Introduction

Lecture Notes for CS 190
Winter 2020
John Ousterhout

Course Background and Goals

  • This may be the only course in the world dedicated entirely to software design:
    • How should large systems be structured?
    • What does good code look like?
    • What is the role of design in the software development process?
  • Goal for the class: enhance design awareness
    • Change the way you think about programming
    • Provide vocabulary for talking about design
    • Present a set of principles of software design
    • Develop critical skills: recognize good and bad designs (red flags)

Complexity

  • Why does design matter? Too hard to build and evolve large software systems:
    • Can't understand how they work
    • Can't figure out how to fix bugs or implement new features
    • Hard to produce code without bugs (can't predict implications of a change)
  • Complexity: anything related to the structure of a system that makes it hard to understand and modify the system.
  • No-one can fully understand a large software system: can't keep it all in your mind at once
  • What makes systems complicated?
    • Lots of little things: complexity is incremental
    • Examples: dependencies, inconsistencies, special cases
  • Complexity increases as a system ages
  • Most important overall goal for design: reduce apparent complexity
    • Eliminate complexity
    • Hide complexity: modular design
      • Decompose large system into pieces that are mostly independent
      • Each module is relatively small
      • Most coding tasks involve only a few modules
      • Allows more functionality before system becomes unmaintainable
  • Result: make it easier to work with large systems that have lots of functionality.

Role of Design in the Software Development Process

  • Software design is a continuous process: different from other kinds of engineering
    • Most engineering areas: design is only done at the beginning of a project
    • Early days of software: waterfall model: divide development into distinct phases: requirements definition, design, coding, testing, maintenance.
    • Problem: can't visualize a large software system well enough to design it all in advance
    • Approaches today tend to be more incremental (e.g. agile development).
    • Eliminate all up-front design?
    • Compromise approach works best:
      • Design a bit
      • Implement and evaluate
      • Refactor to fix problems
      • Design a bit more, and repeat
    • What you learn in earlier phases reduces mistakes in later phases
    • Will still make mistakes: must constantly be looking for and fixing design problems.

Course Structure

  • Iteration
    • Create a nontrivial application from scratch
    • Receive criticism
    • Rewrite to fix problems
  • Criticism:
    • I will read all projects
    • You will read each other's projects
    • In-class code reviews
  • Goal for course: minimize apparent complexity
    • Clean, simple, obvious
    • Correctness less important (within reason)
  • Class organized around 3 projects:
    • Projects 1 and 2: Raft protocol for distributed consensus
      • Project 1: leader election mechanism, communication
      • Project 2:
        • Revise, based on code reviews
        • Add log mechanism, implement replicated shell
    • Project 3: simple bash-like shell
    • Work in teams of 2
    • Code in C++
  • Class format: studio (discussion, interaction):
    • Lectures (only a few): around 50 minutes
    • Studios (code reviews, exercises, etc.): 80 minutes
    • Some classes cancelled to make time for programming
    • Class attendance is mandatory
    • Grading policy:
      • No exams
      • Grades based on project work and class participation
    • 3 late days
    • Office hours
    • Piazza for communication
  • Class participation is essential
  • Giving and receiving criticism is very important
    • Expect to receive lots of feedback, may be intense:
      • Unlikely that any of you is already a good programmer
      • Small details matter
    • Keep criticism constructive:
      • It's about the code, not the person
      • Remember that everyone is learning
    • Growth mindset
  • Book: read Chapters 1-17 before Friday.
  • I'm interested in feedback on the class
  • Get started right away!
    • Pick partners immediately
      • Meet after class
      • Send me info: Google ids, GitHub ids
    • Read Raft paper, Project 1 writeup, start designing/coding
    • Read APOSD Chapters 1-18 before class on Friday
  • This week: discuss Raft (read the paper by Wednesday)
  • Next week: review material from APOSD