- You can work in groups of 2 students.
- It is up to you if you want to fork this repo, or if you prefer to work in a private repo. However, you have to use exactly the same directory structure for the validation procedure to work.
- We expect that you will have more issues and questions than with other labs (because we have a left some questions open on purpose). Please ask your questions on Telegram / Teams, so that everyone in the class can benefit from the discussion.
This lab has 4 objectives:
-
The first objective is to design and implement a simple application protocol on top of UDP. It will be very similar to the protocol presented during the lecture (where thermometers were publishing temperature events in a multicast group and where a station was listening for these events).
-
The second objective is to get familiar with several tools from the JavaScript ecosystem. You will implement two simple Node.js applications. You will also have to search for and use a couple of npm modules (i.e. third-party libraries).
-
The third objective is to continue practicing with Docker. You will have to create 2 Docker images (they will be very similar to the images presented in class). You will then have to run multiple containers based on these images.
-
Last but not least, the fourth objective is to work with a bit less upfront guidance, as compared with previous labs. This time, we do not provide a complete webcast to get you started, because we want you to search for information (this is a very important skill that we will increasingly train). Don't worry, we have prepared a fairly detailed list of tasks that will put you on the right track. If you feel a bit overwhelmed at the beginning, make sure to read this document carefully and to find answers to the questions asked in the tables. You will see that the whole thing will become more and more approachable.
In this lab, you will write 2 small NodeJS applications and package them in Docker images:
-
the first app, Musician, simulates someone who plays an instrument in an orchestra. When the app is started, it is assigned an instrument (piano, flute, etc.). As long as it is running, every second it will emit a sound (well... simulate the emission of a sound: we are talking about a communication protocol). Of course, the sound depends on the instrument.
-
the second app, Auditor, simulates someone who listens to the orchestra. This application has two responsibilities. Firstly, it must listen to Musicians and keep track of active musicians. A musician is active if it has played a sound during the last 5 seconds. Secondly, it must make this information available to you. Concretely, this means that it should implement a very simple TCP-based protocol.
The following table gives you the mapping between instruments and sounds. Please use exactly the same string values in your code, so that validation procedures can work.
| Instrument | Sound |
|---|---|
piano |
ti-ta-ti |
trumpet |
pouet |
flute |
trulu |
violin |
gzi-gzi |
drum |
boum-boum |
- The auditor should include a TCP server and accept connection requests on port 2205.
- After accepting a connection request, the auditor must send a JSON payload containing the list of active musicians, with the following format (it can be a single line, without indentation):
[
{
"uuid" : "aa7d8cb3-a15f-4f06-a0eb-b8feb6244a60",
"instrument" : "piano",
"activeSince" : "2016-04-27T05:20:50.731Z"
},
{
"uuid" : "06dbcbeb-c4c8-49ed-ac2a-cd8716cbf2d3",
"instrument" : "flute",
"activeSince" : "2016-04-27T05:39:03.211Z"
}
]
You should be able to start an Auditor container with the following command:
$ docker run -d -p 2205:2205 res/auditor
You should be able to connect to your Auditor container over TCP and see that there is no active musician.
$ telnet IP_ADDRESS_THAT_DEPENDS_ON_YOUR_SETUP 2205
[]
You should then be able to start a first Musician container with the following command:
$ docker run -d res/musician piano
After this, you should be able to verify two points. Firstly, if you connect to the TCP interface of your Auditor container, you should see that there is now one active musician (you should receive a JSON array with a single element). Secondly, you should be able to use tcpdump to monitor the UDP datagrams generated by the Musician container.
You should then be able to kill the Musician container, wait 5 seconds and connect to the TCP interface of the Auditor container. You should see that there is now no active musician (empty array).
You should then be able to start several Musician containers with the following commands:
$ docker run -d res/musician piano
$ docker run -d res/musician flute
$ docker run -d res/musician flute
$ docker run -d res/musician drum
When you connect to the TCP interface of the Auditor, you should receive an array of musicians that corresponds to your commands. You should also use tcpdump to monitor the UDP trafic in your system.
| # | Topic |
|---|---|
| Question | How can we represent the system in an architecture diagram, which gives information both about the Docker containers, the communication protocols and the commands? |
 |
|
| Question | Who is going to send UDP datagrams and when? |
| Each Musician sends one every second. | |
| Question | Who is going to listen for UDP datagrams and what should happen when a datagram is received? |
| Each Auditor listens for them. When he recieves one, the auditor updates his list of active Musicians. | |
| Question | What payload should we put in the UDP datagrams? |
| The UUID of the Musician and the sound of his instrument. | |
| Question | What data structures do we need in the UDP sender and receiver? When will we update these data structures? When will we query these data structures? |
We need tha map data structure. In fact I used 3 of them. The first is in the UDP sender to get the sound from an instrument. The second is in the UDP reciever to get the instrument from a sound. The third is in the reciever to have all musicians in it. I use the UUID as a key for each musician. |
| # | Topic |
|---|---|
| Question | In a JavaScript program, if we have an object, how can we serialize it in JSON? |
Javascript and JSON (Javascript Object Notation) are like brother and sister. The standard way to do it is this function call : JSON.stringify(object); |
|
| Question | What is npm? |
NPM means Node Package Manager. It's the official package manager of Node.js. With it we can easily download all libraries (packages) that we need for our app. |
|
| Question | What is the npm install command and what is the purpose of the --save flag? |
It's to save the dependencies in the package.json file. That way if we want to download the dependencies again, we have them listed in that file. Since NPM 5 (I'm not exactly sure since what version) the --save flag is put by default. (So we don't need to write it manually). |
|
| Question | How can we use the https://www.npmjs.com/ web site? |
| With a web browser. You can find there all node packages (libraries) you need. | |
| Question | In JavaScript, how can we generate a UUID compliant with RFC4122? |
I used the UUID package on npm. (Use the v4 function) |
|
| Question | In Node.js, how can we execute a function on a periodic basis? |
I used the "classic way" in js, it's the setInterval(callback, milliseconds); function call. |
|
| Question | In Node.js, how can we emit UDP datagrams? |
| Taken from Lecture 3 about UDP : |
// Sending a message to all nodes on the local network
var dgram = require('dgram');
var s = dgram.createSocket('udp4');
s.bind(0, '', function() {
s.setBroadcast(true);
});
var payload = "Node.js rocks, everybody should know!";
message = new Buffer(payload);
s.send(message, 0, message.length, 4411, "255.255.255.255", function(err, bytes) {
console.log("Sending ad: " + payload + " via port " + s.address().port);
});| # | Topic |
|---|---|
| Question | In Node.js, how can we access the command line arguments? |
Simply with process.argv. |
| # | Topic |
|---|---|
| Question | How do we define and build our own Docker image? |
You have to make a Dockerfile file in it you configure how it should be, then you build the image with the command docker build. |
|
| Question | How can we use the ENTRYPOINT statement in our Dockerfile? |
It is used to know what command the container should run on startup (to launch you app). I used like this with 2 arguments : ENTRYPOINT ["npm", "start"] |
|
| Question | After building our Docker image, how do we use it to run containers? |
Command : docker run [OPTIONS] IMAGE [COMMAND] [ARG...] |
|
| Question | How do we get the list of all running containers? |
With the command : docker ps |
|
| Question | How do we stop/kill one running container? |
Command : docker kill [OPTIONS] CONTAINER [CONTAINER...] |
|
| Question | How can we check that our running containers are effectively sending UDP datagrams? |
| With Wireshark we can capture traffic and see what is happening in the docker network. |
| # | Topic |
|---|---|
| Question | With Node.js, how can we listen for UDP datagrams in a multicast group? |
| Taken from Lecture 3 about UDP : |
var dgram = require('dgram');
var s = dgram.createSocket('udp4');
s.bind(protocol.PROTOCOL_PORT, function() {
console.log("Joining multicast group");
s.addMembership(protocol.PROTOCOL_MULTICAST_ADDRESS);
});
// This call back is invoked when a new datagram has arrived.
s.on('message', function(msg, source) {
console.log("Ad has arrived: '" + msg + "'. Source address: " + source.address + ", source port: " + source.port);
});| # | Topic |
|---|---|
| Question | How can we use the Map built-in object introduced in ECMAScript 6 to implement a dictionary? |
We can pass a bi-dimensionnal array to the Map constructor |
|
| Question | How can we use the Moment.js npm module to help us with date manipulations and formatting? |
I don't know. I used another one called dateformat. (In fact it's only used for my own logging...) |
|
| Question | When and how do we get rid of inactive players? |
When their last seen time is too old. now - lst <= INTERVAL if he's in the interval we keep him. |
|
| Question | How do I implement a simple TCP server in Node.js? |
With the net module. An example is in the Lecture 2 about TCP. |
| # | Topic |
|---|---|
| Question | How do we validate that the whole system works, once we have built our Docker image? |
With the validate.sh script. Or simply create manually some containers and see how they behave by removing and adding musicians. |
Please be careful to adhere to the specifications in this document, and in particular
- the Docker image names
- the names of instruments and their sounds
- the TCP PORT number
Also, we have prepared two directories, where you should place your two Dockerfile with their dependent files.
Have a look at the validate.sh script located in the top-level directory. This script automates part of the validation process for your implementation (it will gradually be expanded with additional operations and assertions). As soon as you start creating your Docker images (i.e. creating your Dockerfiles), you should try to run it.