Do not redeploy apps from dir on every node

lib/livebook/application.ex

@@ -333,7 +333,8 @@ defmodule Livebook.Application do
 
       Livebook.Apps.deploy_apps_in_dir(apps_path,
         password: Livebook.Config.apps_path_password(),
-        warmup: warmup
+        warmup: warmup,
+        start_only: true
       )
     end
   end

lib/livebook/apps.ex

@@ -22,10 +22,14 @@ defmodule Livebook.Apps do
     * `:files_source` - a location to fetch notebook files from, see
       `Livebook.Session.start_link/1` for more details
 
+    * `:start_only` - when `true`, deploys only if the app does not
+      exist already. Defaults to `false`
+
   """
-  @spec deploy(Livebook.Notebook.t(), keyword()) :: {:ok, pid()} | {:error, term()}
+  @spec deploy(Livebook.Notebook.t(), keyword()) ::
+          {:ok, pid()} | {:error, :already_started} | {:error, term()}
   def deploy(notebook, opts \\ []) do
-    opts = Keyword.validate!(opts, warnings: [], files_source: nil)
+    opts = Keyword.validate!(opts, warnings: [], files_source: nil, start_only: false)
 
     slug = notebook.app_settings.slug
     name = name(slug)
@@ -41,18 +45,12 @@ defmodule Livebook.Apps do
               end
 
             pid ->
-              App.deploy(pid, notebook,
-                warnings: opts[:warnings],
-                files_source: opts[:files_source]
-              )
-
-              {:ok, pid}
+              redeploy_app(pid, notebook, opts)
           end
         end)
 
       pid ->
-        App.deploy(pid, notebook, warnings: opts[:warnings], files_source: opts[:files_source])
-        {:ok, pid}
+        redeploy_app(pid, notebook, opts)
     end
   end
 
@@ -77,6 +75,15 @@ defmodule Livebook.Apps do
     end
   end
 
+  defp redeploy_app(pid, notebook, opts) do
+    if opts[:start_only] do
+      {:error, :already_started}
+    else
+      App.deploy(pid, notebook, warnings: opts[:warnings], files_source: opts[:files_source])
+      {:ok, pid}
+    end
+  end
+
   @doc """
   Returns app process pid for the given slug.
   """
@@ -166,10 +173,14 @@ defmodule Livebook.Apps do
       This can be used to warmup apps without deployment. Defaults
       to `false`
 
+    * `:start_only` - when `true`, deploys only if the app does not
+      exist already. Defaults to `false`
+
   """
   @spec deploy_apps_in_dir(String.t(), keyword()) :: :ok
   def deploy_apps_in_dir(path, opts \\ []) do
-    opts = Keyword.validate!(opts, [:password, warmup: true, skip_deploy: false])
+    opts =
+      Keyword.validate!(opts, [:password, warmup: true, skip_deploy: false, start_only: false])
 
     infos = import_app_notebooks(path)
 
@@ -226,7 +237,11 @@ defmodule Livebook.Apps do
 
         warnings = Enum.map(info.import_warnings, &("Import: " <> &1))
 
-        {:ok, _} = deploy(notebook, warnings: warnings, files_source: info.files_source)
+        deploy(notebook,
+          warnings: warnings,
+          files_source: info.files_source,
+          start_only: opts[:start_only]
+        )
       end
     end
 

test/livebook/apps_test.exs

@@ -261,5 +261,29 @@ defmodule Livebook.AppsTest do
 
       Livebook.App.close(app.pid)
     end
+
+    @tag :tmp_dir
+    test "skips existing apps when :start_only is enabled", %{tmp_dir: tmp_dir} do
+      app_path = Path.join(tmp_dir, "app.livemd")
+
+      File.write!(app_path, """
+      <!-- livebook:{"app_settings":{"access_type":"public","slug":"app"}} -->
+
+      # App
+      """)
+
+      Livebook.Apps.subscribe()
+
+      Livebook.Apps.deploy_apps_in_dir(tmp_dir)
+      assert_receive {:app_updated, app}
+
+      Livebook.Apps.deploy_apps_in_dir(tmp_dir)
+      assert %{version: 2} = Livebook.App.get_by_pid(app.pid)
+
+      Livebook.Apps.deploy_apps_in_dir(tmp_dir, start_only: true)
+      assert %{version: 2} = Livebook.App.get_by_pid(app.pid)
+
+      Livebook.App.close(app.pid)
+    end
   end
 end