ECE419S: Distributed Systems Assignment 2


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1 Introduction
In the first assignment, you were introduced to basic techniques for building distributed systems. You implemented
a simple Broker system providing stock quotes to users. In the next two related assignments, you are to design and
implement distributed versions for a multiplayer game, Mazewar. The goals are twofold. First, you will need to
understand a given codebase and extend it by adding network communication support and making it distributed –
Assignment 2. Second, now that you have more experience in the tools necessary for building distributed systems,
as well as the knowledge of the concepts from the lectures, you will design and implement a distributed system
on your own. That is, you will have the freedom to reason about the design of a distributed system, to pick the
design choices that you think are best and to produce a specification and implementation showing your creativity
– Assignment 3.
1.1 Assignment Requirements
In this assignment, you are to implement a client-server version of the multiplayer game, Mazewar, that is described
below. You have been provided with a version of Mazewar where you can play against simple computer controlled
opponents. This way you can concentrate on designing and implementing the client-server protocol rather than the
user interface and other framework issues.
You are required to use the ug computers in the GB 243 lab to implement and run your game in a client-server
fashion – i.e., playing against your team member or other classmates and relying on a central server to aid with
communication and managing game events/state. You need basic knowledge of Java to implement the assignment
The code and starter files are placed on the ug machines, at /cad2/ece419s/ referred as ${ECE419_HOME}.
The source code for MAZEWAR is under ${ECE419_HOME}/labs/lab2/mazewar/. We provide a basic Makefile that
compiles the single-player version of MAZEWAR. Please update it as necessary.
NOTE: If you plan to run the code on your home machine, you will have to update the path to
Java (i.e. ${ECE419_HOME} and ${JAVA_HOME}).
1.2 How to Work on the Lab
Both Assignment 2 and Assignment 3 consist of two parts: design and implementation. For this assignment alone,
you are required to implement a client-server solution following the design given below. For Assignment 3, you will
fully design and implement a decentralized version of the distributed game.
Students should form both design teams and programming teams. A programming team should have two people,
similar to Assignment 1 – there will be no exceptions. A design team should consist of up to 3 programming teams
i.e., up to 6 people. In the worst case, you can have the design team be the same as the programming team.
The design teams are for the purpose of discussing the given codebase, designing and specifying a protocol for
the game, not for jointly writing code. You may not share code with other programming teams. For this assignment,
design team members can discuss the codebase and the given design. For the next assignment, all members of a
design team can devise and implement a common design.
I suggest that you stick to the rules provided in the Mazewar game description, but you are free to make any
additions that you think improve the game, as long as you document them clearly.
Your game should not allow inconsistent states to be seen by different players. Specifically, all player moves
should be displayed in a consistent order on all player displays. Assuming there are only two players, e.g., the
players of your team, you should not allow that the display of one player shows that the local player has made a
move before its opponent, while the other display shows the reverse.
2 MAZEWAR Game Description
You are given a template for a simple multi-player game called MAZEWAR. MAZEWAR is a distributed, multiplayer game
that allows each player to control a client in a maze. A player receives points for vaporizing other clients with a
projectile and loses points for being vaporized. The game is based on the X Window System version of Mazewar,
which is in turn based on the classic game of Mazewar that ran on the Xerox Alto in the 1970s.
When you first start Mazewar, you will be presented with a dialog asking you your name. After that, a game
window will be displayed.
Figure 1: Mazewar Screenshot
There are three parts to the window (shown in Figure 1):
• A view of the maze showing the position and orientation of the clients from above.
• A scoreboard, listing the score of all the clients connected.
• A console, where debugging and other status information may be printed.
The green client is the local GUI controlled client. Robot clients are red, and remote clients are magenta.
The user interface operates as follows:
Key Action
Left-arrow Turn 90 degrees counter-clockwise
Right-arrow Turn 90 degrees-clockwise
Up-arrow Move forward
Down-arrow Move backward
Spacebar Fire
q Quit
Table 1: Player actions
2.1 Rules of the Game
The rules of the game are as follows.
• The maze consists of a 20×10 array of cells. Each cell is either occupied by a client, projectile, or is empty.
You may adjust the size and layout the maze if you so desire, but all clients playing together during a given
session must use the same maze for consistency purposes.
• Projectiles have a finite speed, advancing one cell every 200 milliseconds. You may adjust this if you desire,
however, again for consistency purposes all clients must behave the same in a given session.
• A client needs some amount of time to reload after launching a projectile. During this time it can do anything
except for launch another projectile.
• A client hit by an opponent’s projectile is vaporized instantly. Projectiles/missiles are the full width of the
corridor, so that if a projectile passes through any cell, it certainly hits the client in that cell.
• A client that is hit loses 5 points. A client that successfully hits another gains 11 points. Firing a projectile
costs 1 point.
• When a client is hit, it reappears at a random position (in an unoccupied cell) with a random orientation
(though not facing a wall) in the maze.
2.2 Game Design
Alice’s Screen Bob’s Screen
Mazewar Server
Bob Bob
Alice Alice
Alice Alice
Figure 2: A Walkthrough
Figure 2 goes through an example of what types of messages are exchanged during the game in the client-server
architecture. It shows a game played between two players, Alice and Bob. Following Mazewar’s specifications,
on Alice’s screen, Alice is represented in green since she is the local player and Bob is represented in magenta.
Similarly, on Bob’s screen, Bob is represented in green and Alice is represented in magenta. The design includes
the central server Mazewar Server (MazewarServer) which sequences the actions taken by Alice and Bob.
Figure 2 shows an example of what actions are taken to notify the other player of the move and how to coordinate
the messages so both players see a consistent game window. To clarify, if two windows are consistent, then they
should look the same. Suppose, during the game, Alice moves forward.
1. A notification is generated when Alice presses the up-arrow (↑).
2. At this point, a message is sent to the MazewarServer indicating Alice moved forward. The server enqueues
Alice’s request in a local queue. This establishes a global order of player actions since the enqueue operation
occuring at the server is atomic (even if there are multiple threads at the server to handle connections). The
server tags each action with a sequence number.
3. MazewarServer then dequeues an operation and broadcasts to all clients.
4. Upon receiving a message from MazewarServer, all clients enqueue the action in their local queue based on
the sequence number, such that lower sequence number messages are at the front of the queue.
5. Each client dequeues the action message at the front of the queue if it is the next one in sequence i.e., there
are no gaps in the order of sequence numbers for messages, and displays the action by moving the client icon
forward accordingly.
NOTE: In this assignment, you should implement the design described above.
2.3 Description of the Class Structure in Mazewar
GUIClient RobotClient
Now that you have seen a high level description of the actions taken by the game, we also guide you through
the class structure to aid you in your implementation. We present the class structure of MAZEWAR. The players in
the maze are instances of the Client class. The Client class has methods to implement all of the actions defined
in Table 1. The methods have meaningful names and it is easy to understand what action they implement. For
example, the method forward() implements the code to move the client forward in the maze. Other methods are
The Client class is extended by both RemoteClient and Localclient. There is one instance of LocalClient
(specifically GUIClient) representing the local player. The different keypresses are handled by GUIClient which
in turn invokes methods in LocalClient. So, In Figure 2, when Alice pressed the up-arrow key (↑), GUIClient
generates an event saying “Alice pressed the Up-Arrow key”. This subsequently calls the method forward() in
LocalClient. The LocalClient then generates a message indicating “Alice moved forward” and sends it to the
MazewarServer which is then broadcast to all clients.
Similarly, each instance of RemoteClient represents a player playing from a remote machine. Naturally, this
means that any action taken by the remote player will invoke a method in this class. In Figure 2, on Bob’s screen
Alice is represented as a RemoteClient object. Thus, when the message from MazewarServer arrives to Bob saying
“Alice moved forward”, the forward() method is invoked in the RemoteClient object representing Alice.
2.4 Files
In the ECE419 directory (${ECE419_HOME}), we have provided some relevant files to you. These include a sample
Makefile as well as the code for the single player version of MAZEWAR. You may create new files and modify the given
files. Also make sure that you update the Makefile such that everything compiles properly.
3 Grading
This lab assignment represents 6% of your final grade for the course and is due by Thursday February 23th 2012
11:59 PM.
You must implement your solution to:
1. Be a consistent game
2. Be playable, i.e. the game must respond quickly to player moves
3. Handle up to 4 players
The lab will be graded according to the following scheme:
• Full credit for an implementation having all components working according to the objectives above.
• 80% if your game is playable, but has a minor functional or logical mistake, e.g., inconsistencies or visible
• 40% if your program is partly working but has several or major functional or logical mistakes.
• Otherwise, 20% if the code compiles successfully. To receive this credit, your code has to show “reasonable
effort” towards the objective of the lab part and any compilable code will not award you this credit 🙂
4 Submission
Only one team member needs to submit the assignment. Include the names and student numbers for both team
members in a README file. Your submission should consist of the source files, a Makefile, and the README. Feel
free to use/update the sample Makefile provided to you, or write your own Makefile.
Your submission must be in the form of one compressed archive named Lastname1.Lastname2.tar.gz. The
directory structure of the archive should be as follows:
README (short description of how to run your code and other aspects you feel worth mentioning)

If you need to provide additional explanations about your solution, please do so in the general README file.
Once you have a compressed archive in the .tar.gz format, you may submit your solution by the deadline using
the submitece419s command, located under /local/bin on the ug machines:
/local/bin/submitece419s 2 Lastname1.Lastname2.tar.gz