- 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? |
 Représentation de l'architecture dans le cas où un auditeur et trois musiciens (jouant respectivement du piano, de la trompette et de la flûte) sont lancés. |
|
| Question | Who is going to send UDP datagrams and when? |
| Les musiciens envoient les diagrammes UDP toutes les secondes. | |
| Question | Who is going to listen for UDP datagrams and what should happen when a datagram is received? |
| L'auditeur écoute pour les datagrammes UDP. Quand il en reçoit un, il convertit le son entendu en instrument et met à jour la liste des musiciens actifs. | |
| Question | What payload should we put in the UDP datagrams? |
| Le payload contient le son de l'instrument et un identifiant unique pour permettre à l'auditeur de distinguer les musiciens. | |
| 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? |
| Les musiciens ont besoin d'un tableau permettant de mapper un instrument à un son. L'auditeur a quant à lui besoin d'un tableau permettant de mapper un son à un instrument. Il a également besoin de maintenir une liste de musiciens actifs qui sera mise à jour lorsqu'un nouveau musicien arrive ou lorsqu'un musicien n'est plus actif depuis 5 secondes. |
| # | Topic |
|---|---|
| Question | In a JavaScript program, if we have an object, how can we serialize it in JSON? |
En utilisantJSON.stringify(object) |
|
| Question | What is npm? |
| npm est le gestionnaire de paquets officiel de Node.js. | |
| Question | What is the npm install command and what is the purpose of the --save flag? |
La commande npm installpermet d'installer les dépendances dans le dossier local node_modules. Avant la version de npm 5.0.0, le flag --save permettait d'ajouter les dépendances dans le package.json. |
|
| Question | How can we use the https://www.npmjs.com/ web site? |
| On peut rechercher des packages d'un outil dont nous avons besoin (par exemple UUID). On y retrouve leur documentation, ce qui s'avère pratique. | |
| Question | In JavaScript, how can we generate a UUID compliant with RFC4122? |
On utilise le package uuid qui fournit une fonction v4() pour générer un id |
|
| Question | In Node.js, how can we execute a function on a periodic basis? |
On utilise la fonction setInterval(callback, milliseconds, [args]) |
|
| Question | In Node.js, how can we emit UDP datagrams? |
On commence par créer un socket à l'aide du package dgram. La fonction send utilisée sur le socket permet d'envoyer un datagramme UDP. On lui fournit notamment le message, la longuer du message, le numéro de port et l'adresse ip. |
|
| Question | In Node.js, how can we access the command line arguments? |
| On utilise le tableau process.argv qui permet de récupérer les arguments. |
| # | Topic |
|---|---|
| Question | How do we define and build our own Docker image? |
A l'aide d'un Dockerfile permettant de contruire une image avec la commande build depuis le dossier où se trouve le Dockerfile. Exemple: docker build -t res/musician . |
|
| Question | How can we use the ENTRYPOINT statement in our Dockerfile? |
On utilise l'ENTRYPOINT pour exécuter une commande au démarrage du container. |
|
| Question | After building our Docker image, how do we use it to run containers? |
On lance un conteneur à l'aide de la commande run . Exemple: docker run res/musician <instrument>, ici un paramètre est également passé. |
|
| Question | How do we get the list of all running containers? |
Avec la commande docker ps |
|
| Question | How do we stop/kill one running container? |
Pour stopper un container, avec la commande docker stop <container name>. Pour tuer un container, avec la commande docker kill <container name>. |
|
| Question | How can we check that our running containers are effectively sending UDP datagrams? |
On peut utiliser l'outil Wireshark pour voir le trafic des containers ou en utilisant tcpdump. |
| # | Topic |
|---|---|
| Question | With Node.js, how can we listen for UDP datagrams in a multicast group? |
On commence par créer un socket à l'aide du package dgram. On lie ensuite le socket à un port et la fonction addMembership permet de s'ajouter au groupe multicast grâce à l'adresse ip. On peut ensuite définir un comportement à chaque fois qu'un datagramme UDP arrive. |
|
| Question | How can we use the Map built-in object introduced in ECMAScript 6 to implement a dictionary? |
| L'objet Map représente un dictionnaire composé de clé/valeurs. Dans notre cas, l'uuid est la clé et l'instrument + la date de dernière activité sont les valeurs. | |
| Question | How can we use the Moment.js npm module to help us with date manipulations and formatting? |
Pour manipuler les dates, nous n'avons pas utiliser Moment.js et nous nous sommes contentées d'utiliser des objets Javascript Date et ses fonctions. |
|
| Question | When and how do we get rid of inactive players? |
A l'aide de la fonction setInterval on parcourt chaque seconde la liste de musicians pour détecter si un musicien n'a pas été actif depuis 5 secondes. Si c'est le cas on le supprime de la liste. |
|
| Question | How do I implement a simple TCP server in Node.js? |
On peut créer un serveur TCP à l'aide du package net et de la fonction createServer. La fonction listen permet d'écouter sur le port fourni. |
| # | Topic |
|---|---|
| Question | How do we validate that the whole system works, once we have built our Docker image? |
| On lance le script de validation fourni. |
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.