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MESSAGE PASSING MECHANISM
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1. INTRODUCTION
2. MESSAGE
3. ROUTING
4. SERIALIZING PRIMITIVES
5. SERIALIZING CUSTOM DATA TYPES


1. INTRODUCTION

In server.txt we describe the datatypes involved at both client and server, and
the related messages that are passed from client to server and back. In this
file we will describe how the actual messages are composed and how they are
routed from where they come from to where they are taken care of.

 PLEASE NOTE: Everything in this file is open for discussion. This file is
 meant to get ideas acros, find flaws in the reasoning and generally get
 comments on the system before somebody sets out to implement it.


2. MESSAGE

The protocol is described in terms of messages. Each message has a certain
type, a length, and a certain number of values in primitive data types:

   { C msgId, S length, ... }

The length stores the length of the remaining data. We need this so that the
message can be cut off from the incoming stream of bytes and be encapsulated
on its own to be further processed.

The primitive data types are the same as those described in server.txt. The
first thing that is done to the incoming message in to put it in an instance
of the Message class. Below we outline the Message interface:

  class Message
  {
    public:
      Message(int length, void* data);
      ~Message();

      int getChar();
      int getShort();
      int getLong();
      std::string getString();

    private:
      int length, readPointer;
      void* data;
  }


3. ROUTING

After the Message instance has been constructed, it is routed to the message
handler that has registered itself to handle this message type. If no handler
has been registered a warning should probably be issued. A message handler
implements the following interface:

  class MessageHandler
  {
    public:
      void receiveMessage(int msgId, Message *msg) = 0;
  }

A mapping is made from message types to message handlers. One handler could
be registered to handle a group of message types. This mapping can be stored
in a std::map as follows:

  std::map<int, MessageHandler*> messageHandlers;

This will be a member of the class that processes the incoming byte stream,
which will implement an interface to allow registration of message handlers,
for example:

  void registerHandler(int msgId, MessageHandler *handler);


4. SERIALIZING PRIMITIVES

Here we will describe for each primitive data type how it is serialized in
bytes.

  char:    1 byte  (direct copy)
  short:   2 bytes (now need to keep in mind endian order)
  long:    4 bytes (see short)
  string:  2 bytes for length (short) + X bytes (characters)


5. SERIALIZING CUSTOM DATA TYPES

Custom data types will be serialized using a composition of the primitive data
types, or other custom data types. They will need to implement the following
interface:

  class Serializable
  {
    public:
      void serialize(Message *msg);
      static Serializable *unserialize(Message *msg);
  }