Introduction

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)
• Expensive to build and maintain

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: simplified version of make utility
• 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
• Ed 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: making mistakes is OK

Book: read all but Chapters 19-20 by next Monday.

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

This week: discuss Raft (read the paper by Wednesday)

Next week: review material from APOSD