Code for step 4

Author: akoutmos

lib/book_store/application.ex

@@ -13,6 +13,8 @@ defmodule BookStore.Application do
       BookStoreWeb.Telemetry,
       # Start the PubSub system
       {Phoenix.PubSub, name: BookStore.PubSub},
+      # Start the Book Store Registry
+      BookStore.BookRegistry.child_spec(),
       # Start the Endpoint (http/https)
       BookStoreWeb.Endpoint
       # Start a worker by calling: BookStore.Worker.start_link(arg)

lib/book_store/book_registry.ex

@@ -0,0 +1,19 @@
+defmodule BookStore.BookRegistry do
+  def child_spec do
+    Registry.child_spec(
+      keys: :unique,
+      name: __MODULE__,
+      partitions: System.schedulers_online()
+    )
+  end
+
+  def lookup_book(book_id) do
+    case Registry.lookup(__MODULE__, book_id) do
+      [{book_pid, _}] ->
+        {:ok, book_pid}
+
+      [] ->
+        {:error, :not_found}
+    end
+  end
+end