lab 04

labs/04/lab4/arithmetic.ex

@@ -0,0 +1,90 @@
+defmodule Arithmetic.Worker do
+  use GenServer
+
+  def start_link() do
+    GenServer.start_link(__MODULE__, nil)
+  end
+
+  ### Server Impl ###
+  @impl true
+  def init(_arg) do
+    {:ok, nil}
+  end
+
+  @impl true
+  def handle_cast({:square, x, from}, state) do
+    # The server dispatches work asynchronously, so workers handle casts and
+    # reply directly to the client that made the original GenServer.call/2.
+    GenServer.reply(from, {self(), x * x})
+    {:noreply, state}
+  end
+
+  @impl true
+  def handle_cast({:sqrt, x, from}, state) do
+    Process.sleep(2000)
+    GenServer.reply(from, {self(), if(x < 0, do: :error, else: :math.sqrt(x))})
+    {:noreply, state}
+  end
+end
+
+defmodule Arithmetic.Server do
+  use GenServer
+
+  ### Client API ###
+  def start(n) do
+    GenServer.start(__MODULE__, n, name: __MODULE__)
+  end
+
+  def square(x) do
+    GenServer.call(__MODULE__, {:square, x})
+  end
+
+  def sqrt(x) do
+    GenServer.call(__MODULE__, {:sqrt, x})
+  end
+
+  ### Server Impl ###
+  @impl true
+  def init(n) do
+    Process.flag(:trap_exit, true)
+
+    workers =
+      Enum.map(1..n, fn _ ->
+        {:ok, pid} = Arithmetic.Worker.start_link()
+        IO.inspect(pid)
+        pid
+      end)
+
+    {:ok, {workers, 0}}
+  end
+
+  @impl true
+  def handle_call({:square, x}, from, {workers, index}) do
+    worker = Enum.at(workers, index)
+
+    GenServer.cast(worker, {:square, x, from})
+
+    next = rem(index + 1, length(workers))
+    # Do not reply here; the selected worker replies to `from` after the cast.
+    {:noreply, {workers, next}}
+  end
+
+  @impl true
+  def handle_call({:sqrt, x}, from, {workers, index}) do
+    worker = Enum.at(workers, index)
+
+    GenServer.cast(worker, {:sqrt, x, from})
+
+    next = rem(index + 1, length(workers))
+    {:noreply, {workers, next}}
+  end
+
+  @impl true
+  def handle_info({:EXIT, dead, reason}, {workers, index}) do
+    {:ok, new} = Arithmetic.Worker.start_link()
+    IO.puts("Worker #{inspect(dead)} died (#{inspect(reason)}), replacement: #{inspect(new)}")
+
+    workers = Enum.map(workers, fn pid -> if pid == dead, do: new, else: pid end)
+    {:noreply, {workers, index}}
+  end
+end