- Every student must do the implementation and have a repo with the code at the end of the process.
- 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.
- There will be no grade for this lab. However, if you work on it seriously, the next challenge will be very easy (just be careful: the challenge will be done on a short period, so don't be late!)
- 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, 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 10 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? |
| Musicians will send UDP datagrams every second. | |
| Question | Who is going to listen for UDP datagrams and what should happen when a datagram is received? |
| Auditor is going to listen for UDP datagrams and will update his active musicians list when a datagram is received. | |
| Question | What payload should we put in the UDP datagrams? |
| We should put musician id, instrument and the sound of the 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? |
| The auditor must maintain a list of active musicians. He will add a new musician in the list or update the last time the musician has emitted a sound. |
| # | Topic |
|---|---|
| Question | In a JavaScript program, if we have an object, how can we serialize it in JSON? |
We can serialize it using JSON.stringify() method. |
|
| Question | What is npm? |
| npm is a package manager for the JavaScript programming language. It helps you to build applications using modern open source code. | |
| Question | What is the npm install command and what is the purpose of the --save flag? |
npm install command is used to install a package. The --save flag instructed NPM to include the package inside of the dependencies section of your package.json. |
|
| Question | How can we use the https://www.npmjs.com/ web site? |
| We can use this website to search for packages and install them simply by doing a keyword search. | |
| Question | In JavaScript, how can we generate a UUID compliant with RFC4122? |
First we have to install uuid package with the npm install --save uuid command. Then we add these two instructions in our code : const uuid = require('uuid/v1'); and const id = uuid(); |
|
| Question | In Node.js, how can we execute a function on a periodic basis? |
We can do that using setInterval(callback, delay[, ...args]) function. It schedules repeated execution of callback every delay milliseconds. |
|
| Question | In Node.js, how can we emit UDP datagrams? |
We can do that with the standard Node.js module dgram. We create a socket with dgram.createSocket('udp4') and then we can emit UDP datagrams. |
|
| Question | In Node.js, how can we access the command line arguments? |
We can access them with the process.argv property. It returns an array containing the command line arguments passed when the Node.js process was launched. |
| # | Topic |
|---|---|
| Question | How do we define and build our own Docker image? |
| We create our Dockerfile file and extend the Node base image. Then we add our JavaScript sources in the image file system. | |
| Question | How can we use the ENTRYPOINT statement in our Dockerfile? |
We will us this statement for musicians containers like that : ENTRYPOINT ["node", "/opt/app/musician.js"]. It allows you to configure a container that will run as an executable. |
|
| Question | After building our Docker image, how do we use it to run containers? |
We run the following command: docker run -d res/musician <instrument>. <instrument> can be piano, trumpet, flute, violin or drum. |
|
| Question | How do we get the list of all running containers? |
We run the following command: docker ps |
|
| Question | How do we stop/kill one running container? |
We run the following command: docker kill <container> |
|
| Question | How can we check that our running containers are effectively sending UDP datagrams? |
We can check the logs by running the following command: docker logs -f <container>. |
| # | Topic |
|---|---|
| Question | With Node.js, how can we listen for UDP datagrams in a multicast group? |
We create a datagram socket and bind it to listen for datagram messages on a named port and optional address. Then we have to tell the kernel to join a multicast group with this instruction: s.addMembership(PROTOCOL_MULTICAST_ADDRESS). |
|
| Question | How can we use the Map built-in object introduced in ECMAScript 6 to implement a dictionary? |
With the following instruction we create a map: const activeMusicians = new Map();. Different properties and methods exist for Map object. More infos here. |
|
| Question | How can we use the Moment.js npm module to help us with date manipulations and formatting? |
We have to install moment package with npm install --save moment command. Then we read moment documentation here. |
|
| Question | When and how do we get rid of inactive players? |
| Before sending active musicians, we check that the last time the musician has emitted a sound is less than 5 seconds. | |
| Question | How do I implement a simple TCP server in Node.js? |
We implement a TCP server using standard Node.js module net. |
| # | Topic |
|---|---|
| Question | How do we validate that the whole system works, once we have built our Docker image? |
We validate it by executing the validate.sh script. |
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.