First: Homework Questions

  • Memory Game with Server-Side State is due today.

Project 1

  • Project 1: Make a multi-player game.
  • You can pick what game you want to make.
    • Turn based games should be at least as complex as Connect4.
    • Real-time games should be at least as complex as a 2-player breakout (think Pong with blocks in the middle).
  • You'll work with a partner.
  • Make sure you've talked to your partner for Project 1 - you want to be on a team by tomorrow.
  • Project 1 is due in 2 weeks.
    • If homeworks normally take 8 hours, that means your team has at least 36 hours to spend on this project.
    • For students who claim that homework takes 23 hours...

OTP: Processes & State

# Can compile / recompile from iex
> c("stack.ex")

The problem:

  • Stack of integers: push, pop, print
  • More than one stack at a time.
  • Every five seconds, If the top integer is odd, add 2.

Supervisors and Supervision Trees


  • Manual state process.


  • Use the built-in GenServer behavior.


  • Add a Registry
  • What do we do when we want a bunch of processes?
  • We can register by arbitrary term with the Registry module.


  • Add a Supervisor
  • Supervisor: Start & Monitor processes, potentially restart them when they crash.
  • In this case, DynamicSupervisor

Supervisor Concepts

Supervision Tree

  • Supervisor module takes a static list of children and keeps them running.
  • DynamicSupervisor handles workers spawned at runtime.
  • Spin up hangman & show tree in observer.
    • iex -S mix phx.server
    • :observer.start()
    • install and use kmag to zoom in


If a child is linked to a parent, the parent gets an exit message when the parent dies.

Trapping exit?

A process that gets an exit message will exit itself unless it's configured to trap exits. Trapping exits is what lets supervisors work.

Multiplayer Hangman

  • Current Hangman design:
    • User connects to channel.
    • Game state is stored in channel and backed up to agent
    • When a move comes in, the next state is produced by channel process.
  • Two-player Hangman:
    • Two users each connect to the same channel.
    • They each get their own, seperate, channel process.
    • We can't store or update the state in the channels, because then the game states could get out of sync.
    • Solution: Add a process to manage the game state.
      • Channel processes sends move messages to game process.
      • Game process broadcasts updated state to the channel.
    • If the process crashes, the game state will be lost.
    • Could keep the backup agent as well.

Let's try building that. We could add turns, but instead we'll just allow either player to guess whenever they want.


  • Write Hangman.Server
  • Write Hangman.GameSup
  • Add Hangman.GameSup to the application child list.


$ iex -S mix
iex> Hangman.GameServer.start("foo")
iex> Hangman.GameServer.guess("foo", "l")
iex> Hangman.GameServer.guess("foo", "z")
iex> Hangman.GameServer.guess("foo", "b")