Supervisor and GenServer Drills

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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Drills

Drills help you develop familiarity and muscle memory with syntax through repeated exercises. Unlike usual problems, Drills are not intended to develop problem solving skills, they are purely for developing comfort and speed.

This set of drills is for Supervisors and GenServers and using them with Supervised Mix Projects.

> A GenServer is a process like any other Elixir process and it can be used to keep state, execute code asynchronously and so on. The advantage of using a generic server process (GenServer) implemented using this module is that it will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into a supervision tree. > > HexDocs: GenServer

> A supervisor is a process which supervises other processes, which we refer to as child processes. Supervisors are used to build a hierarchical process structure called a supervision tree. Supervision trees provide fault-tolerance and encapsulate how our applications start and shutdown. > > HexDocs: GenServer

GenServer

Create a Zero Genserver that does nothing other than store the integer 0 in its state. Use GenServer.start_link/3 to start the Zero process. Use :sys.get_state/1 to view the state of your Zero process.

Example Solution

defmodule Zero do
  use GenServer

  @impl true
  def init(_opts) do
    {:ok, 0}
  end
end

{:ok, pid} = GenServer.start_link(Zero, "init_arg")
:sys.get_state(pid)

Use GenServer.start_link/3 to start your Zero GenServer as a named process.

Example Solution

GenServer.start_link(Zero, "init_arg", [name: :my_name])

Create a SimpleCounter GenServer whose state starts as 0. Implement a GenServer.handle_call/3 callback function which accepts the :increment message and increments the state by 1 and returns :ok.

Use GenServer.start_link/3 and GenServer.call/3 to spawn a SimpleCounter process and send it an :increment message.

Use :sys.get_state/1 to see that the state of the counter has incremented.

Example Solution

defmodule SimpleCounter do
  use GenServer do
  
  def init(_opts) do
    {:ok, 0}
  end
  
  def handle_call(:increment, _from, state) do
    {:reply, :ok, state + 1}
  end
end

{:ok, pid} = GenServer.start_link(SimpleCounter, [])
GenServer.call(pid, :increment)
:sys.get_state(pid)

Create an InitialState GenServer whose initial state can be configured. Call GenServer.start_link/3 to spawn a State GenServer with an initial state. Use :sys.get_state/1 to confirm the state matches your configured state.

Example Solution

defmodule InitialState do
  use GenServer

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

{:ok, pid} = GenServer.start_link(InitialState, "my initial state")

:sys.get_state(pid)

Create a State module.

It should:

• Define a State.get/1 function that uses GenServer.call/3 and a GenServer.handle_call/3 callback function to retrieve state.
• Define a State.set/2 function that uses GenServer.cast/2 and a GenServer.handle_cast/2 callback function to update state.
• Define a State.start_link/3 function with a GenServer.init/1 callback function to initialize the GenServer with a configurable state.

Manually test each function (State.set/2, State.get/1 and State.start_link/3) to confirm they work as expected.

Example Solution

defmodule State do
  use GenServer

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

  @impl true
  def handle_cast({:set, new_state}, state) do
    {:noreply, new_state}
  end

  @impl true
  def handle_call(:get, _from, state) do
    # response is not specified
    {:reply, state, state}
  end

  def start_link(state) do
    GenServer.start_link(pid, state)
  end

  def set(pid, new_state) do
    GenServer.cast(pid, {:set, new_state})
  end

  def get(pid) do
    GenServer.call(pid, :get)
  end
end

{:ok, pid} = State.start_link("initial state")
"initial state" = State.get(pid)
State.set(pid, "updated state")
"updated state" = State.get(pid)

Create a minimal Named GenServer that can be started and configured as a named process using Named.start_link/1. Start a Named process and use GenServer.whereis/1 and/or Process.whereis/1 to find the pid of the named process.

Example Solution

defmodule Named do
  use GenServer

  def start_link(opts) do
    name = Keyword.get(opts, :name, __MODULE__)
    GenServer.start_link(__MODULE__, [], name: name)
  end

  def init(_opts) do
    {:ok, "any state!"}
  end
end

Named.start_link([name: :my_configured_name])
Process.whereis(:my_configured_name)

Create a minimal NamedState GenServer that can be started and configured as a named process with a configurable state using NamedState.start_link. Use :sys.get_state/1 to confirm the initial state is as expected and GenServer.whereis/1 and/or Process.whereis/1 to find the pid of the named process.

Example Solution

There are many ways to configure NamedState.start_link/1

Using multiple parameters:

defmodule NamedState do
  use GenServer

  def start_link(state, opts \\ []) do
    name = Keyword.get(opts, :name, __MODULE__)
    GenServer.start_link(__MODULE__, state, name: name)
  end

  def init(state) do
    {:ok, state}
  end
end

NamedState.start_link("initial state", name: :multi_arg_example)

Process.whereis(:multi_arg_example)

Using a keyword list with different keys:

defmodule NamedState do
  use GenServer

  def start_link(opts) do
    name = Keyword.get(opts, :name, __MODULE__)
    state = Keyword.get(opts, :state)
    GenServer.start_link(__MODULE__, state, name: name)
  end

  def init(state) do
    {:ok, state}
  end
end

NamedState.start_link(state: "initial state", name: :keyword_list_example)
Process.whereis(:keyword_list_example)

Supervisor

We’ve created a Worker GenServer for you to use with the following Supervisor drills.

defmodule Worker do
  use GenServer

  def start_link(state, opts \\ []) do
    GenServer.start_link(__MODULE__, state, opts)
  end

  def init(state) do
    {:ok, state}
  end
end

Use Supervisor.start_link/3 to start a supervisor process with no children. Use Supervisor.which_children/1 to see the empty list of children.

Example Solution

We often bind children to a variable in demonstrations.

children = []

Supervisor.start_link(children, strategy: :one_for_one)

But that’s not enforced, just idiomatic.

Supervisor.start_link([], strategy: :one_for_one)

Use Supervisor.start_link/3 function to start a supervisor with the Worker as a child using the map syntax.

Example Solution

children = [
  %{
    id: :worker1,
    start: {Worker, :start_link, ["initial state"]}
  } 
]

Supervisor.start_link(children, strategy: :one_for_one)

Use Supervisor.start_link/3 function to start a supervisor three Worker children using the map syntax.

Example Solution

children = [
  %{
    id: :worker1,
    start: {Worker, :start_link, ["initial state"]}
  },
  %{
    id: :worker2,
    start: {Worker, :start_link, ["initial state"]}
  },
  %{
    id: :worker3,
    start: {Worker, :start_link, ["initial state"]}
  }
]

Supervisor.start_link(children, strategy: :one_for_one)

Use Supervisor.start_link/3 function to start a supervisor with the Worker as a child using the tuple syntax.

Example Solution

children = [
  {Worker, "initial state"}
]

Supervisor.start_link(children, strategy: :one_for_one)

Use Supervisor.start_link/3 function to start a supervisor with the Worker as a child using the map syntax to start the Worker process as a named process.

Example Solution

children = [
  %{
    id: :worker1,
    start: {Worker, :start_link, ["initial state", [name: :named_worker1]]}
  }
]

Supervisor.start_link(children, strategy: :one_for_one)

Usually worker GenServers should define a start_link/1 function to start under a supervision tree. However, this is idiomatic and we can (but usually shouldn’t) break this rule.

Start the following NonIdiomatic GenServer under a supervision tree using the starter/3 function.

Example Solution

children = [
  %{
    id: :never_do_this,
    start: {NonIdiomatic, :starter, ["arg", "arg", "arg"]}
  }
]

Supervisor.start_link(children, strategy: :one_for_one)

defmodule NonIdiomatic do
  use GenServer

  def starter(_arg1, _arg2, _arg3) do
    GenServer.start_link(__MODULE__, [])
  end

  def init(_opts) do
    {:ok, "initial state"}
  end
end

Use Supervisor.start_link/3 function to start a supervisor with three Worker children using the tuple syntax.

This should cause the supervisor to crash because each Worker has the same id.

Comment your solution when finished to avoid crashing this livebook.

Example Solution

children = [
  {Worker, "initial state"},
  {Worker, "initial state"},
  {Worker, "initial state"}
]

Supervisor.start_link(children, strategy: :one_for_one)

Mix Projects

1. Create a new mix empty mix project using the --sup flag.

2. Create a new unsupervised mix project without the --sup flag and configure it with a supervisor.

• Create an Application module in a lib/my_app/application.ex file.

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = []
    Supervisor.start_link(children, strategy: :one_for_one)
  end
end

• Configure the mix project in mix.exs to use the application module.

def application do
  # make sure to keep the existing options and add the following:
  [mod: {MyApp.Application, []}]
end

Mark As Completed

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

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "supervisor_and_genserver_drills_reading"
    "exercises" -> "supervisor_and_genserver_drills_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 supervisor-and-genserver-drills-exercise
$ git add .
$ git commit -m "finish supervisor and genserver drills exercise"
$ git push origin supervisor-and-genserver-drills-exercise

Create a pull request from your supervisor-and-genserver-drills-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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