Dominoes

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.8.0", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

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Dominoe

Dominoes are square tiles often placed near each other to make satisfying effects when it falls and knocks each other over.

We’ve created a named Domino process that can receive a generic :fall message to cause it to crash. It will also print a message every time it’s started with the name of the process.

defmodule Domino do
  use GenServer

  def start_link(opts) do
    IO.inspect(opts, label: "STARTED DOMINO")
    GenServer.start_link(__MODULE__, opts, name: opts[:name])
  end

  @impl true
  def init(opts) do
    {:ok, opts}
  end

  @impl true
  def handle_info(:fall, _state) do
    raise "error"
  end
end

You’re going to start three Domino processes under a supervisor. When one crashes, they should restart sequentially in the order they are defined.

flowchart TD 
    Supervisor
    Supervisor --> Domino1
    Supervisor --> Domino2
    Supervisor --> Domino3

    classDef crashed fill:#fe8888;
    classDef terminated fill:#fbab04;
    classDef restarted stroke:#0cac08,stroke-width:4px

    class Domino2 crashed
    class Domino3 terminated
    class Domino2,Domino3 restarted

For example, if Domino2 crashes, then Domino3 should also be restarted. If Domino3 crashes, no other processes are affected.

Hint Use the :rest_for_one strategy for your supervisor.

Example Solution

children = [
  %{
    id: :domino1,
    start: {Domino, :start_link, [[name: :domino1]]}
  },
  %{
    id: :domino2,
    start: {Domino, :start_link, [[name: :domino2]]}
  },
  %{
    id: :domino3,
    start: {Domino, :start_link, [[name: :domino3]]}
  }
]

Supervisor.start_link(children, strategy: :rest_for_one)

Keep in mind, if you have already started a named process, the supervisor might crash when you attempt to start it again. Re-evaluate the cell after the livebook crashes to resolve this issue.

Send your dominos messages to ensure they are crashing in the correct order. They will log a message that demonstrates the Domino.start_link/1 function was called again.

Example Solution

Replace :domino_name with the name of a domino process you started in the supervisor above.

Process.send(:domino_name, :fall, [])

Test sending each Domino process a message individually.

Mark As Completed

file_name = Path.basename(Regex.replace(~r/#.+/, __ENV__.file, ""), ".livemd")

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "dominoes_reading"
    "exercises" -> "dominoes_exercise"
  end

progress_path = __DIR__ <> "/../progress.json"
existing_progress = File.read!(progress_path) |> Jason.decode!()

default = Map.get(existing_progress, save_name, false)

form =
  Kino.Control.form(
    [
      completed: input = Kino.Input.checkbox("Mark As Completed", default: default)
    ],
    report_changes: true
  )

Task.async(fn ->
  for %{data: %{completed: completed}} <- Kino.Control.stream(form) do
    File.write!(
      progress_path,
      Jason.encode!(Map.put(existing_progress, save_name, completed), pretty: true)
    )
  end
end)

form

Commit Your Progress

Run the following in your command line from the curriculum folder to track and save your progress in a Git commit. Ensure that you do not already have undesired or unrelated changes by running git status or by checking the source control tab in Visual Studio Code.

$ git checkout -b dominoes-exercise
$ git add .
$ git commit -m "finish dominoes exercise"
$ git push origin dominoes-exercise

Create a pull request from your dominoes-exercise branch to your solutions branch. Please do not create a pull request to the DockYard Academy repository as this will spam our PR tracker.

DockYard Academy Students Only:

Notify your teacher by including @BrooklinJazz in your PR description to get feedback. You (or your teacher) may merge your PR into your solutions branch after review.

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