Update README.md

README.md

@@ -29,8 +29,18 @@ curl -X POST http://localhost:4000/api/messages -H "Content-Type: application/js
 curl -X POST http://localhost:4001/api/messages -H "Content-Type: application/json" -d '{"from": "2-nibbler", "to": "1-bender", "message": "I am Nibbler, agent of the Nibblonian fleet."}'
 ```
 
+### Notes on queueing failed messages
+When messages fail to be delivered they are queued for retry at 30 second interval with a maximum of 5 retries by default (can be configured)
+
 ## Security
 This application uses a shared auth key (`AUTH_KEY`) to authenticate requests between servers. The key is provided as an environment variable and must be the same on all servers.
 
-### Next steps
+## Next steps
+
+### Security - HTTPS with client certificates
 HTTPS with client certificates should be implemented to ensure secure communication between servers and prevent unauthorized access and possible man-in-the-middle attacks.
+
+### Testing
+Full suite of unit tests should be implemented to ensure the correctness of the application's logic and behavior.
+
+### Finish implementation of Message Queue to queue failed messages for retry

lib/message_server/application.ex

@@ -14,6 +14,7 @@ defmodule MessageServer.Application do
     children = [
       {MessageServer.ServerRegistry, {server_id, servers}},
       {MessageServer.Storage, {server_id}},
+      {MessageServer.MessageQueue, [retry_interval: 30_000]},
       {Bandit, plug: MessageServer.Router, port: port}
     ]
 

lib/message_server/message_queue.ex

@@ -0,0 +1,106 @@
+defmodule MessageServer.MessageQueue do
+  use GenServer
+  require Logger
+  alias MessageServer.RemoteClient
+
+  @type queued_message :: %{
+          target_server: String.t(),
+          payload: map(),
+          timestamp: DateTime.t(),
+          retry_count: non_neg_integer()
+        }
+
+  @spec start_link(keyword()) :: GenServer.on_start()
+  def start_link(opts \\ []) do
+    {retry_interval, opts} = Keyword.pop(opts, :retry_interval, 30_000)
+    {queue_name, _opts} = Keyword.pop(opts, :name, __MODULE__)
+    GenServer.start_link(__MODULE__, retry_interval, name: queue_name)
+  end
+
+  @spec queue_message(String.t(), map()) :: :ok
+  def queue_message(target_server, payload) do
+    GenServer.cast(__MODULE__, {:queue_message, target_server, payload})
+  end
+
+  @spec process_queue() :: :ok
+  def process_queue do
+    GenServer.cast(__MODULE__, :process_queue)
+  end
+
+  @impl true
+  def init(retry_interval) do
+    :timer.send_interval(retry_interval, :process_queue)
+    {:ok, %{queue: [], processing: false}}
+  end
+
+  @impl true
+  def handle_cast({:queue_message, target_server, payload}, %{queue: queue} = state) do
+    message = %{
+      target_server: target_server,
+      payload: payload,
+      timestamp: DateTime.utc_now(),
+      retry_count: 0
+    }
+
+    Logger.info("Queueing message for offline server #{target_server}")
+    {:noreply, %{state | queue: [message | queue]}}
+  end
+
+  @impl true
+  def handle_cast(:process_queue, %{processing: true} = state) do
+    {:noreply, state}
+  end
+
+  @impl true
+  def handle_cast(:process_queue, %{queue: []} = state) do
+    {:noreply, state}
+  end
+
+  @impl true
+  def handle_cast(:process_queue, %{queue: queue} = state) do
+    {:noreply, %{state | processing: true}}
+
+    Task.start(fn ->
+      process_queued_messages(queue)
+      GenServer.cast(__MODULE__, {:processing_complete, []})
+    end)
+
+    {:noreply, state}
+  end
+
+  @impl true
+  def handle_cast({:processing_complete, failed_messages}, state) do
+    {:noreply, %{state | queue: failed_messages, processing: false}}
+  end
+
+  @impl true
+  def handle_info(:process_queue, state) do
+    handle_cast(:process_queue, state)
+  end
+
+  @spec process_queued_messages([queued_message()]) :: [queued_message()]
+  defp process_queued_messages(messages) do
+    Enum.reduce(messages, [], fn message, failed_acc ->
+      case attempt_delivery(message) do
+        :ok ->
+          Logger.info("Successfully delivered queued message to #{message.target_server}")
+          failed_acc
+
+        {:error, _reason} ->
+          updated_message = %{message | retry_count: message.retry_count + 1}
+
+          if updated_message.retry_count < 5 do
+            [updated_message | failed_acc]
+          else
+            Logger.error("Dropping message to #{message.target_server} after 5 retries")
+            failed_acc
+          end
+      end
+    end)
+  end
+
+  @spec attempt_delivery(queued_message()) :: :ok | {:error, String.t()}
+  defp attempt_delivery(%{target_server: target_server, payload: payload}) do
+    RemoteClient.send_message_to_server(target_server, payload)
+  end
+end

lib/message_server/remote_client.ex

@@ -1,7 +1,7 @@
 defmodule MessageServer.RemoteClient do
   require Logger
 
-  alias MessageServer.{MessageRequest, ServerRegistry, Auth}
+  alias MessageServer.{MessageRequest, ServerRegistry, Auth, MessageQueue}
 
   @spec send_message_to_server(String.t(), MessageRequest.t()) :: :ok | {:error, String.t()}
   def send_message_to_server(target_server_id, payload) do
@@ -15,6 +15,7 @@ defmodule MessageServer.RemoteClient do
 
       {:error, reason} = error ->
         Logger.error("Failed to send message to server #{target_server_id}: #{reason}")
+        MessageQueue.queue_message(target_server_id, payload)
         error
     end
   end

lib/message_server/router.ex

@@ -60,11 +60,6 @@ defmodule MessageServer.Router do
           |> send_resp(400, Jason.encode!(%{error: reason}))
       end
     else
-      {:error, :auth_failure, reason} ->
-        conn
-        |> put_resp_content_type("application/json")
-        |> send_resp(401, Jason.encode!(%{error: "Unauthorized", message: reason}))
-
       {:error, reason} ->
         conn
         |> put_resp_content_type("application/json")