Week 6


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Week 6

  1. 1. Software Engineering Software Design Slide 1 Software Engineering Software Design
  2. 2. Software Engineering Software Design Slide 2 Software Design Deriving a solution which satisfies software requirements
  3. 3. Software Engineering Software Design Slide 3 Stages of Design • Problem understanding – Look at the problem from different angles to discover the design requirements. • Identify one or more solutions – Evaluate possible solutions and choose the most appropriate depending on the designer's experience and available resources. • Describe solution abstractions – Use graphical, formal or other descriptive notations to describe the components of the design. • Repeat process for each identified abstraction until the design is expressed in primitive terms.
  4. 4. Software Engineering Software Design Slide 4 The Design Process • Any design may be modelled as a directed graph made up of entities with attributes which participate in relationships. • The system should be described at several different levels of abstraction. • Design takes place in overlapping stages. It is artificial to separate it into distinct phases but some separation is usually necessary.
  5. 5. Software Engineering Software Design Slide 5 Phases in the Design Process Architectural design Abstract specificatio n Interface design Component design Data structure design Algorithm design System architecture Software specification Interface specification Component specification Data structure specification Algorithm specification Requirements specification Design activities Design products
  6. 6. Software Engineering Software Design Slide 6 Design Phases • Architectural design: Identify sub-systems. • Abstract specification: Specify sub-systems. • Interface design: Describe sub-system interfaces. • Component design: Decompose sub-systems into components. • Data structure design: Design data structures to hold problem data. • Algorithm design: Design algorithms for problem functions.
  7. 7. Software Engineering Software Design Slide 7 Design • Computer systems are not monolithic: they are usually composed of multiple, interacting modules. • Modularity has long been seen as a key to cheap, high quality software. • The goal of system design is to decode: – What the modules are; – What the modules should be; – How the modules interact with one-another
  8. 8. Software Engineering Software Design Slide 8 Modular programming • In the early days, modular programming was taken to mean constructing programs out of small pieces: “subroutines” • But modularity cannot bring benefits unless the modules are – autonomous, – coherent and – robust
  9. 9. Software Engineering Software Design Slide 9 Procedural Abstraction • The most obvious design methods involve functional decomposition. • This leads to programs in which procedures represent distinct logical functions in a program. • Examples of such functions: – “Display menu” – “Get user option” • This is called procedural abstraction
  10. 10. Software Engineering Software Design Slide 10 Programs as Functions • Another view is programs as functions: input output x → f → f (x) the program is viewed as a function from a set I of legal inputs to a set O of outputs. • There are programming languages (ML, Miranda, LISP) that directly support this view of programming
  11. 11. Software Engineering Software Design Slide 11 Object-oriented design – The system is viewed as a collection of interacting objects. – The system state is decentralized and each object manages its own state. – Objects may be instances of an object class and communicate by exchanging methods.
  12. 12. Software Engineering Software Design Slide 12 Five Criteria for Design Methods • We can identify five criteria to help evaluate modular design methods: – Modular decomposability; – Modular composability; – Modular understandability; – Modular continuity; – Modular protection.
  13. 13. Software Engineering Software Design Slide 13 Modular Decomposability • This criterion is met by a design method if the method supports the decomposition of a problem into smaller sub-problems, which can be solved independently. • In general method will be repetitive: sub- problems will be divided still further • Top-down design methods fulfil this criterion; stepwise refinement is an example of such method
  14. 14. Software Engineering Software Design Slide 14 Hierarchical Design Structure System level Sub-system level
  15. 15. Software Engineering Software Design Slide 15 Top-down Design • In principle, top-down design involves starting at the uppermost components in the hierarchy and working down the hierarchy level by level. • In practice, large systems design is never truly top-down. Some branches are designed before others. Designers reuse experience (and sometimes components) during the design process.
  16. 16. Software Engineering Software Design Slide 16 Modular Composability • A method satisfies this criterion if it leads to the production of modules that may be freely combined to produce new systems. • Composability is directly related to the issue of reusability • Note that composability is often at odds with decomposability; top-down design, – for example, tends to produce modules that may not be composed in the way desired • This is because top-down design leads to modules which fulfil a specific function, rather than a general one
  17. 17. Software Engineering Software Design Slide 17 Examples • The Numerical Algorithm Group (NAG) libraries contain a wide range of routines for solving problems in linear algebra, differential equations, etc. • The Unix shell provides a facility called a pipe, written “−”, whereby – the standard output of one program may be redirected to the standard input of another; this convention favours composability.
  18. 18. Software Engineering Software Design Slide 18 Modular Understandability • A design method satisfies this criterion if it encourages the development of modules which are easily understandable. • EXAMPLE. Take a thousand lines program, containing no procedures; it’s just a long list of sequential statements. Divide it into twenty blocks, each fifty statements long; make each block a method.
  19. 19. Software Engineering Software Design Slide 19 Understandability • Related to several component characteristics – Can the component be understood on its own? – Are meaningful names used? – Is the design well-documented? – Are complex algorithms used? • Informally, high complexity means many relationships between different parts of the design.
  20. 20. Software Engineering Software Design Slide 20 Modular Continuity • A method satisfies this criterion if it leads to the production of software such that a small change in the problem specification leads to a change in just one (or a small number of ) modules. • EXAMPLE. Some projects enforce the rule that no numerical or textual literal should be used in programs: only symbolic constants should be used
  21. 21. Software Engineering Software Design Slide 21 Modular Protection • A method satisfied this criterion if it yields architectures in which the effect of an abnormal condition at run-time only effects one (or very few) modules • EXAMPLE. Validating input at source prevents errors from propagating throughout the program.
  22. 22. Software Engineering Software Design Slide 22 Five principles for Good Design • From the discussion above, we can distil five principles that should be adhered to: – Linguistic modular units; – Few interfaces; – Small interfaces – Explicit interfaces; – Information hiding.
  23. 23. Software Engineering Software Design Slide 23 Linguistic Modular Units • A programming language (or design language) should support the principle of linguistic modular units: – Modules must correspond to linguistic units in the language used • EXAMPLE. Java methods and classes
  24. 24. Software Engineering Software Design Slide 24 Few Interfaces • This principle states that the overall number of communication channels between modules should be as small as possible: – Every module should communicate with as few others as possible. • So, in the system with n modules, there may be a minimum of n-1 and a maximum of links; your system should stay closer to the minimum 2 )1( −nn
  25. 25. Software Engineering Software Design Slide 25 Few Interfaces
  26. 26. Software Engineering Software Design Slide 26 Small Interfaces (Loose Coupling) • This principle states: – If any two modules communicate, they should exchange as little information as possible. • COUNTER EXAMPLE. Declaring all instance variables as public!
  27. 27. Software Engineering Software Design Slide 27 • A measure of the strength of the inter-connections between system components. • Loose coupling means component changes are unlikely to affect other components. – Shared variables or control information exchange lead to tight coupling. – Loose coupling can be achieved by state decentralization (as in objects) and component communication via parameters or message passing. Coupling
  28. 28. Software Engineering Software Design Slide 28 Tight Coupling Module A Module B Module C Module D Shared data area
  29. 29. Software Engineering Software Design Slide 29 Loose Coupling Module A A’s data Module B B’s data Module D D’s data Module C C’s data
  30. 30. Software Engineering Software Design Slide 30 • Object-oriented systems are loosely coupled because there is no shared state and objects communicate using message passing. • However, an object class is coupled to its super-classes. Changes made to the attributes or operations in a super-class propagate to all sub-classes. Coupling and Inheritance
  31. 31. Software Engineering Software Design Slide 31 Reusability • A major obstacle to the production of cheap quality software is the intractability of the reusability issue. • Why isn’t writing software more like producing hardware? Why do we start from scratch every time, coding similar problems time after time after time? • Obstacles: – Economic; – Organizational;
  32. 32. Software Engineering Software Design Slide 32 Stepwise Refinement • The simplest realistic design method, widely used in practice. • Not appropriate for large-scale, distributed systems: mainly applicable to the design of methods. • Basic idea is: – Start with a high-level spec of what a method is to achieve; – Break this down into a small number of problems (usually no more than 10) – For each of these problems do the same; – Repeat until the sub-problems may be solved immediately.
  33. 33. Software Engineering Software Design Slide 33 Explicit Interfaces • If two modules must communicate, they must do it so that we can see it: – If modules A and B communicate, this must be obvious from the text of A or B or both. • Why? If we change a module, we need to see what other modules may be affected by these changes.
  34. 34. Software Engineering Software Design Slide 34 Information Hiding • This principle states: – All information about a module, (and particularly how the module does what it does) should be private to the module unless it is specifically declared otherwise. • Thus each module should have some interface, which is how the world sees it anything beyond that interface should be hidden. • The default Java rule: – Make everything private
  35. 35. Software Engineering Software Design Slide 35 Cohesion A measure of how well a component “fits together”. • A component should implement a single logical entity or function. • Cohesion is a desirable design component attribute as when a change has to be made, it is localized in a single cohesive component. • Various levels of cohesion have been identified.