COMP90015: Distributed Systems – Assignment 1
Multi-threaded Dictionary Server (15 marks)
Using a client-server architecture, design and implement a multi-threaded server that allows concurrent
clients to search the meaning(s) of a word, add a new word, and remove an existing word.
This assignment has been designed to demonstrate the use of two fundamental technologies that have
been discussed during the lectures:
Hence, the assignment must make an EXPLICIT use of the two above. By explicit, we mean that in your
application, sockets and threads must be the lowest level of abstraction for network communication and
The system will follow a client-server architecture in which multiple clients can connect to a
(single) multi-threaded server and perform operations concurrently.
The multi-threaded server may implement a thread-per-request, thread-per-connection, or
worker pool architecture. This is a design decision for you to make.
All communication will take place via sockets. These sockets can be either TCP or UDP, however,
keep in mind that all communication between clients and server is required to be reliable.
You are responsible for designing your own message exchange protocol. Some data formats that
you may use include XML, JSON, Java Object Serialization, or you may choose to design your own.
All communication between components has to be reliable. If you are using TCP, then the
reliability guarantees offered by the protocol are sufficient. If you decide to use UDP, then you
have to implement an infrastructure that offers reliable communication over UDP. For those of
you with previous experience using TCP, using UDP may be a rewarding challenge.
It is expected that, on both the server and the client side, errors (by means of exception handling)
are properly managed. The errors include the following:
Input from the console for what concerns the parameters passed as command line.
Network communication (address not reachable, bad data...).
I/O to and from disk (cannot find the dictionary file, error reading the file, etc...).
Other errors you might come up with.
The application will be tested and validated against all these errors.
Query the meaning(s) of a given word
The client should implement a function that is used to query the dictionary with the following
minimum (additional input/output parameters can be used as required) input and output:
Input: Word to search
Output: Meaning(s) of the word
Error: The client should clearly indicate if the word was not found or if an error occurred. In case
of an error, a suitable description of the error should be given to the user.
Add a new word
Add a new word and one or more of its meanings to the dictionary. For the word to be added
successfully it should not exist already in the dictionary. Also, attempting to add a word without
an associated meaning should result in an error. A new word added by one client should be visible
to all other clients of the dictionary server. The minimum input and output parameters are as
Input: Word to add, meaning(s)
Output: Status of the operation (e.g., success, duplicate)
Error: The user should be informed if any errors occurred while performing the operation.
Remove an existing word
Remove a word and all of its associated meanings from the dictionary. A word deleted by one
client should not be visible to any of the clients of the dictionary server. If the word does not exist
in the dictionary then no action should be taken. The minimum input and output parameters are
Input: Word to remove
Output: Status of the operation (e.g., success, not found)
Error: The user should be informed if any errors occurred while performing the operation.
A Graphical User Interface (GUI) is required for this project. You are free to design it and to use any
existing tools and libraries for this purpose.
These are guidelines only, you are allowed and encouraged to come up with your own design and
When the server is launched, it loads the dictionary data from a file containing the initial list of
words and their meanings. These data is maintained in memory in a structure that enables an
efficient word search. When words are added or removed, the data structure is updated to
reflect the changes.
A sample command to start the server is:
> java –jar DictionaryServer.jar <port> <dictionary-file>
Where <port> is the port number where the server will listen for incoming client connections
and <dictionary-file> is the path to the file containing the initial dictionary data.
When the client is launched, it creates a TCP socket bound to the server address and port
number. This socket remains open for the duration of the client-server interaction. All messages
are sent/received through this socket.
A sample command to start the client is:
> java –jar DictionaryClient.jar <server-address> <server-port>
The assignment should be implemented in Java. Utilization of technologies such as RMI and JMS are not
You should write a report describing your system and discussing your design choices. Your report should
The problem context in which the assignment has been given.
A brief description of the components of the system.
An overall class design and an interaction diagram.
A critical analysis of the work done followed by the conclusions.
Please mind that the report is a WRITTEN document, do not put only graphs. A report without any
descriptive text addressing the problem, architecture, protocols, and the analysis of the work done will
not be considered valid.
The length of the report is not fixed. A good report is auto-consistent and contains all the required
information for understanding and evaluating the work done. Given the level of complexity of the
assignment, a report in the range of 4 to 6 pages is reasonable. Please mind that the length of the report
is simply a guideline to help you in avoiding writing an extremely long or incomplete report.
It is important to put your details (name, surname, student id) in:
The head page of the report.
As a header in each of the files of the software project.
This will help to avoid any mistakes in locating the assignment and its components on both sides.
You need to submit the following via LMS:
Your report in PDF format only.
Two executable jar files (DictionaryClient.jar and DictionaryServer.jar) that will be used to run
your system on the day of the demo.
Your source files in a .ZIP or .TAR archive only.
Submissions will be due on: Thursday 5 of September (5 pm).
The marking process will be structured by first evaluating whether the assignment (application + report)
is compliant with the specification given. This implies the following:
Use of TCP/UDP sockets and threads for the application;
proper use of concurrency in the server;
proper handling of the errors;
a report with the requested content and format (PDF);
timeliness in submitting the assignment in the proper format (names, directory structure, etc.).
All of the above elements will earn you a maximum of 10 (out of 15) marks. The rest of the marks are
assigned to two elements:
Excellence (3 marks - 2 for implementation, 1 for report). The assignment has not only been
implemented by meeting the minimum requirements but the design has been well thought and a
proper interaction with the user has been provided (notification of errors, which really help to
understand what went wrong). For what concerns the report excellence, the report is complete
and well written with the proper graphs and a discussion and analysis of the system implemented
including advantages and disadvantages of your design choices. You need to explicitly write in
the Report all Excellence elements (point-by-point listing with brief illustration/discussion
included) in a separate section.
Creativity (2 marks). Any additional implementation such as a GUI for managing the server or
other enhancements to the code introducing advanced features (such as processing of user
requests within a server in scalable, reliable, and efficient manner) will be considered a plus. For
example, using your own implementation of a thread pool (not Java’s) would earn you these
marks. You need to explicitly write in the Report all Creativity elements (point-by-point listing
with brief illustration/discussion included) in a separate section.
NOTE 1 (EXTREMELY IMPORTANT): The excellence and the creativity marks (5 marks) cannot
compensate any lack in the first part (the one that scores up to 10 marks). Please concentrate
your efforts in getting the first 10 marks done and then proceed with the rest.
Note 2: Don’t document anything you haven’t implemented in the report. This is misconduct and will
result in severe penalties.
Demonstration Schedule and Venue
You are required to provide a demonstration of the working application and will have the opportunity to
discuss with the tutors the design and implementation choices made during the demo.
You are free to develop your system where you are more comfortable (at home, on one pc, on your laptop,
in the labs...) but keep in mind that the assignment is meant to be a distributed system that works on at
least two different machines in order to separate the client from the server.
We will announce the demo date, time, and venue closer to the due date. Each tutor will hold 2-3 demo
sessions and you will be required to showcase your system in one of the sessions held by the tutor of the
workshop in which you are enrolled.
If you need any clarification on the assignment, kindly ask questions during the tutorials or in the LMS
forum, so that all students benefit from it.
Note: You need to submit a printed copy of your report during the demonstration.
Penalties for late submissions of assignments
Assignments submitted late will be penalized in the following way:
1 day late: -1 mark
2 days late: -3 marks (-1 - 2)
3 days late: -6 marks (-3 - 3)
4 days late: -10 marks (-6 - 4)