David Evans CS201j: Engineering Software University of Virginia Computer Science Lecture 8: Designing for Indestructability

24 September 2002CS 201J Fall Menu Design –Why it matters? –What makes a good design? Modular Dependency Diagrams –Documenting Designs How to Design Systems –How to Design Programs

24 September 2002CS 201J Fall 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)

24 September 2002CS 201J Fall 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

24 September 2002CS 201J Fall The Browser Wars 1996: –Netscape Navigator: 73% –Microsoft IE: ~20% August 2002: –Microsoft IE: 96% –Netscape: ~1%

24 September 2002CS 201J Fall 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 : both browsers grew uncontrollably –Communicator 4.0: 120 developer, 3M lines of code Based on Daniel Jackson’s Notes

24 September 2002CS 201J Fall 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

24 September 2002CS 201J Fall 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

24 September 2002CS 201J Fall 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

24 September 2002CS 201J Fall 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

24 September 2002CS 201J Fall How should we describe designs?

24 September 2002CS 201J Fall 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?

24 September 2002CS 201J Fall 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?

24 September 2002CS 201J Fall MDD Notation StringTable Module Usually a Java class

24 September 2002CS 201J Fall MDD Notation Module Usually a Java class depends on the specification of TableEntry StringTable

24 September 2002CS 201J Fall Code  MDD If A calls a method of B, then A depends on the specification of B –If the specification of B changes, we need to reconsider A If A mentions the class B, but doesn’t call any methods or access fields, then A weakly depends on B –If the specification of B changes, we don’t need to reconsider A. A B A B

24 September 2002CS 201J Fall PS2 Module Dependencies TableEntry StringTable NameTrends StringIterator If the specification of StringTable changes, do we have to reconsider NameTrends ? Yes, dependencies reveal dependencies.

24 September 2002CS 201J Fall PS2 Module Dependencies TableEntry StringTable NameTrends StringIterator If the specification of TableEntry changes, do we have to reconsider NameTrends ? No, we only have to consider StringTable.

24 September 2002CS 201J Fall PS2 Module Dependencies TableEntry StringTable NameTrends StringIterator 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.

24 September 2002CS 201J Fall PS1 Module Dependency Diagram Grid CellAutomata Cell CellState GridDisplay ConwayLifeCell is a subtype of (extends) What’s bad about this design?

24 September 2002CS 201J Fall Evaluating Designs Grid Cell Circular Dependency: Grid depends on Cell Cell depends on Grid

24 September 2002CS 201J Fall 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 (with 100 bonus points!)

24 September 2002CS 201J Fall Evaluating Designs Conflicting demands: Dependencies are bad, but reuse is good StringTable NameTrends StringIterator

24 September 2002CS 201J Fall Evaluating Designs Too many modules: lots of dependencies, overall design too complex to understand Too few modules: hard to divide, each module may be too big to implement Guideline: humans can only understand about 7 things at once – if you have more than 7 modules, make the modules bigger (could then subdivide them)

24 September 2002CS 201J Fall Evaluating Designs No absolute rules Important thing is that you can justify your design by explaining why it is better than alternatives A good design will make implementation (relatively) easy, a bad design will make implementation impossible

24 September 2002CS 201J Fall Charge PS4: Design Document –Due Thursday Wednesday, 8pm: AC’s will hold recitation on useful programming techniques –Not intended to help with design for PS4 –But, will be helpful for coding for PS4 and beyond…