Allow protocol to be configured, so either UDP or TCP sockets can be used

config/config.exs

@@ -1 +1,5 @@
 use Mix.Config
+
+config :logger,
+       # disable console backend, so it does not clutter `mix test` output
+       backends: []

lib/logger_logstash_backend.ex

@@ -14,33 +14,80 @@
 # limitations under the License.
 ################################################################################
 defmodule LoggerLogstashBackend do
+  alias LoggerLogstashBackend.Socket
+
   use GenEvent
   use Timex
 
+  # Struct
+
+  defstruct ~w(
+               host
+               level
+               metadata
+               name
+               port
+               protocol
+               socket
+               type
+             )a
+
+  # Functions
+
+  ## GenEvent callbacks
+
   def init({__MODULE__, name}) do
-    {:ok, configure(name, [])}
+    # trap exits, so that cleanup can occur in terminate/2
+    Process.flag(:trap_exit, true)
+
+    configure(name, [])
   end
 
   def handle_call({:configure, opts}, %{name: name}) do
-    {:ok, :ok, configure(name, opts)}
+    case configure(name, opts) do
+      {:ok, state} ->
+        {:ok, :ok, state}
+      reply = {:error, _reason} ->
+        {:remove_handler, reply}
+    end
   end
 
   def handle_event(
     {level, _gl, {Logger, msg, ts, md}}, %{level: min_level} = state
   ) do
     if is_nil(min_level) or Logger.compare_levels(level, min_level) != :lt do
       log_event level, msg, ts, md, state
+    else
+      {:ok, state}
     end
+  end
+
+  def handle_info({:tcp_closed, socket}, state = %__MODULE__{socket: socket}) do
+    case configure_socket(state) do
+      {:error, _} -> {:ok, %{state | socket: nil}}
+      other -> other
+    end
+  end
+
+  def handle_info(_, state) do
     {:ok, state}
   end
 
+  @doc """
+  Closes socket when the backend is removed
+  """
+  def terminate(_, state)  do
+    :ok = Socket.close(state)
+
+    state
+  end
+
+  ## Private Functions
+
   defp log_event(
-    level, msg, ts, md, %{
-      host: host,
-      port: port,
+    level, msg, ts, md, state = %{
       type: type,
-      metadata: metadata,
-      socket: socket
+      metadata: metadata
     }
   ) do
     fields = md
@@ -56,10 +103,11 @@ defmodule LoggerLogstashBackend do
       message: to_string(msg),
       fields: fields
     }
-    :gen_udp.send socket, host, port, to_char_list(json)
+
+    send_with_retry(state, json)
   end
 
-  defp configure(name, opts) do
+  defp configure(name, opts) when is_atom(name) and is_list(opts) do
     env = Application.get_env :logger, name, []
     opts = Keyword.merge env, opts
     Application.put_env :logger, name, opts
@@ -69,16 +117,34 @@ defmodule LoggerLogstashBackend do
     type = Keyword.get opts, :type, "elixir"
     host = Keyword.get opts, :host
     port = Keyword.get opts, :port
-    {:ok, socket} = :gen_udp.open 0
-    %{
-      name: name,
-      host: to_char_list(host),
-      port: port,
-      level: level,
-      socket: socket,
-      type: type,
-      metadata: metadata
-    }
+    protocol = Keyword.get opts, :protocol, :udp
+
+    state = %__MODULE__{level: level, metadata: metadata, name: name, protocol: protocol, type: type}
+            |> put_destination(host, port)
+
+    new_state = case configure_socket(state) do
+      {:error, _} -> state
+      {:ok, configured_state} -> configured_state
+    end
+
+    {:ok, new_state}
+  end
+
+  defp configure_socket(state = %__MODULE__{}) do
+    case Socket.open state do
+      reply = {:error, reason} ->
+        IO.puts :stderr, "Could not open socket (#{url(state)}) due to reason #{inspect reason}"
+
+        reply
+      other ->
+        other
+    end
+  end
+
+  defp fallback_log(state, json, reason) do
+    IO.puts :stderr,
+            "Could not log message (#{json}) to socket (#{url(state)}) due to #{inspect reason}.  " <>
+            "Check that state (#{inspect state}) is correct."
   end
 
   # inspects the argument only if it is a pid
@@ -91,4 +157,36 @@ defmodule LoggerLogstashBackend do
       {key, inspect_pid(value)}
     end
   end
+
+  defp put_destination(state = %__MODULE__{}, host, port) when is_nil(host) or is_nil(port), do: state
+  defp put_destination(state = %__MODULE__{}, host, port), do: %{state | host: to_char_list(host), port: port}
+
+  defp send_with_retry(state = %__MODULE__{socket: nil}, json), do: send_with_new_socket(state, json)
+  defp send_with_retry(state, json) do
+    case Socket.send(state, [json, "\n"]) do
+      :ok ->
+        {:ok, state}
+      {:error, :closed} ->
+        send_with_new_socket(state, json)
+      {:error, reason} ->
+        fallback_log(state, json, reason)
+
+        {:ok, state}
+    end
+  end
+
+  defp send_with_new_socket(state, json) do
+    case configure_socket(state) do
+      {:error, reason} ->
+        fallback_log(state, json, reason)
+
+        {:ok, state}
+      {:ok, new_state} ->
+        send_with_retry(new_state, json)
+    end
+  end
+
+  defp url(%__MODULE__{host: host, port: port, protocol: protocol}) do
+    "#{protocol}://#{host}:#{port}"
+  end
 end

lib/logger_logstash_backend/socket.ex

@@ -0,0 +1,50 @@
+defmodule LoggerLogstashBackend.Socket do
+  @moduledoc """
+  Hides differences between `:udp` and `:tcp` sockets, so that protocol can be chosen in configuration
+  """
+
+  # Types
+
+  @type protocol :: :tcp | :udp
+
+  # Functions
+
+  @doc """
+  Closes the `protocol` `socket`
+  """
+  @spec close(map) :: :ok
+
+  def close(%{socket: socket}), do: :inet.close(socket)
+
+  @doc """
+  Opens a `protocol` socket using [`:gen_tcp.connect/3`](http://erlang.org/doc/man/gen_tcp.html#connect-3) or
+  [`:gen_udp.open/1`](http://erlang.org/doc/man/gen_udp.html#open-1).
+  """
+  @spec open(map) :: {:ok, map} | {:error, :inet.posix}
+
+  def open(map = %{host: host, port: port, protocol: :tcp}) do
+    host
+    |> :gen_tcp.connect(port, [{:active, true}, :binary, {:keepalive, true}])
+    |> put_socket(map)
+  end
+
+  def open(map = %{protocol: :udp}) do
+    0
+    |> :gen_udp.open
+    |> put_socket(map)
+  end
+
+  @doc """
+  Sends a `protocol` message over `socket`.
+  """
+  @spec send(map, iodata) :: :ok | {:error, :closed | :not_owner | :inet.posix}
+  def send(%{protocol: :tcp, socket: socket}, packet), do: :gen_tcp.send(socket, packet)
+  def send(%{host: host, port: port, protocol: :udp, socket: socket}, packet) do
+    :gen_udp.send(socket, host, port, packet)
+  end
+
+  ## Private Functions
+
+  defp put_socket({:ok, socket}, map), do: {:ok, Map.put(map, :socket, socket)}
+  defp put_socket(other, _), do: other
+end