Revision as of 00:04, 19 September 2010

My Project - Instant Messenger Client

This design study was introduced by my ENEL428 software assignment. The purpose of the assignment was to design a prototype of an Instant Messenger System using concurrent programming. The system was broken into two separate parts a client and a server. This design study is regarding the client program.

Design Study

The initial server-client system uses a simple login model, a user attempts to log in with just a username. The server accepts a login attempt if the username is not already taken. When a user logs out their username is freed for anyone else to use. The client and server communicate using a basic protocol. Commands are serailised packets containing a command type and any required data (username, message, chat ID's etc).

Functional requirements of the client:

• Connect to a server.
• Login.
• Logout.
• Display the other users online - receive updates from the server.
• Start a conversation with another online user - started by the local user, or an online user.
• Post messages in a conversation.
• Receive messages in a conversation - from other users via the server.
• Invite other online users to a conversation.
• Leave a conversation.

Constraints:

• C++
• Uses POSIX threads for concurrency.

Room for Expansion:

• Other protocols, XMPP etc.
• Other GUI implementations, currently uses fltk-1.1.9

Initial Design

This is the design which is fully functional.

Figure 1: The initial working design for the Instant Messenger Client

General Description

The initial design is event driven. There are two threads that respond to events, the Inbox and the GUI.

• The Inbox responds to messages from the server. It receives, decodes and executes packets that it gets from the socket. It executes the packets by calling operations on the UserManager and Chat objects.
• The GUI responds to input from the user. It calls operations on the UserManager and Chat objects according to the input from the user.

The UserManager and Chats use Mutexs to synchronise access to their data. They also use the Outbox in some operations to write packets to the socket.

Class Descriptions

• Socket/ClientSocket: The Socket encapsulates a TCP Socket. It provides an interface to read/write packets to the socket. The ClientSocket has a different constructor because it uses the IP address of the server to connect. There are bad things about this part of the design and will be discussed below.
• Inbox: Inherits from a thread class. Reads packets from the ClientSocket, unpacks them and executes operations on the Chats and the UserManager. The Inbox also contains a map that links Chat objects to their ChatID's (This is nasty, the inbox should not have this map).
• Chat: Each chat represents a conversation with another user. The chat maintains a list (history) of messages but not the participants of the conversation (YAGNI vs OCP?). Chats are subjects in the Observer pattern so they notify their observers whenever their state changes.
• UserManager: The UserManager keeps a record of the other users online. The server pushes updates to the client when users log on and off so they can be added and removed from the UserManager. The user manager also handles the login/logout actions of the local user. The UserManager is a subject in the Observer pattern so it notifies its observers if its state changes.
• Outbox: The outbox builds packets to write to the socket. It is used by Chats and the UserManager to send information to the server. The outbox currently records the userID of the local user so it can tag outgoing packets, this seems like it shouldn't be there. (If it wasn't there then the outbox is just behavior without state, except a reference to the Socket)

GUI:

• Factory/Fl_Factory: Sort of implements the factory pattern, in a strange way. It makes Chats and UserManagers and also makes the associated Fl_Window'ed objects (Fl_WindowedChat and FL_WindowedUserManager) which are then linked with the observer pattern.
• Observer/Subject: These classes implement the GoF observer design pattern. C++ allows multiple inheritance so the Observer and Subject base classes can be inherited without burning my base class of Chat, UserManager or the Fl_Window'd classes.
• Fl_WindowedChat: Encapsulates the GUI for a Chat. In an observer of a Chat. Implemented using fltk-1.1.9
• Fl_WindowedUserManager: Encapsulates the GUI for a UserManager. Is an observer of a UserManager. Implemented using fltk-1.1.9

Concurrency:

• Thread: The thread class wraps the POSIX threads library. It is inherited by Inbox so that so that it can run concurrently.
• Mutex: Wraps a POSIX threads mutex and condition variable. The interface can lock and unlock the mutex. I also implemented an interface to wait and signal the condition variable, but that is unused in the current design. This breaks YAGNI but makes the Mutex class more reusable for further concurrent designs.

Initial Design Review

Good Things

The use of the Observer and Abstract Factory patterns completely removes the implementation of the GUI from the Client implementation. The GUI objects are observers of the client model objects and the concrete factory is responsible for building the GUI objects and linking them to their subjects.

Observer Pattern

This pattern is used to allow the GUI windows to observe the Client's state without coupling.

Participants:

• Subject - Subject.
• Concrete Subject - Chat and UserManager.
• Observer - Observer
• Concrete Observer - Fl_WindowedChat and Fl_WindowedUserManager

Abstract Factory Pattern

This implementation of an abstract factory doesn't follow the exact GoF architecture, however the intent is still to decouple the interface for making a Chat or UserManager from its implementation. There are no abstract products, just concrete Chat and concrete UserManager. However, the Factory is also responsible for constructing the GUI object linked to the concrete products using the Observer Pattern.

Participants:

• Abstract Factory - Factory
• Concrete Factory - Fl_Factory
• Abstract Product - None
• Concrete Product - Chat (+ Fl_WindowedChat), UserManager (+ Fl_WindowedUserManger)

Tell, Don't ask

The whole design follows the tell, don't ask maxim. All operations either take no parameters or return void. This helps ensure that all behavior and state are in the appropriate place.

Acyclic dependancies

The relationships between the classes are designed to remove any cyclic dependancies from the chain of operations triggered by an event from the GUI or server.

Inbox->Socket

Inbox/GUI->UserManager/Chat->Outbox->Socket

The acyclic dependancies principle concerns packages not classes, but it is still good that there are no cyclic relationships between classes. It also helps with the concurrent analysis of the safety and liveliness of the program.

Bad Things

Single Responsibility Broken

• Inbox contains a map of Chat objects. This breaks the Single responsibility principle. Inbox should be responsible for executing incoming packets, not constructing and mapping Chat objects to ChatID's. Removing this responsibility from the Inbox requires a new class to manage Chat objects, a ChatManager. The interface of the ChatManager follows the Tell, don't ask principle by passing operation calls to the correct Chat without revealing the actual Chat objects.
• UserManager handles local User login status/operations. This also breaks the single responsibility principle. The UserManager should only maintain a record of online users not the Login behavior of the local user. This extra responsibilty can be encapsulated in a LocalUser class.

God-like Manager Class

Solving the above Single responsibility issues leads to a ChatManager class. This class is responsible for constructing Chats and mapping Chats to ChatID's. In order to follow the [[Tell, Don't Ask|tell, don't ask maxim the interface of the ChatManager needs to support the same operations as a Chat object, it merely passes the operation call to the appropriate Chat object. This leads to a God-like manager class that contains a collection of Chat objects. While this seems bad, and there are alternative solutions, I think it is the best I have thought of so far. Other solutions are:

• Use the chain of responsibility pattern (Thanks Jimmy for this suggestion). Make Chat objects into a linked list so that a operation can be called on the first Chat, if the ChatID doesn't match that Chat then the operation call is passed on down the chain. This is a elegant way of removing the need of a ChatManager and would be a good solution if the program wasn't concurrent. The problem arises when one thread (the Inbox) is moving down the list calling operations and the other (the GUI) jumps in and removes a Chat causing a segfault. This could be solved with a global ChatList mutex but thats nasty and we may as well use the ChatManager to protect the list of Chats.
• Another solution is to remove a lot of the functionality from the ChatManager so it is just a ChatRecord. The ChatRecord would return a reference to a Chat object associated given a ChatID. This protects the Chat list but also breaks the tell, don't ask principle.

UserManager has the same smell of God-like to it but it is just a list of UserID's, which are just strings so its actually just a UserList, and the name of it should reflect that.

ClientSocket Weirdly Inherits Concrete Socket Base Class

The Socket class was shared across the client and server programs, it has concrete implementation of the Send() and Receive() operations. The ClientSocket only has a different constructor, so it can connect to an existing socket. This breaks the Avoid concrete base classes principle. To make this design extensible polymorphism should be used to change the behavior of the Socket. Different protocols may have very different implementation of socket connection, sending and receiving, so a abstract base class is better. Socket should only define the interface.

Inbox Switch Smell for Executing Packets

The current solution is a switch statement based on the command type of Packet received. A Packet is read from the socket as a string, the string is tokenised into the Packet data structure. One of the fields in the Packet is the command type enumeration which is used to decide how to execute the command. The Packet data structure is also used by the Outbox which builds Packets with the appropriate command type and send them to the server.

Polymorphism is an alternative solution, make Packet execute itself. Use an abstract base class with concrete sub classes for each Packet type. However, there are several things to consider:

• The switch smell would still exist where the Packets were constructed, in the ClientSocket.
• Packets would need added behavior to execute or serialise themselves. This introduces dependancies on the UserManager, Chats etc, and dependency on the serialisation strategy.
• Some packet types are only received, or only sent, some both. C++ allows multiple inheritance so we could make two Packet base classes (Send and Receive), or have one base class and some Packet types where Serialise() or Execute() do nothing.

My chosen solution is to make a single abstract Packet base class that has an interface for Execution and Serialisation of a Packet. Each Packet type will be a concrete Packet subclass. On the downside, some of the Packets will have unimplemented/unused, Execute() or Serialise() operations. But this removes the requirement for a type switch in Inbox and Socket, and the requirement of a complex multiple inheritance hierarchy. It also makes the program easier to extend because new Packet types with different execution behavior, contained data, or serialisation strategies could be implemented and used in the program following

This leads into the serialisation strategy. It comes to mind that the strategy pattern could be used to remove the concrete implementation of the serialisation from each Packet subclass. However, the serialisation process needs access to the data in the Packet which is achieved by either:

• Defining the interface of the strategy operation so the data is passed as arguments to the serialisation process, OR
• Providing a common interface for all Packets that lets the serialisation operation get the data it needs.

Both these approaches restrict the possible data contained in a Packet subclass to adhere to the interface. This removes any gains in extensibility. Therefore each Packet subclass will have its own concrete implementation for serialisation. Other strategies will require new Packet subclasses.

Stuff that needs fixing, maybe?

• Outbox shouldn't record who is logged in.
• Names of classes are a bit meh.
• State switch in Fl_WindowedUserManager, switch statement smell -> State pattern...

• Burning base class with observer, subject and thread?
• Data and functionality not is same place for GUI, observer pattern requires getters on subject class. Alright because the GUI functionality is not related to the actual Chat's or UserManager

Good Stuff:

• observer decouples GUI from implementation, coupled with Factory means you can build whatever kind of GUI you fancy.
• command query separation used, all functions that return something take no parameters. Works because its an event driven system, most operations are void-return commands.

TODO

• Make ChatManager
• Rename getters and setters
• Rearrange socket and client socket

Other Stuff

Simple chat Log