Start with initial children

.formatter.exs

@@ -1,4 +1,7 @@
 # Used by "mix format"
 [
-  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+  inputs: [
+    "{mix,.formatter}.exs",
+    "{config,lib,test,test_app,test_app_with_children}/**/*.{ex,exs}"
+  ]
 ]

lib/pogo/dynamic_supervisor.ex

@@ -30,6 +30,13 @@ defmodule Pogo.DynamicSupervisor do
 
   use GenServer, type: :supervisor
 
+  @type option :: {:name, Supervised.name()}
+  @type init_option ::
+          Supervisor.init_option()
+          | {:scope, term}
+          | {:sync_interval, pos_integer()}
+          | {:children, [Supervisor.child_spec()]}
+
   @sync_interval 5_000
 
   defstruct [:scope, :supervisor, :sync_interval]
@@ -48,8 +55,14 @@ defmodule Pogo.DynamicSupervisor do
     * `sync_interval` - interval in milliseconds, how often the supervisor should
       synchronize its local state with the cluster by processing requests to
       start and terminate child processes, defaults to `5_000`
+    * `children` - list of child specifications to automatially start in the cluster
+
+  Children specified at startup are handled in the same way as children started
+  dynamically. The only difference is that when `Pogo.DynamicSupervisor` runs under
+  a supervision tree and is restarted, it will automatically restart all these
+  children as well.
   """
-  @spec start_link(keyword) :: GenServer.on_start()
+  @spec start_link([option | init_option]) :: GenServer.on_start()
   def start_link(opts) do
     {name, opts} = Keyword.pop(opts, :name, __MODULE__)
     GenServer.start_link(__MODULE__, opts, name: name)
@@ -93,6 +106,7 @@ defmodule Pogo.DynamicSupervisor do
   def init(opts) do
     scope = Keyword.fetch!(opts, :scope)
     {sync_interval, opts} = Keyword.pop(opts, :sync_interval, @sync_interval)
+    {children, opts} = Keyword.pop(opts, :children, [])
 
     opts = Keyword.put(opts, :strategy, :one_for_one)
 
@@ -109,7 +123,18 @@ defmodule Pogo.DynamicSupervisor do
 
     Process.send_after(self(), :sync, sync_interval)
 
-    {:ok, state}
+    {:ok, state, {:continue, {:start_children, children}}}
+  end
+
+  @impl true
+  def handle_continue({:start_children, children}, %{scope: scope} = state) do
+    for child_spec <- children do
+      child_spec = Supervisor.child_spec(child_spec, [])
+      :ok = validate_child(child_spec)
+      make_request(scope, {:start_child, child_spec})
+    end
+
+    {:noreply, state}
   end
 
   @impl true

mix.exs

@@ -33,6 +33,7 @@ defmodule Pogo.MixProject do
       {:libring, "~> 1.6.0"},
       {:local_cluster, "~> 1.2.1", only: [:test]},
       {:test_app, path: "test_app", only: [:test]},
+      {:test_app_with_children, path: "test_app_with_children", only: [:test]},
       {:ex_doc, "~> 0.29", only: [:dev], runtime: false},
       {:credo, "~> 1.7", only: [:dev, :test], runtime: false}
     ]

test/pogo/dynamic_supervisor_test.exs

@@ -2,7 +2,7 @@ defmodule Pogo.DynamicSupervisorTest do
   use ExUnit.Case
   import AssertAsync
 
-  @supervisor TestApp.DistributedSupervisor
+  @supervisor PogoTest.DistributedSupervisor
 
   setup do
     on_exit(fn ->
@@ -15,7 +15,7 @@ defmodule Pogo.DynamicSupervisorTest do
   end
 
   test "starts child on a single node in the cluster" do
-    [node1, node2] = start_nodes("foo", 2)
+    [node1, node2] = start_nodes(:test_app, "foo", 2)
 
     child_spec = Pogo.Worker.child_spec(1)
 
@@ -31,7 +31,7 @@ defmodule Pogo.DynamicSupervisorTest do
   end
 
   test "terminates child running in the cluster" do
-    [node1, node2] = nodes = start_nodes("foo", 2)
+    [node1, node2] = nodes = start_nodes(:test_app, "foo", 2)
 
     [child_spec1, _child_spec2, child_spec3] =
       for id <- 1..3 do
@@ -80,7 +80,7 @@ defmodule Pogo.DynamicSupervisorTest do
   end
 
   test "keeps track of new pid when child process crashes and gets restarted" do
-    [node] = start_nodes("foo", 1)
+    [node] = start_nodes(:test_app, "foo", 1)
 
     child_spec = Pogo.Worker.child_spec(1)
     start_child(node, child_spec)
@@ -99,7 +99,7 @@ defmodule Pogo.DynamicSupervisorTest do
   end
 
   test "moves children between nodes when cluster topology changes" do
-    [node1] = start_nodes("foo", 1)
+    [node1] = start_nodes(:test_app, "foo", 1)
 
     start_child(node1, Pogo.Worker.child_spec(1))
     start_child(node1, Pogo.Worker.child_spec(2))
@@ -113,7 +113,7 @@ defmodule Pogo.DynamicSupervisorTest do
              } = local_children([node1])
     end
 
-    [node2] = start_nodes("bar", 1)
+    [node2] = start_nodes(:test_app, "bar", 1)
 
     assert_async do
       assert %{
@@ -135,7 +135,7 @@ defmodule Pogo.DynamicSupervisorTest do
   end
 
   test "starts process on another node when the node it was scheduled on goes down" do
-    [node1, node2, node3] = start_nodes("foo", 3)
+    [node1, node2, node3] = start_nodes(:test_app, "foo", 3)
 
     start_child(node2, Pogo.Worker.child_spec(1))
 
@@ -161,10 +161,53 @@ defmodule Pogo.DynamicSupervisorTest do
     end
   end
 
+  test "starts distributed supervisor with initial list of children" do
+    [node1, node2, node3] = start_nodes(:test_app_with_children, "foo", 3)
+
+    assert_async do
+      assert %{
+               ^node1 => [{{Pogo.Worker, 2}, _, :worker, _}],
+               ^node2 => [
+                 {{Pogo.Worker, 1}, _, :worker, _},
+                 {{Pogo.Worker, 3}, _, :worker, _}
+               ],
+               ^node3 => []
+             } = local_children([node1, node2, node3])
+    end
+  end
+
+  test "initial list of children is automatically started after supervisor crash" do
+    [node] = start_nodes(:test_app_with_children, "foo", 1)
+
+    assert_async do
+      assert %{
+               ^node => [
+                 {{Pogo.Worker, 1}, _, :worker, _},
+                 {{Pogo.Worker, 2}, _, :worker, _},
+                 {{Pogo.Worker, 3}, _, :worker, _}
+               ]
+             } = local_children([node])
+    end
+
+    # kill supervisor
+    pid = :rpc.call(node, Process, :whereis, [PogoTest.DistributedSupervisor])
+    :rpc.call(node, Process, :exit, [pid, :brutal_kill])
+
+    assert_async do
+      assert %{
+               ^node => [
+                 {{Pogo.Worker, 1}, _, :worker, _},
+                 {{Pogo.Worker, 2}, _, :worker, _},
+                 {{Pogo.Worker, 3}, _, :worker, _}
+               ]
+             } = local_children([node])
+    end
+  end
+
   @tag chaos: true
   test "chaos" do
     # start nodes
-    nodes = start_nodes("foo", 10)
+    nodes = start_nodes(:test_app, "foo", 10)
 
     # start children
     specs = for id <- 1..60, do: Pogo.Worker.child_spec(id)
@@ -192,7 +235,7 @@ defmodule Pogo.DynamicSupervisorTest do
     :timer.sleep(1000)
 
     # start some more nodes
-    nodes = nodes ++ start_nodes("bar", 10)
+    nodes = nodes ++ start_nodes(:test_app, "bar", 10)
 
     :timer.sleep(1000)
 
@@ -270,7 +313,7 @@ defmodule Pogo.DynamicSupervisorTest do
 
   describe "which_children/1" do
     test "returns children running on the node when called with :local" do
-      [node1, node2] = nodes = start_nodes("foo", 2)
+      [node1, node2] = nodes = start_nodes(:test_app, "foo", 2)
 
       start_child(node1, Pogo.Worker.child_spec(1))
       start_child(node1, Pogo.Worker.child_spec(2))
@@ -299,7 +342,7 @@ defmodule Pogo.DynamicSupervisorTest do
     end
 
     test "returns all children running in cluster when called with :global" do
-      [node1, node2] = start_nodes("foo", 2)
+      [node1, node2] = start_nodes(:test_app, "foo", 2)
 
       start_child(node1, Pogo.Worker.child_spec(1))
       start_child(node1, Pogo.Worker.child_spec(2))
@@ -334,9 +377,9 @@ defmodule Pogo.DynamicSupervisorTest do
     end
   end
 
-  defp start_nodes(prefix, n) do
+  defp start_nodes(app, prefix, n) do
     LocalCluster.start_nodes(prefix, n,
-      applications: [:test_app],
+      applications: [app],
       files: ["test/support/pogo/worker.ex"]
     )
   end

test_app/lib/test_app/application.ex

@@ -9,7 +9,7 @@ defmodule TestApp.Application do
   def start(_type, _args) do
     children = [
       {Pogo.DynamicSupervisor,
-       name: TestApp.DistributedSupervisor, scope: :test, sync_interval: 100}
+       name: PogoTest.DistributedSupervisor, scope: :test, sync_interval: 100}
     ]
 
     # See https://hexdocs.pm/elixir/Supervisor.html

test_app_with_children/lib/test_app_with_children/application.ex

@@ -0,0 +1,27 @@
+defmodule TestAppWithChildren.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 = [
+      {Pogo.DynamicSupervisor,
+       name: PogoTest.DistributedSupervisor,
+       scope: :test,
+       sync_interval: 100,
+       children: [
+         Pogo.Worker.child_spec(1),
+         Pogo.Worker.child_spec(2),
+         {Pogo.Worker, 3}
+       ]}
+    ]
+
+    # See https://hexdocs.pm/elixir/Supervisor.html
+    # for other strategies and supported options
+    opts = [strategy: :one_for_one, name: TestAppWithChildren.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end

test_app_with_children/mix.exs

@@ -0,0 +1,29 @@
+defmodule TestAppWithChildren.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :test_app_with_children,
+      version: "0.1.0",
+      elixir: "~> 1.11",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {TestAppWithChildren.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