Java vs Erlang.

.gitignore

*~
*.beam

README.md

<!-- -*- mode: html; coding: utf-8 -*- -->

After I saw an Java/Appia implementation for the Print module
described in [Introduction to Reliable and Secure Distributed
Programming](http://www.amazon.com/Introduction-Reliable-Secure-Distributed-Programming/dp/3642152597).

I was blown away by all the code that was needed to pass a message to
a concurrent actor.  I hope that [Akka](http://akka.io/) presents
better APIs for doing Actor-based concurrency.

## Interface of the printing module

<dl>
    <dt>Module:<dt>
    <dd>Name: Print.</dd>
    
    <dt>Events:</dt>
    <dd>
        <dl>
           <dt>Request</dt>
           <dd>〈PrintRequest | rqid, str〉: Requests a string to be printed. The token `rqid` is an identifier of the request </dd>
           
           <dt>Confirmation</dt>
           <dd>〈PrintConfirm | rqid〉: Used to confirm that the printing request with identifier `rqid` succeeded</dd>
        </dl>
    </dd>
</dl>

## Algorithim of the printing module

<dl>
    <dt>Implements</dt>
    <dd>Print.</dd>

    <dt>upon event 〈PrintRequest | rqid, str〉 do</dt>
    <dd>
      <pre><code>
	print str;
	trigger 〈PrintConfirm | rqid〉;
      </code></pre>
    </dd>
</dl>

## Java/Appia implementation

So without further ado, here's the Java code with the Erlang code.

To implement the algorithm described in the pseudo-code presented above.  First you need to implement the Event classes:

* [PrintRequestEvent](#file-printrequestevent-java)
* [PrintConfirmEvent](#file-printconfirmevent-java)

Then you need to create a Layer class that describes the Events the module has: [PrintLayer](#file-printlayer-java)

Then you actually get to build the event handlers in a Session class [PrintSession](#file-printsession-java)

## Simple Erlang implementation

I'm not even going to introduce this:

```erlang
-module(simple_print).
%%%%
%% Public API
%%%%
-export([
	 start/0,
	 print/2
	]).

start() ->
    spawn(fun print_process/0).

print(Pid, Str) ->
    From = self(),
    Rqid = make_ref(),
    % directly from the interface spec: 
    Request = {print_request, Rqid, Str},

    % Send the message
    Pid ! {From, Request},

    % wait for the print_confirm message
    receive
        {print_confirm, Rqid} ->
            ok
    end.

%%%%
%% Private
%%%%
print_process() ->
    % wait for print_request messages
    receive 
        {From, {print_request, RqId, Str}} ->
            io:format("[Print] ~s~n", [Str]),
            From ! {print_confirm, RqId}
    end,
    % loop after handling a request
    print_process().
		
```

## gen_server implementation

This request/response loop as been abstracted away in OTP's `gen_server` behaviour.  Here's the OTP gen_server version of the previous
example:


```erlang
-module(print_server).

-behaviour(gen_server).

%% Public API
-export([start/0, print/2]).

%% gen_server callbacks
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
	 terminate/2, code_change/3]).

%%%%
%% Public API
%%%%

%% starts the server
start() ->
    gen_server:start(?MODULE, [], []).

%% prints a string
print(Pid, Str) ->
    %% gen_server:call handles the rqid for us, we just send the message
    gen_server:call(Pid, {print_request, Str}).

%%%%
%% gen_server callbacks
%%%%

init([]) ->
    {ok, undefined}.

handle_call({print_request, Str}, _From, State) ->
    io:format("[Print] ~s~n", [Str]),
    {reply, ok, State}.

handle_cast(_Msg, State) ->
    {noreply, State}.

handle_info(_Info, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

```
      12 text files.

## Cloc

```
http://cloc.sourceforge.net v 1.56  T=0.5 s (10.0 files/s, 628.0 lines/s)
-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
Java                            4             32            154             88
simple_print.erl                1              6             11             23
print_server.erl                1             14             11             22
-------------------------------------------------------------------------------
```

print_server.erl

-module(print_server).

-behaviour(gen_server).

%% Public API
-export([start/0, print/2]).

%% gen_server callbacks
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
	 terminate/2, code_change/3]).

%%%%
%% Public API
%%%%

%% starts the server
start() ->
    gen_server:start(?MODULE, [], []).

%% prints a string
print(Pid, Str) ->
    %% gen_server:call handles the rqid for us, we just send the message
    gen_server:call(Pid, {print_request, Str}).

%%%%
%% gen_server callbacks
%%%%

init([]) ->
    {ok, undefined}.

handle_call({print_request, Str}, _From, State) ->
    io:format("[Print] ~s~n", [Str]),
    {reply, ok, State}.

handle_cast(_Msg, State) ->
    {noreply, State}.

handle_info(_Info, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

PrintConfirmEvent.java

/*
 *
 * Hands-On code of the book Introduction to Reliable Distributed Programming
 * by Christian Cachin, Rachid Guerraoui and Luis Rodrigues
 * Copyright (C) 2005-2011 Luis Rodrigues
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 *
 * Contact
 * 	Address:
 *		Rua Alves Redol 9, Office 605
 *		1000-029 Lisboa
 *		PORTUGAL
 * 	Email:
 * 		[email protected]
 * 	Web:
 *		http://homepages.gsd.inesc-id.pt/~ler/
 * 
 */

package irdp.protocols.tutorialDA.print;

import net.sf.appia.core.Event;


/**
 * Print request confirmation event.
 * 
 * @author alexp
 */
public class PrintConfirmEvent extends Event {
  int rqid;

  void setId(int rid) {
    rqid = rid;
  }

  int getId() {
    return rqid;
  }
}

PrintLayer.java

/*
 *
 * Hands-On code of the book Introduction to Reliable Distributed Programming
 * by Christian Cachin, Rachid Guerraoui and Luis Rodrigues
 * Copyright (C) 2005-2011 Luis Rodrigues
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 *
 * Contact
 * 	Address:
 *		Rua Alves Redol 9, Office 605
 *		1000-029 Lisboa
 *		PORTUGAL
 * 	Email:
 * 		[email protected]
 * 	Web:
 *		http://homepages.gsd.inesc-id.pt/~ler/
 * 
 */

package irdp.protocols.tutorialDA.print;

import net.sf.appia.core.Layer;
import net.sf.appia.core.Session;
import net.sf.appia.core.events.channel.ChannelInit;


/**
 * Layer of the Print protocol.
 * @author alexp
 */
public class PrintLayer extends Layer {

  public PrintLayer() {
    /* events that the protocol will create */
    evProvide = new Class[1];
    evProvide[0] = PrintConfirmEvent.class;

    /*
     * events that the protocol requires to work This is a subset of the
     * accepted events.
     */
    evRequire = new Class[0];

    /* events that the protocol will accept */
    evAccept = new Class[2];
    evAccept[0] = PrintRequestEvent.class;
    evAccept[1] = ChannelInit.class;
  }

  public Session createSession() {
    return new PrintSession(this);
  }
}

PrintRequestEvent.java

/*
 *
 * Hands-On code of the book Introduction to Reliable Distributed Programming
 * by Christian Cachin, Rachid Guerraoui and Luis Rodrigues
 * Copyright (C) 2005-2011 Luis Rodrigues
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 *
 * Contact
 * 	Address:
 *		Rua Alves Redol 9, Office 605
 *		1000-029 Lisboa
 *		PORTUGAL
 * 	Email:
 * 		[email protected]
 * 	Web:
 *		http://homepages.gsd.inesc-id.pt/~ler/
 * 
 */

package irdp.protocols.tutorialDA.print;

import net.sf.appia.core.Event;


/**
 * Print request event.
 * 
 * @author alexp
 */
public class PrintRequestEvent extends Event {
  int rqid;
  String str;

  void setId(int rid) {
    rqid = rid;
  }

  void setString(String s) {
    str = s;
  }

  int getId() {
    return rqid;
  }

  String getString() {
    return str;
  }
}

PrintSession.java

/*
 *
 * Hands-On code of the book Introduction to Reliable Distributed Programming
 * by Christian Cachin, Rachid Guerraoui and Luis Rodrigues
 * Copyright (C) 2005-2011 Luis Rodrigues
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 *
 * Contact
 * 	Address:
 *		Rua Alves Redol 9, Office 605
 *		1000-029 Lisboa
 *		PORTUGAL
 * 	Email:
 * 		[email protected]
 * 	Web:
 *		http://homepages.gsd.inesc-id.pt/~ler/
 * 
 */

package irdp.protocols.tutorialDA.print;

import net.sf.appia.core.AppiaEventException;
import net.sf.appia.core.Direction;
import net.sf.appia.core.Event;
import net.sf.appia.core.Layer;
import net.sf.appia.core.Session;
import net.sf.appia.core.events.channel.ChannelInit;


/**
 * Session implementing the Print protocol.
 * <br>
 * Receives print requests and prints them on screen.
 * 
 * @author alexp
 */
public class PrintSession extends Session {

  public PrintSession(Layer layer) {
    super(layer);
  }

  public void handle(Event event) {
    if (event instanceof ChannelInit)
      handleChannelInit((ChannelInit) event);
    else if (event instanceof PrintRequestEvent) {
      handlePrintRequest((PrintRequestEvent) event);
    }
  }

  private void handleChannelInit(ChannelInit init) {
    try {
      init.go();
    } catch (AppiaEventException e) {
      e.printStackTrace();
    }
  }

  private void handlePrintRequest(PrintRequestEvent request) {
    try {
      PrintConfirmEvent ack = new PrintConfirmEvent();

      System.out.println();
      System.out.println("[Print] " + request.getString());
      request.go();

      ack.setChannel(request.getChannel()); // set the Appia channel where the
                                            // event will travel
      ack.setDir(Direction.UP); // set events direction
      ack.setSourceSession(this); // set the session that created the event
      ack.setId(request.getId()); // set the request ID
      // initializes the event, defining all internal structures,
      // for instance the path the event will take (sessions to visit)
      ack.init();
      ack.go(); // send the event
    } catch (AppiaEventException e) {
      e.printStackTrace();
    }
  }
}

simple_print.erl

-module(simple_print).
%%%%
%% Public API
%%%%
-export([
	 start/0,
	 print/2
	]).

start() ->
    spawn(fun print_process/0).

print(Pid, Str) ->
    From = self(),
    Rqid = make_ref(),
    % directly from the interface spec: 
    Request = {print_request, Rqid, Str},

    % Send the message
    Pid ! {From, Request},

    % wait for the print_confirm message
    receive
       {print_confirm, Rqid} ->
           ok
    end.

%%%%
%% Private
%%%%
print_process() ->
    % wait for print_request messages
    receive 
	{From, {print_request, RqId, Str}} ->
	    io:format("[Print] ~s~n", [Str]),
	    From ! {print_confirm, RqId}
    end,
    % loop after handling a request
    print_process().