation Domain Logic Data Source

  42. Presentation Domain Technical services Presentation Application Domain (logic) Low-level domain logic Technical services Foundation. General Layering Scheme Refined

  43. Presentation Domain Technical services Presentation Application Business services Low-level business services Technical services Low-level technical services General Layering Scheme Refined

  44. Layering (Larman) • See Larman Sect 13.6

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