Lecture 8 Designing for Indestructability CS201j Engineering Software University of Virginia Computer Science David Evans http www cs virginia edu evans Menu Design Why it matters What makes a good design Modular Dependency Diagrams Documenting Designs How to Design Systems How to Design Programs 25 September 2003 CS 201J Fall 2003 2 Announcements Schedule changes including Exam 1 on notes today Bring today s notes to section tomorrow Read through the problem before section PS4 out tomorrow You will be assigned into teams of 3 If you turned in a PS3 25 September 2003 CS 201J Fall 2003 3 Why Does Design Matter How is a taste in this beautiful art to be formed in our countrymen unless we avail ourselves of every occasion when public buildings are to be erected of presenting to them models for their study and imitation You see I am an enthusiast on the subject of the arts But it is an enthusiasm of which I am not ashamed as its object is to improve the taste of my countrymen to increase their reputation to reconcile them to the rest of the world and procure them its praise Thomas Jefferson letter to James Madison 1785 25 September 2003 CS 201J Fall 2003 4 Software Design Matters Cost Time to Implement Some of you learned this the hard way for PS3 Independence of Modules Decoupled modules can be developed and tested independently Ability to Change Requirements for software change poorly designed software is hard impossible to change 25 September 2003 CS 201J Fall 2003 5 The Browser Wars 1996 Netscape Navigator 73 Microsoft IE 20 August 2002 Microsoft IE 96 Netscape 1 25 September 2003 CS 201J Fall 2003 6 Browser History NCSA Mosaic first widely used browser no design quick and dirty implementation Dec 1994 developed into Netscape Navigator V 1 0 100K lines of code Oct 1994 Microsoft starts developing IE 1995 1997 both browsers grew uncontrollably Communicator 4 0 120 developers 3M lines of code Based on Daniel Jackson s Notes 25 September 2003 CS 201J Fall 2003 7 Microsoft Componentizes IE 3 0 Microsoft does complete restructuring Jan Aug 96 Designed around clear modules Team of 3 4 people designed component architecture Modules URL high level services low level services MSHTML HTML rendering shell document security services etc Easy to customize browser by replacing components 25 September 2003 CS 201J Fall 2003 8 What went wrong for Netscape 1997 Communicator is impossible to develop Without clear interfaces and modules can t divide work All 120 developers had to work together Most of the code is self contained spaghetti The core functionality works but it s the little squeaks everywhere that break Aleks Totic Netscape July 1997 We re in a really bad situation with our current code base we re paying the price for going fast And when you go fast you don t architect Michael Toy Netscape July 1997 25 September 2003 CS 201J Fall 2003 9 Netscape s Downfall Netscape tries for 2 months to re architect browser but failed Tried starting from scratch for Communicator 6 0 never completed Eventually give up and release it as open source Mozilla Nobody can understand the code so no one develops anything interesting for it Based on Daniel Jackson s Notes 25 September 2003 CS 201J Fall 2003 10 Microsoft v Netscape Microsoft knew design mattered Designed IE 3 0 around clear modules Easy to add new features fix problems Won AOL deal because they could customize appearance of browser Netscape grew a quick and dirty implementation without clear modules and interfaces until it was impossible to develop Netscape sold to AOL IE is the only browser that matters today 25 September 2003 CS 201J Fall 2003 11 How should we describe designs 25 September 2003 CS 201J Fall 2003 12 Modular Dependency Diagrams Show the component modules How is the program organized Show the dependencies between them How do modules depend on each other Why do we want to know 25 September 2003 CS 201J Fall 2003 13 Using MDDs Design Time Consider different designs If the MDD has lots of cycles crossings etc the design is not decoupled enough Implementation Organize the implementation Testing Where do you look when a test fails Maintenance What modules must be considered when specification of one changes 25 September 2003 CS 201J Fall 2003 14 MDD Notation StringTable 25 September 2003 Module Usually a Java class CS 201J Fall 2003 15 MDD Notation StringTable Module Usually a Java class depends on the specification of TableEntry 25 September 2003 CS 201J Fall 2003 16 Code MDD A If A calls a method of B then A depends on the specification of B B If the specification of B changes we need to reconsider A A If A mentions the class B but doesn t call any methods or access fields then A weakly depends on B B If the specification of B changes we don t need to reconsider A 25 September 2003 CS 201J Fall 2003 17 PS2 Module Dependencies NameTrends StringTable StringIterator TableEntry If the specification of StringTable changes do we have to reconsider NameTrends Yes dependencies reveal dependencies 25 September 2003 CS 201J Fall 2003 18 PS2 Module Dependencies NameTrends StringTable StringIterator TableEntry If the specification of TableEntry changes do we have to reconsider NameTrends No we only have to consider StringTable 25 September 2003 CS 201J Fall 2003 19 PS2 Module Dependencies NameTrends StringTable StringIterator TableEntry 25 September 2003 If the implementation of StringIterator changes what classes must be reconsidered None Trick question if specification contract is followed we only care when spec changes CS 201J Fall 2003 20 PS1 Module Dependency Diagram CellAutomata GridDispla y Grid Cell CellState is a subtype of extends ConwayLifeCell What s bad about this design 25 September 2003 CS 201J Fall 2003 21 Evaluating Designs Cell Grid 25 September 2003 Circular Dependency Grid depends on Cell Cell depends on Grid CS 201J Fall 2003 22 Why are circular dependencies bad Need to finish both modules before you can test them can use stub versions for testing If a test fails may be hard to figure out which module is at fault If spec of one changes need to reconsider other module but what if its spec changes as a result But sometimes unavoidable Challenge come up with a better design for PS1 worth 100 bonus points 25 September 2003 CS 201J Fall 2003 23 Evaluating Designs Conflicting demands Dependencies are bad but reuse is good NameTrends StringTable StringIterator 25 September 2003 CS 201J Fall 2003 24 Evaluating Designs Too many