Software design principles


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Software design principles represent a set of guidelines that helps us to avoid having a bad design

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Software design principles

  1. 1. Software Design Principles Software design principles represent a set of guidelines that helps us to avoid having a bad design
  2. 2. Why Design?  Manage complexity of a system  Improve software quality factors  Manage changes easily with minimum cost  Facilitate reusability
  3. 3. Abstract Design Guideline High Cohesion Low Coupling
  4. 4. Art of Unix Programming  Rule of Clarity: Clarity is better than cleverness.  Rule of Composition: Design programs to be connected to other programs.  Rule of Separation: Separate policy from mechanism; separate interfaces from engines  Rule of Simplicity: Design for simplicity; add complexity only where you must
  5. 5. Art of Unix Programming  Rule of Transparency: Design for visibility to make inspection and debugging easier  Rule of Robustness: Robustness is the child of transparency and simplicity  Rule of Least Surprise: In interface design, always do the least surprising thing  Rule of Silence: When a program has nothing surprising to say, it should say nothing
  6. 6. Art of Unix Programming  Rule of Repair: When you must fail, fail noisily and as soon as possible  Rule of Economy: Programmer time is expensive; conserve it in preference to machine time  Rule of Optimization: Prototype before polishing. Get it working before you optimize it  Rule of Extensibility: Design for the future, because it will be here sooner than you think
  7. 7. Symptoms of Rotting Design Rigidity Fragility Immobility Viscosity
  8. 8. Rigidity • Difficult to change • Ripple Effect • Entering roach motel A software can be called a rigid when it is difficult to change even in a sample way When the manager’s fears become so acute that they refuse to allow changes to software, official rigidity sets in. Thus, what starts as a design deficiency, winds up being adverse management policy.
  9. 9. Fragility • Conceptually irrelevant pieces breaking in multiple places • Such software is impossible to maintain. Every fix makes it worse, introducing more problems than are solved • Managers and Customers have a feeling that Developers have lost control over the project Closely related to rigidity is fragility. Fragility is the tendency of the software to break in many places every time it is changed. Often the breakage occurs in areas that have no conceptual relationship with the area that was changed
  10. 10. Immobility • Inability to reuse software • Software is simply rewritten instead of reused Immobility is the inability to reuse software from other projects or from parts of the same project. It often happens that one engineer will discover that he needs a module that is similar to one that another engineer wrote I am asking for someone NOT everyone
  11. 11. Viscosity • It is easy to do the wrong thing, but hard to do the right thing • When the design preserving methods are harder to employ than the hacks, then the viscosity of the design is high • Viscosity of environment comes about when the development environment is slow and inefficient
  12. 12. These four symptoms are the tell-tale signs of poor architecture. Any application that exhibits them is suffering from a design that is rotting from the inside out. But what causes that rot to take place?  Changing Requirements  Changing Technologies  Human Error  Dependency Management
  13. 13. OO Design Principles • Single Responsibility Principle (SRP) • Open Closed Principle (OCP) • Liskov Substitution Principle (LSP) • Interface Segregation Principle (ISP) • Dependency Inversion Principle (DIP) • Tell Don’t Ask • DRY – Don’t Repeat yourself • Once and only once • The Law of Demeter • Package Principles • DRY • YAGNI
  14. 14. Single Responsibility Principle (SRP) There should never be more than one reason for a class to change.
  15. 15. SRP (Cont’d)
  16. 16. SRP (Cont’d)
  17. 17. Open Closed Principle (OCP) • A module should be open for extension but closed for modification
  18. 18. OCP (Cont’d)
  19. 19. OCP (Cont’d)
  20. 20. Liskov Substitution Principle (LSP) • Subclasses should be substitutable for their base classes.
  21. 21. LSP (Cont’d)
  22. 22. Interface Segregation Principle (ISP) • Many client specific interfaces are better than one general purpose interface
  23. 23. ISP (Cont’d)
  24. 24. Dependency Inversion Principle (DIP) • Depend upon Abstractions. Do not depend upon concretions
  25. 25. DIP (Cont’d)
  26. 26. DIP (Cont’d)
  27. 27. Tell Don’t Ask • Don't call us we will call you • Hollywood Principle
  28. 28. Tell Don’t Ask (Cont’d) if(getStateOfSomething()) doThis(); else doThat(); object.doSomethingForMe(); V/S
  29. 29. DRY – Don’t Repeat yourself Once and only once Every piece of knowledge must have a single, unambiguous, authoritative representation within a system Refactoring has as a primary objective getting each piece of functionality to exist in exactly one place in the software. –Ron Jeffries
  30. 30. The Law of Demeter • Any method of an object should only call methods belonging to: – itself – any composite objects – any parameters that were passed in to the method – any objects it created
  31. 31. Train Wreck? • Methods belonging to objects that were returned from some other call. my_television.front_panel.swit ches.power.on(); v/s my_television.power_up(); • Talk only to your neighbors, not with their neighbors.
  32. 32. YAGNI (You ain’t gonna need it) "Always implement things when you actually need them, never when you just foresee that you need them.” – Ron Jeffries The need for combining it with the supporting practices, rather than using it standalone
  33. 33. Package Principles  Cohesion Principles  Coupling Principles
  34. 34. Package Cohesion Principles Classes are a necessary, but insufficient, means of organizing a design. The larger granularity of packages are needed to help bring order. But how do we choose which classes belong in which packages. There are three principles known as the Package Cohesion Principles, that attempt to help the software architect.  Release Reuse Equivalency Principle (REP) - The granule of reuse is the granule of release.  Common Closure Principle (CCP) - Classes that change together, belong together.  Common Reuse Principle (CRP) - Classes that aren’t reused together should not be grouped together.
  35. 35. Package Coupling Principles three packages govern the interrelationships between packages. Applications tend to be large networks of interrelated packages. The rules that govern these interrelationship are some of the most important rules in object oriented architecture.  Acyclic Dependencies Principle (ADP) -The dependencies between packages must not form cycles.  Stable Dependencies Principle (SDP) - Depend in the direction of stability.  Stable Abstractions Principle (SAP) - Stable packages should be abstract packages.
  36. 36. DIP IoC DI & Factory Pattern • Dependency Injection Principle • Inversion of Control • Dependency Injection • Practice: Information retrieving from several file types. By applying DIP, IoC, DI and Factory Pattern
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