Use custom serializer like MessagePack with Phoenix Framework

Phoenix 1.3
Elixir 1.5.2 compiled with Erlang 20

I have an Android mobile app which sends and receives MsgPack with a HTTP server. Now we are planning to integrate Phoenix framework for some parts. But sending binaries specifically MsgPack is troublesome on Phoenix framework.

How to use custom (de-)serializers like MsgPack in Phoenix?

If it is of any relevance: 1. Using MsgPax library
2. Checked this link on stoiximan.gr

I've used MsgPax to do this, and it works just fine. First add it to your mix.exs file.

To use a custom serializer:

• Phoenix v1.4+
  configure serlirizer in your Phoenix.Endpoint

  socket "/socket", YourApp.UserSocket,
    websocket: [
      connect_info: [:peer_data, :x_headers, :uri, session: @session_options],
      serializer: [{YourApp.MsgpaxSerializer, "2.0.0"}]
    ]
  

• Phoenix v1.3 and older
  use Phoenix.Socket.transport macro in your user_socket.ex file, like so:

    defmodule YourApp.UserSocket do
      use Phoenix.Socket
  
      # Your channels here
  
      # Take out whichever version you're not using
      ## Transports for Phoenix 1.2
      transport :websocket, Phoenix.Transports.WebSocket, timeout: 45_000,
                serializer: YourApp.MsgpaxSerializer
      ## Transports for Phoenix 1.3
      transport :websocket, Phoenix.Transports.WebSocket, timeout: 45_000,
                serializer: [{YourApp.MsgpaxSerializer, "~> 2.0.0"}]
  
      # Other user socket stuff
    end
  

Then, as according to the docs, your serializer has to have 3 functions: decode!/2, encode!/1, and fastlane!/1. So, copying mostly from phoenix's own implementation, your serializer might look something like this:

defmodule YourApp.MsgpaxSerializer do
  @behaviour Phoenix.Transports.Serializer

  alias Phoenix.Socket.Reply
  alias Phoenix.Socket.Message
  alias Phoenix.Socket.Broadcast

  def fastlane!(%Broadcast{} = msg) do
    msg = %Message{topic: msg.topic, event: msg.event, payload: msg.payload}

    {:socket_push, :binary, encode_to_binary(msg)}
  end

  def encode!(%Reply{} = reply) do
    msg = %Message{
      topic: reply.topic,
      event: "phx_reply",
      ref: reply.ref,
      payload: %{status: reply.status, response: reply.payload}
    }
    {:socket_push, :binary, encode_to_binary(msg)}
  end
  def encode!(%Message{} = msg) do
    {:socket_push, :binary, encode_to_binary(msg)}
  end

  defp encode_to_binary(msg) do
    msg |> Map.from_struct() |> Msgpax.pack!()
  end

  def decode!(message, _opts) do
    message
    |> Msgpax.unpack!()
    |> Phoenix.Socket.Message.from_map!()
  end
end

Finally, if you are encoding structs, you may have to add @derive Msgpax.Packer attribute to the modules containing the structs as detailed in Msgpax's documentation.