Grading procedure


First deadline: 26th of February, 23:59.
Final deadline: 15 of March, 23:59.

Demo sessions

Sessions: 26th of February - (8:00-9:45), (13:15-15:00) and (15:15-17:00).

chat bubbles

Introduction to CCHAT

Check how to install Erlang/OTP in your computer!

Building a "real" concurrent system is not easy. In this lab, we aims put you through some of that experience. You will need to make design decisions, evaluate the trade-off between performance and simplicity, face debugging, and much more. While challenging, it is plenty of fun!

The topic

Nowadays, messaging systems are very popular (Whatsapp, Snapchat, Facebook chat, etc.). They provide great connectivity to users as well as different features like stickers, videos clips, etc. In this lab, you will build a simple (but still quite real) text-based messaging system called CCHAT (the first C is from Chalmers). CCHAT is very much inspired by IRC, an old but still valid standard designed for group discussions. For simplicity, and reasons of time, your implementation of CCHAT is not going to use IRC's protocol nor low-level TCP/IP communication. Instead, it will leverage Erlang's processes and message passing features.

A first look

Below, you can find a short video of how CCHAT will look and work.

Here, some details about the demo.


CCHAT will use a client-server architecture. It consists on a client part, which runs the GUI and a client process, and a server which runs the chat service (server process) and host the discussions groups. The graphic below illustrates the situation.


In the graphic above, the client process (blue circle with double lines) has the goal to be a bridge between the GUI and the server process (the other blue circle with double lines).

The GUI and the client process

The protocol

The protocol between the GUI and the client process is partly fixed. The reason for that is two fold. Firstly, by following the protocol, you will be able to use the GUI without knowing its internal implementation details. Secondly, and more importantly, we will test your code assuming that your client process follow the protocol. If you do not follow it, your code will not pass the tests (see the test section below) and your submission will be immediately rejected.

The protocol scheme is as follows.


The GUI sends a message Message requesting some operations. Then, the Client either replies with the atom ok or {error, Atom, Text}. Atom ok indicates that the operation succeeded. Tuple {error, Atom, Text} denotes that something went wrong while processing the request. These errors are not fatal and the GUI can recover from them. Variable Text contains the text to be reported in the System tab of the GUI. You are free to choose the value of Text. However, you should strictly use the values for Atom as described by the protocol.

Until this point, the protocol describes communications initiated by the GUI. There is only one occasion when the client process starts communication with the GUI: when something is written to a channel, the client needs to tell the GUI to display the new text. The client process sends the message {msg_to_GUI, Chatroom, Msg} when it wishes to print out the line Msg in the chat room Chatroom.

Client to GUI

For that, we use the following line of code

gen_server:call(list_to_atom(GUIName), {msg_to_GUI, Chatroom, Name++"> "++Msg})

Where Name is the nick of the author and Msg is their message. Observe that gen_server is not the same as genserver (the module that has been shown in class). Module gen_server is the OTP implementation of a generic server..

Variable GUIName contains the name for the GUI process, which is generated in the gui.erl module and passed onto your client in client:initial_state/2.


Fatal errors in the protocol

Code skeleton

Get the skeleton code here! If clone the repository in git, make sure to check out the proper branch vt2015lp3.

In this lab, you are required to build the client and server processes based on the structure in the skeleton code. The main reason for that is to make it easier to have your lab up and running. You could use any Erlang code that we saw at the lectures if it helps you.

We will give you the following files:

Component Files Description
 gui.erl lexgrm.erl grm.yrl lex.xrl 
These files contain the implementation of the GUI. Do not modify them.
 test_client.erl dummy_gui.erl 
Files used for testing. Do not modify them.
Record definitions
This file contain record definitions. The state of the client, server, or any other entity that you wish to add should represent its state as a record. We provide you with some incomplete definitions already. For instance, -record(cl_st, {gui}). defines the record cl_st (client state) where field gui stores the name of the GUI.
Client process
The exported function client:main/1 is responsible for launching the client process and handling messages. It should: receive a message from the GUI, handle it using the loop function, and repeat with a possibly updated state. The loop function handles each different kind of request, returning a tuple of the response and the updated state. You also need to implement initial_state(Nick, GUIName) which generates the initial state for the client.
Server process
The exported function server:main/1 should launch the server process, handling messages in a loop in a similar way to the client. It is up to you to decide the protocol between the client and the server! You also need to implement initial_state(ServerName) which generates the initial state for the server. For the purpose of the lab, the server name will always be "shire" (although you should not hardcode it anywhere).
This is the top level module. It is used to launch the server and several clients with their respective GUIs. Do not modify this file.
This file contains functions for spawning and running Erlang processes as servers (as in gen_server), and implementing synchronous message passing. It is used internally, so do not modify this file... but you might want to use its functions yourselves!

To download it, click on the button that says "Download ZIP", or you can clone the entire repository using git clone.

After you download it (and extract the files) you should compile everything with make and start to perform the same demo as in the first video. You should be able to start the server and open up chat windows, but Bilbo and Frodo will not able to communicate because most of the functions are not implemented. It is your task to make sure that they can!

If you do not have GNU Make, you should run the following commands in the Erlang shell:

  cd("the directory where the skeleton code is").


Test cases

All unit tests are contained in the file test_client.erl. Tests are carried out using EUnit. We have created entries in the Makefile to make life easier for you (see below for alternatives to using make). There are two sets of tests, correctness and performance tests.

The video below shows how to use the test suite (includes audio):


There are positive and negative tests which check that your solution follows the requirements and protocol as specified above. To run these tests, execute the following:

$ make -s run_tests


We have also included two cases which test the performance of your solution by spawning large numbers of clients and channels simultaneously. To run them, use the command:

$ make -s run_perf_tests

This will run the 2 performance tests 3 times each: once using 4 cores, once with 2 cores, and once with only 1 core enabled. In each case you will see the time elapsed for the test case to complete. To test your use of concurrency, we expect that your solution will be significantly faster with more cores. The actual times do not matter much, as long as there is a noticeable improvement. Of course if you don't have 4 cores, then the first set of results will not be taken into consideration.

Alternatives to make

If your system doesn't have the make command, you can run the test suites like so:


$ erl +P 1000000 -eval "cover:compile_directory(), eunit:test(test_client), halt()"


The number 4 below specifies the number of cores to use. Make sure to run this command with values 4, 2, and 1:

$ erl -smp +S 4 +P 1000000 -eval "cover:compile_directory(), eunit:test([{timeout, 60, {test,test_client,many_users_one_channel}},{timeout, 60, {test,test_client,many_users_many_channels}}]),halt()"

Turning off colour codes

If you don't have a colour-enabled terminal, you will see a lot of ugly colour codes in your test output. You can disable these by commenting out the colour function in test_client.erl and replacing it with:

colour(Num,S) -> S.


You should submit your code based on the skeleton code and whatever other files are needed for your solution to work. In addition, you should comment your code so that graders can easily review your submission.