elixir-otp-patterns

# Elixir OTP Patterns

Master OTP (Open Telecom Platform) patterns to build concurrent,
fault-tolerant Elixir applications. This skill covers GenServer,
Supervisor, Agent, Task, and other OTP behaviors.

## GenServer Basics

```elixir
defmodule Counter do
  use GenServer

  # Client API

  def start_link(initial_value \\ 0) do
    GenServer.start_link(__MODULE__, initial_value, name: __MODULE__)
  end

  def increment do
    GenServer.cast(__MODULE__, :increment)
  end

  def get_value do
    GenServer.call(__MODULE__, :get_value)
  end

  # Server Callbacks

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

  @impl true
  def handle_call(:get_value, _from, state) do
    {:reply, state, state}
  end

  @impl true
  def handle_cast(:increment, state) do
    {:noreply, state + 1}
  end
end

# Usage
{:ok, _pid} = Counter.start_link(0)
Counter.increment()
Counter.get_value()  # => 1
```

## GenServer with State Management

```elixir
defmodule UserCache do
  use GenServer

  # Client API

  def start_link(_opts) do
    GenServer.start_link(__MODULE__, %{}, name: __MODULE__)
  end

  def put(user_id, user_data) do
    GenServer.cast(__MODULE__, {:put, user_id, user_data})
  end

  def get(user_id) do
    GenServer.call(__MODULE__, {:get, user_id})
  end

  def delete(user_id) do
    GenServer.cast(__MODULE__, {:delete, user_id})
  end

  def all do
    GenServer.call(__MODULE__, :all)
  end

  # Server Callbacks

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

  @impl true
  def handle_call({:get, user_id}, _from, state) do
    {:reply, Map.get(state, user_id), state}
  end

  @impl true
  def handle_call(:all, _from, state) do
    {:reply, state, state}
  end

  @impl true
  def handle_cast({:put, user_id, user_data}, state) do
    {:noreply, Map.put(state, user_id, user_data)}
  end

  @impl true
  def handle_cast({:delete, user_id}, state) do
    {:noreply, Map.delete(state, user_id)}
  end
end
```

## Supervisor Strategies

```elixir
de