lecture 06

lectures/06/counter/dynamic/client/dynamic_supervisor.ex

@@ -0,0 +1,13 @@
+defmodule Counter.DynamicSupervisor do
+  @name  {:global, __MODULE__}
+
+  def start_link(_) do
+    DynamicSupervisor.start_link(__MODULE__, nil, name: @name)
+  end
+
+  def start_worker(name) do
+    DynamicSupervisor.start_child(@name, %{
+      id: Counter.Worker, 
+      start: {Counter.Worker, :start_link, [name]}})
+  end
+end

lectures/06/counter/dynamic/client/worker.ex

@@ -0,0 +1,18 @@
+defmodule Counter.Worker do
+  # client
+  def start_link(name) do
+    GenServer.start_link(__MODULE__, name, name: via(name))
+  end
+
+  def value(name) do
+    GenServer.call(via(name), :value)
+  end
+
+  def inc(name, amt \\ 1) do
+    GenServer.cast(via(name), {:inc, amt})
+  end
+
+  defp via(name) do
+    {:global, name}
+  end
+end

lectures/06/counter/dynamic/server/.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

lectures/06/counter/dynamic/server/.gitignore

@@ -0,0 +1,24 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Temporary files, for example, from tests.
+/tmp/
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+dynamic_counters-*.tar
+

lectures/06/counter/dynamic/server/README.md

@@ -0,0 +1,21 @@
+# Counter
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `dynamic_counters` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:dynamic_counters, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/dynamic_counters>.
+

lectures/06/counter/dynamic/server/lib/counter.ex

@@ -0,0 +1,18 @@
+defmodule Counter do
+  @moduledoc """
+  Documentation for `Counter`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> Counter.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

lectures/06/counter/dynamic/server/lib/counter/application.ex

@@ -0,0 +1,26 @@
+defmodule Counter.Application do
+  # See https://hexdocs.pm/elixir/Application.html
+  # for more information on OTP Applications
+  @moduledoc false
+
+  @registry  Counter.Registry
+  @table     Counter.Table
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+      {Registry, [name: @registry, keys: :unique]},
+      # Starts a worker by calling: Counter.Worker.start_link(arg)
+      # {DynamicSupervisor, name: Counter.DynamicSupervisor, strategy: :one_for_one}
+      Counter.DynamicSupervisor 
+    ]
+
+    :ets.new(@table, [:named_table, :public])
+    # See https://hexdocs.pm/elixir/Supervisor.html
+    # for other strategies and supported options
+    opts = [strategy: :one_for_one, name: Counter.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end

lectures/06/counter/dynamic/server/lib/counter/dynamic_supervisor.ex

@@ -0,0 +1,17 @@
+defmodule Counter.DynamicSupervisor do
+  use DynamicSupervisor
+  @name  {:global, __MODULE__}
+
+  def start_link(_) do
+    DynamicSupervisor.start_link(__MODULE__, nil, name: @name)
+  end
+
+  def start_worker(name) do
+    DynamicSupervisor.start_child(@name, {Counter.Worker, name})
+  end
+
+  @impl true
+  def init(_) do
+    DynamicSupervisor.init(strategy: :one_for_one)
+  end
+end

lectures/06/counter/dynamic/server/lib/counter/worker.ex

@@ -0,0 +1,47 @@
+defmodule Counter.Worker do
+  use GenServer
+  @table     Counter.Table
+
+  # client
+  def start_link(name) do
+    GenServer.start_link(__MODULE__, name, name: via(name))
+  end
+
+  def value(name) do
+    GenServer.call(via(name), :value)
+  end
+
+  def inc(name, amt \\ 1) do
+    GenServer.cast(via(name), {:inc, amt})
+  end
+
+  defp via(name) do
+    {:global, name}
+  end
+
+  # server
+  @impl true
+  def init(arg) do
+    name = {__MODULE__, arg}
+    value = case :ets.lookup(@table, name) do
+      [] -> 0
+      [{_, v}] -> v
+    end
+    {:ok, {name, value}}
+  end
+
+  @impl true
+  def handle_call(:value, _from, {_, value} = state) do
+    {:reply, value, state}
+  end
+
+  @impl true
+  def handle_cast({:inc, amt}, {name, value}) do
+    {:noreply, {name, value + amt}}
+  end
+
+  @impl true
+  def terminate(_reason, state) do
+    :ets.insert(@table, state)
+  end
+end

lectures/06/counter/dynamic/server/mix.exs

@@ -0,0 +1,29 @@
+defmodule Counter.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :dynamic_counters,
+      version: "0.1.0",
+      elixir: "~> 1.20",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {Counter.Application, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end

lectures/06/counter/dynamic/server/test/counter_test.exs

@@ -0,0 +1,8 @@
+defmodule CounterTest do
+  use ExUnit.Case
+  doctest Counter
+
+  test "greets the world" do
+    assert Counter.hello() == :world
+  end
+end

lectures/06/counter/dynamic/server/test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()

lectures/06/counter/static/.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

lectures/06/counter/static/.gitignore

@@ -0,0 +1,24 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Temporary files, for example, from tests.
+/tmp/
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+counter-*.tar
+

lectures/06/counter/static/client/worker.ex

@@ -0,0 +1,17 @@
+defmodule Counter.Worker do
+  @name   {:global, __MODULE__}
+
+  # client
+  def start_link(n \\ 0) do
+    GenServer.start_link(__MODULE__, n, name: @name)
+  end
+
+  def inc(amt \\ 1) do
+    GenServer.cast(@name, {:inc, amt})
+  end
+
+  def value() do
+    GenServer.call(@name, :value)
+  end
+end
+

lectures/06/counter/static/server/README.md

@@ -0,0 +1,21 @@
+# Counter
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `counter` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:counter, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/counter>.
+

lectures/06/counter/static/server/counter.db

@@ -0,0 +1 @@
+ƒa

lectures/06/counter/static/server/lib/counter.ex

@@ -0,0 +1,18 @@
+defmodule Counter do
+  @moduledoc """
+  Documentation for `Counter`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> Counter.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

lectures/06/counter/static/server/lib/counter/application.ex

@@ -0,0 +1,21 @@
+defmodule Counter.Application do
+  # See https://hexdocs.pm/elixir/Application.html
+  # for more information on OTP Applications
+  @moduledoc false
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+      # Starts a worker by calling: Counter.Worker.start_link(arg)
+      {Counter.Store, "counter.db"},
+      {Counter.Worker, 0}
+    ]
+
+    # See https://hexdocs.pm/elixir/Supervisor.html
+    # for other strategies and supported options
+    opts = [strategy: :one_for_one, name: Counter.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end

lectures/06/counter/static/server/lib/counter/store.ex

@@ -0,0 +1,41 @@
+defmodule Counter.Store do
+  use GenServer
+
+  # client
+  def start_link(file) do
+    GenServer.start_link(__MODULE__, file, name: __MODULE__)
+  end
+
+  def put(value) do
+    GenServer.cast(__MODULE__, {:put, value})
+  end
+
+  def get() do
+    GenServer.call(__MODULE__, :get)
+  end
+
+  # server
+  @impl true
+  def init(file) do
+    {:ok, file}
+  end
+
+  @impl true
+  def handle_call(:get, _from, file) do
+    reply = 
+      case File.read(file) do 
+        {:ok, content} -> :erlang.binary_to_term(content)
+        _ -> nil
+      end
+    {:reply, reply, file}
+  end
+
+  @impl true
+  def handle_cast({:put, value}, file) do
+    File.write(file, :erlang.term_to_binary(value))
+    {:noreply, file}
+  end
+end
+
+
+

lectures/06/counter/static/server/lib/counter/worker.ex

@@ -0,0 +1,47 @@
+defmodule Counter.Worker do
+  use GenServer
+
+  @store  Counter.Store
+  @name   {:global, __MODULE__}
+
+  # client
+  def start_link(n \\ 0) do
+    GenServer.start_link(__MODULE__, n, name: @name)
+  end
+
+  def inc(amt \\ 1) do
+    GenServer.cast(@name, {:inc, amt})
+  end
+
+  def value() do
+    GenServer.call(@name, :value)
+  end
+
+  # server
+  @impl true
+  def init(arg) do
+    {:ok, @store.get() || arg}
+  end
+
+  @impl true
+  def handle_call(:value, _from, state) do
+    {:reply, state, state}
+  end
+
+  @impl true
+  def handle_cast({:inc, amt}, state) do
+    {:noreply, state + amt}
+  end
+
+  # for demonstration only
+  @impl true
+  def handle_info(:reset, _state) do
+    {:noreply, 0}
+  end
+
+  @impl true
+  def terminate(_reason, state) do
+    @store.put(state)
+  end
+end
+

lectures/06/counter/static/server/mix.exs

@@ -0,0 +1,29 @@
+defmodule Counter.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :counter,
+      version: "0.1.0",
+      elixir: "~> 1.20",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {Counter.Application, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end

lectures/06/counter/static/server/test/counter_test.exs

@@ -0,0 +1,8 @@
+defmodule CounterTest do
+  use ExUnit.Case
+  doctest Counter
+
+  test "greets the world" do
+    assert Counter.hello() == :world
+  end
+end

lectures/06/counter/static/server/test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()

lectures/06/echo/echo.ex

@@ -0,0 +1,144 @@
+defmodule Echo.Active do
+  def start(port \\ 6666) do
+    opts = [:binary, active: true, packet: :line, reuseaddr: true]
+    {:ok, listen} = :gen_tcp.listen(port, opts)
+    accept(listen)
+  end
+
+  def accept(listen) do
+    {:ok, socket} = :gen_tcp.accept(listen)  # data delivered as messages in the mailbox of the
+                                             # process that calls accept
+    spawn(fn -> accept(listen) end)
+    loop(socket)
+  end
+
+  defp loop(socket) do
+    receive do
+      {:tcp, ^socket, data} ->
+        :gen_tcp.send(socket, data)
+        loop(socket)
+      {:tcp_closed, ^socket} ->
+        :gen_tcp.close(socket)
+      {:tcp_error, ^socket, _reason} ->
+        :gen_tcp.close(socket)
+    end
+  end
+end
+
+defmodule Echo.Passive do
+  def start(port \\ 6666) do
+    opts = [:binary, active: false, packet: :line, reuseaddr: true]  # note: active: false
+    {:ok, listen} = :gen_tcp.listen(port, opts)
+    accept(listen)
+  end
+
+  def accept(listen) do
+    {:ok, socket} = :gen_tcp.accept(listen)  # data delivered as messages in the mailbox of the
+                                             # process that calls accept
+    spawn(fn -> accept(listen) end)
+    loop(socket)
+  end
+
+  def loop(socket) do
+    case :gen_tcp.recv(socket, 0) do
+      {:ok, data} ->
+        :gen_tcp.send(socket, data)
+        loop(socket)
+      {:error, _reason} ->
+        :gen_tcp.close(socket)
+    end
+  end
+end
+
+defmodule Echo.Hybrid do
+  def start(port \\ 6666) do
+    opts = [:binary, active: :once, packet: :line, reuseaddr: true]
+    {:ok, listen} = :gen_tcp.listen(port, opts)
+    accept(listen)
+  end
+
+  def accept(listen) do
+    {:ok, socket} = :gen_tcp.accept(listen)  # data delivered as messages in the mailbox of the
+                                             # process that calls accept (the controlling process)
+    spawn(fn -> accept(listen) end)
+    loop(socket)
+  end
+
+  defp loop(socket) do
+    receive do
+      {:tcp, ^socket, data} ->
+        :inet.setopts(socket, active: :once)
+        :gen_tcp.send(socket, data)
+        loop(socket)
+      {:tcp_closed, ^socket} ->
+        :gen_tcp.close(socket)
+      {:tcp_error, ^socket, _reason} ->
+        :gen_tcp.close(socket)
+    end
+  end
+end
+
+defmodule Echo do
+  use GenServer
+  require Logger
+
+  def start(port \\ 6666) do
+    GenServer.start(__MODULE__, port)
+  end
+
+  @impl true
+  def init(port) do
+    opts = [:binary, active: true, packet: :line, reuseaddr: true]
+    {:ok, listen} = :gen_tcp.listen(port, opts)
+    send(self(), :accept)
+    {:ok, listen}
+  end
+
+  @impl true
+  def handle_info(:accept, listen) do
+    {:ok, socket} = :gen_tcp.accept(listen)
+    log(socket, "accepted connection")
+    {:ok, pid} = Echo.Worker.start(socket)
+    :gen_tcp.controlling_process(socket, pid)
+    send(self(), :accept)
+    {:noreply, listen}
+  end
+
+  defp log(socket, msg) do
+    Logger.info("#{inspect self()}: #{msg} #{inspect socket}")
+  end
+end
+
+defmodule Echo.Worker do
+  use GenServer
+  require Logger
+
+  def start(socket) do
+    GenServer.start(__MODULE__, socket)
+  end
+
+  @impl true
+  def init(socket) do
+    log(socket, "started worker")
+    {:ok, socket}
+  end
+
+  @impl true
+  def handle_info({:tcp, socket, data}, socket) do
+    :inet.setopts(socket, active: :once)
+    :gen_tcp.send(socket, data)
+    {:noreply, socket}
+  end
+
+  @impl true
+  def handle_info({:tcp_closed, socket}, socket) do
+    :gen_tcp.close(socket)
+    log(socket, "closed connection")
+    {:stop, :normal, socket}
+  end
+
+  defp log(socket, msg) do
+    Logger.info("#{inspect self()}: #{msg} #{inspect socket}")
+  end
+end
+

lectures/06/echo/echo_client.ex

@@ -0,0 +1,31 @@
+defmodule Echo.Client do
+  def connect(host, port \\ 6666) do
+    opts = [active: false, packet: :line]
+    {:ok, socket} = :gen_tcp.connect(host, port, opts)
+    loop(socket)
+  end
+
+  defp loop(socket) do
+    case IO.gets("> ") do
+      :eof ->
+        :gen_tcp.close(socket)
+      line -> 
+        :gen_tcp.send(socket, line)
+        case :gen_tcp.recv(socket, 0) do
+          {:ok, data} -> 
+            IO.write(data) 
+            loop(socket)
+          {:error, _} ->
+            :gen_tcp.close(socket)
+        end
+    end
+  end
+end
+
+with [host, port | _] <- System.argv(),
+     {port, _} <- Integer.parse(port)
+do
+  Echo.Client.connect(String.to_atom(host), port)
+else
+  _ -> Echo.Client.connect(:"127.0.0.1", 6666)
+end

lectures/06/java/EchoClient.java

@@ -0,0 +1,32 @@
+import java.io.*;
+import java.net.*;
+
+class EchoClient {
+  public static void main(String... args) throws IOException {
+    var host = "127.0.0.1";
+    var port = 6666;
+    try ( 
+      var s = new Socket(host, port);
+      var in = 
+        new BufferedReader(new InputStreamReader(s.getInputStream()));
+      var out =
+        new PrintWriter(new OutputStreamWriter(s.getOutputStream()), true);
+      var stdin =
+        new BufferedReader(new InputStreamReader(System.in));
+    ) {
+      String line, reply;
+
+      while (true) {
+        System.out.print("> ");
+        if ((line = stdin.readLine()) == null) {
+          break;
+        }
+        out.println(line);
+        if ((reply = in.readLine()) == null) {
+          break;
+        }
+        System.out.println(reply);
+      }
+    }
+  }
+}

lectures/06/notes.ex

@@ -0,0 +1,160 @@
+# Lecture 06 notes: raw sockets, then splitting a client/server across nodes
+# This file is not meant to be compiled/run as a whole - paste lines into
+# iex (or a shell, where noted) one at a time.
+
+
+### ---------------------------------------------------------------------- ###
+### Part 1: echo/ - plain TCP, no distributed Elixir involved yet
+### ---------------------------------------------------------------------- ###
+
+# load the module definitions into an iex session (run from lectures/06/echo/)
+# iex echo.ex
+
+# Echo.Active / Echo.Passive / Echo.Hybrid all block the calling process
+# forever inside accept/loop, so start them in a spawned process if you
+# want your iex shell back:
+spawn(fn -> Echo.Active.start(6666) end)
+spawn(fn -> Echo.Passive.start(6667) end)
+spawn(fn -> Echo.Hybrid.start(6668) end)
+
+# the GenServer version returns immediately because init/1 just schedules
+# :accept via send(self(), :accept) and returns - no spawn needed
+Echo.start(6666)
+# -> {:ok, #PID<...>}
+
+# quick manual client, no Elixir needed - good for sanity-checking a server
+# nc localhost 6666
+# (type a line, press enter, see it echoed back; Ctrl-D or Ctrl-C to quit)
+
+# or use the provided Elixir client (run from a second terminal):
+# elixir echo_client.ex 127.0.0.1 6666
+# (defaults to 127.0.0.1:6666 if no args given)
+
+# or the Java client (compiles/runs independently - proves the server
+# doesn't care what language/process connects, it's just bytes on a socket)
+# cd ../java && javac EchoClient.java && java EchoClient
+
+
+### ---------------------------------------------------------------------- ###
+### Part 2: distributed Elixir basics - the mechanism the counter/ examples
+### rely on instead of hand-rolled sockets
+### ---------------------------------------------------------------------- ###
+
+# Start two connected BEAM nodes (same machine is fine for testing).
+# --sname gives each node a short name; both MUST share a cookie or they
+# refuse to connect to each other.
+
+# terminal 1 (this will become the "server" node):
+# iex --sname server --cookie lecture06
+
+# terminal 2 (this will become the "client" node):
+# iex --sname client --cookie lecture06
+
+# from either node, connect them:
+Node.connect(:"server@your-hostname")
+# -> true
+# find the exact name to use with: Node.self() (run that on the server node)
+
+Node.list()          # -> [:"client@your-hostname"] (from the server's side)
+Node.self()           # -> the current node's own full name
+
+# :global is what replaces Registry here - Registry is local-to-a-node only,
+# :global registers names cluster-wide once nodes are connected
+:global.registered_names()        # -> [] until something registers
+:global.whereis_name(Counter.Worker)   # -> PID or :undefined
+
+
+### ---------------------------------------------------------------------- ###
+### Part 3: counter/static - one globally-registered counter
+### ---------------------------------------------------------------------- ###
+
+# -- server node --
+# cd counter/static/server
+# mix deps.get        # first time only
+# iex --sname server --cookie lecture06 -S mix
+# this boots Counter.Application, which starts Counter.Store (file-backed,
+# "counter.db") and Counter.Worker (registered as {:global, Counter.Worker})
+
+Counter.Worker.value()    # works locally on the server node too
+Counter.Worker.inc(5)
+
+# -- client node --
+# cd counter/static/client   (NOT a mix project - just loose .ex files)
+# iex --sname client --cookie lecture06
+Node.connect(:"server@your-hostname")
+c "worker.ex"              # compiles/loads Counter.Worker (the thin stub)
+
+# these now run against the REAL process living on the server node -
+# no socket code written by us, distributed Erlang routes the GenServer
+# call/cast over the network
+Counter.Worker.inc()
+Counter.Worker.value()
+# -> whatever the server node's Counter.Worker currently holds
+
+# don't call Counter.Worker.start_link/1 from the client - the server's
+# Application already started (and globally registered) the one that
+# matters; starting another just races/duplicates the name
+
+
+### ---------------------------------------------------------------------- ###
+### Part 4: counter/dynamic - many named counters, started on demand,
+### reachable from a remote node
+### ---------------------------------------------------------------------- ###
+
+# -- server node --
+# cd counter/dynamic/server
+# mix deps.get
+# iex --sname server --cookie lecture06 -S mix
+# boots: Registry (local, unused for cross-node lookup), an ETS table for
+# crash-durable state, and Counter.DynamicSupervisor registered as
+# {:global, Counter.DynamicSupervisor}
+
+# -- client node --
+# cd counter/dynamic/client
+# iex --sname client --cookie lecture06
+Node.connect(:"server@your-hostname")
+c "dynamic_supervisor.ex"
+c "worker.ex"
+
+# ask the REMOTE supervisor (on the server node) to spawn a new worker -
+# the child process is started under the server's supervision tree, not
+# the client's
+Counter.DynamicSupervisor.start_worker("c1")
+# -> {:ok, #PID<...>}   (a server-node PID)
+
+Counter.Worker.inc("c1")
+Counter.Worker.inc("c1", 5)
+Counter.Worker.value("c1")
+# -> 6
+
+# start as many independent counters as you like, all living on the
+# server node, all addressable from the client by name:
+Enum.each(1..5, fn n -> Counter.DynamicSupervisor.start_worker("c#{n}") end)
+Enum.each(1..5, fn n -> Counter.Worker.inc("c#{n}", n) end)
+Enum.map(1..5, fn n -> Counter.Worker.value("c#{n}") end)
+# -> [1, 2, 3, 4, 5]
+
+# kill a worker on the server node and watch it come back with its value
+# intact (DynamicSupervisor restarts it, ETS on the server supplies the
+# last known value in init/1):
+# on the SERVER node:
+Process.exit(:global.whereis_name("c1"), :kill)
+# back on the CLIENT node:
+Counter.Worker.value("c1")
+# -> still 1 (reloaded from the server's ETS table in init/1, same pattern
+# as lecture 05's local DynamicSupervisor/Registry/ETS combo - just now the
+# "local" side is a different machine)
+
+
+### ---------------------------------------------------------------------- ###
+### Misc / troubleshooting
+### ---------------------------------------------------------------------- ###
+
+# cookie mismatch or wrong hostname are the two most common reasons
+# Node.connect/1 silently returns false - check `iex --sname` output for
+# the exact full node name, and make sure both nodes used the same --cookie
+
+# run a mix app's tests / compile without a full iex session:
+# mix test
+# mix compile
+# mix run --no-halt      # boots the Application and just hangs there running