@kbrw/node_erlastic

node_erlastic

Node library to make nodejs gen_server in Erlang/Elixir through Port connection.

This module allows you to :

• decode and encode between Binary Erlang Term and javascript types
• create a simple Erlang port interface through a nodeJS Readable and Writable (Duplex)
• create a "gen_server style" handler to manage your port

Example Usage

Before going through details, lets take an example, write an account manager server, where you can add or remove an amount in the account and get it :

require('@kbrw/node_erlastic').server(function(term,from,current_amount,done){
  if (term == "get") return done("reply",current_amount);
  if (term[0] == "add") return done("noreply",current_amount+term[1]);
  if (term[0] == "rem") return done("noreply",current_amount-term[1]);
  throw new Error("unexpected request")
});

GenServer.start_link(Exos.Proc,{"node calculator.js",0,cd: "/path/to/proj"}, name: Calculator)
GenServer.cast Calculator, {:add, 2}
GenServer.cast Calculator, {:add, 3}
GenServer.cast Calculator, {:rem, 1}
4 = GenServer.call Calculator, :get

defmodule Exos.Proc do
  use GenServer
  @moduledoc """
    Generic port as gen_server wrapper :
    send a message at init, first message is remote initial state
    cast and call encode and decode erlang binary format
  """
  def init({cmd,init,opts}) do
    port = Port.open({:spawn,'#{cmd}'}, [:binary,:exit_status, packet: 4] ++ opts)
    send(port,{self,{:command,:erlang.term_to_binary(init)}})
    {:ok,port}
  end
  def handle_info({port,{:exit_status,0}},port), do: {:stop,:normal,port}
  def handle_info({port,{:exit_status,_}},port), do: {:stop,:port_terminated,port}
  def handle_info(_,port), do: {:noreply,port}
  def handle_cast(term,port) do
    send(port,{self,{:command,:erlang.term_to_binary(term)}})
    {:noreply,port}
  end
  def handle_call(term,_reply_to,port) do
    send(port,{self,{:command,:erlang.term_to_binary(term)}})
    res = receive do {^port,{:data,b}}->:erlang.binary_to_term(b) end
    {:reply,res,port}
  end
end

External Term Format codec (BERT)

var Bert = require('@kbrw/node_erlastic/bert');
// you can configure `convention`, `all_binaries_as_string` , `map_key_as_atom`, see below
Bert.convention = Bert.ELIXIR;
Bert.all_binaries_as_string = true;

Bert.encode({foo: "bar", k2: 4});
Bert.encode({foo: "bar", k2: 4},true);
// with ",true", the result is not copied, the return buffer point always to
// the same allocated memory
Bert.decode(mybuffer);

Bert.decode and Bert.encode use a nodejs Buffer object containing the binary erlang term, converted using the following rules :

• erlang atom foobar is js {type: "Atom",value: "foobar", toString->value} create it with Bert.atom("foobar")
  • the toString() method allows you to match with string myatom == 'foobar'
• erlang list is js list
• erlang tuple {a,b} is js {type: "Tuple",value: [a,b],length: 2, 0: a, 1: b}
  • the js object allows you to access elements by index
• erlang integer is js integer
• erlang float is js float
• other js objects are erlang maps
  • erlang keys are converted to string during decoding (js behavior)
  • js keys are converted to erlang atom if Bert.map_key_as_atom == true
• erlang binary is nodejs "Buffer"
  • but converted into string if Bert.convention == Bert.ELIXIR && Bert.all_binaries_as_string
• js string is
  • UTF8 erlang binary if Bert.convention == Bert.ELIXIR
  • erlang character list if Bert.convention == Bert.ERLANG
• js boolean are true and false atoms
• js null and undefined are
  • nil atom if Bert.convention == Bert.ELIXIR
  • undefined atom if Bert.convention == Bert.ERLANG
  • if Bert.decode_undefined_values == false, then nil and undefined are decoded into atom instead of null

The Port Duplex

Port provides you a Node Duplex stream in object mode which is both Readable and Writable : http://nodejs.org/api/stream.html#stream_class_stream_duplex_1 Through this duplex, you can communicate javascript objects with an erlang node through stdin/out with port.read() and port.write(obj). These objects are converted to erlang external binary format using the Bert encoder described above.

Need {packet,4} openport option on the erlang side

Below a simple "echo" server using this abstraction, read nodejs "readable" documentation to understand it :

var port = require('@kbrw/node_erlastic').port;
port.on('readable', function echo(){
  if(null !== (term = port.read())){
    port.write(term);
    echo();
  }
});

port = Port.open({:spawn,'node calculator.js'}, [:binary, packet: 4])
send(port,{self,{:command,:erlang.term_to_binary( {:hello, 007} )}})
{:hello, 007} = receive do {^port,{:data,b}}->:erlang.binary_to_term(b) end
send(port,{self,{:command,:erlang.term_to_binary( [:foo, :bar]} )}})
[:foo, :bar] = receive do {^port,{:data,b}}->:erlang.binary_to_term(b) end

The Erlang style handler interface to the port event handler

For convenience, you can use the server function to react to the port events in the same fashion as the erlang gen server handler.

It takes as parameter a handler function taking (req_term,from,state,done) parameters. To "unlock" state and continue to read request mailbox (equivalent of the return of the erlang gen_server handle_* function), you need to call done.

done("noreply",newstate); 
done("noreply");
done("reply",reply_term,newstate);
done("reply",reply_term);

Like in erlang, your handler can unlock the state before it replies to the call:

done("noreply",newstate);
// then in some callback
from(myreply);

Before sending request, the first message from the port will be used to define the initial state.

Please see the beginning of this README to find a complete example.

Log function

The port stderr is directly output into the erlang stdout, this library provides a convenient log function allowing you to log something from your node server.

  var log = require("@kbrw/node_erlastic").log;
  log("your log");