Teaching-HEIGVD-RES-2020-Labo-Orchestra
Admin
- 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.
Objectives
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).
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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).
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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.
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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.
Requirements
In this lab, you will write 2 small NodeJS applications and package them in Docker images:
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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.
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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.
Instruments and sounds
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 |
TCP-based protocol to be implemented by the Auditor application
- 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"
}
]
What you should be able to do at the end of the lab
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.
Task 1: design the application architecture and protocols
| # | 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? |
| The musicians are sending the datagrams, every 1000 ms | |
| Question | Who is going to listen for UDP datagrams and what should happen when a datagram is received? |
| The auditor is goint to listen to these datagrams. When received he should add the new musician to the list of currently playing musicians (i guess) | |
| Question | What payload should we put in the UDP datagrams? |
| The sound of the instrument the musician plays and his uuid | |
| 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 json objects for the musician, that will be sent to the auditor through udp. The receiver needs to store this information in a map to know which musician is active at any time. The musician will send this info every 1000 sec. The map must be refreshed periodically to make sure it only stores currently active musicians |
Task 2: implement a "musician" Node.js application
| # | Topic |
|---|---|
| Question | In a JavaScript program, if we have an object, how can we serialize it in JSON? |
| We just enter the values in a json object format | |
| Question | What is npm? |
| a package manager for javascript | |
| Question | What is the npm install command and what is the purpose of the --save flag? |
| This command allows to install all dependencies defined in the package.json file. Before the 5.0.0 version of npm we needed to specify the --save option when doing a npm install if we wanted to save the dependencie in the package.json file. This option is no longer necessary. | |
| Question | How can we use the https://www.npmjs.com/ web site? |
| We can use it to search for packages that we can install with npm install | |
| Question | In JavaScript, how can we generate a UUID compliant with RFC4122? |
| We use the 'uuid' node package | |
| Question | In Node.js, how can we execute a function on a periodic basis? |
| We use the setInterval function | |
| Question | In Node.js, how can we emit UDP datagrams? |
| By using the upd4 package, we create a socket through which we send messages with the send function | |
| Question | In Node.js, how can we access the command line arguments? |
| We use process.argv and specify the id of the argument |
Task 3: package the "musician" app in a Docker image
| # | Topic |
|---|---|
| Question | How do we define and build our own Docker image? |
| we use the command docker build | |
| Question | How can we use the ENTRYPOINT statement in our Dockerfile? |
| It allows us to pass an argument when using docker run to the application laucnhed with ENTRYPOINT. This statement replaces CMD | |
| Question | After building our Docker image, how do we use it to run containers? |
| we use the docker run command, and specify the instrument used | |
| Question | How do we get the list of all running containers? |
| docker ps | |
| Question | How do we stop/kill one running container? |
| docker kill container_name | |
| Question | How can we check that our running containers are effectively sending UDP datagrams? |
| we can use tcp dump to analyse the traffic and check if we see our musicians payloads |
Task 4: implement an "auditor" Node.js application
| # | Topic |
|---|---|
| Question | With Node.js, how can we listen for UDP datagrams in a multicast group? |
| We wait for a 'message' event | |
| Question | How can we use the Map built-in object introduced in ECMAScript 6 to implement a dictionary? |
| We add a Map that will contain all musicians that the auditor hears, and we use the uuid as the key, since it is a unique value | |
| Question | How can we use the Moment.js npm module to help us with date manipulations and formatting? |
| using the format() function to format the date to make it more readable, and we can use functions like subtract() to modify our dates. It makes our task easier than using the Date object | |
| Question | When and how do we get rid of inactive players? |
| We make a function that will be executed every 500 ms, and check for each musician in our musician map when we heard it for the last time. If we haven't heard a musician for more than 5 sec we remove it from our map | |
| Question | How do I implement a simple TCP server in Node.js? |
| We use the net framework. When a client arrives, a socket is created, we send our message through that socket and close it, to ensure the data is correctly sent |
Task 5: package the "auditor" app in a Docker image
| # | Topic |
|---|---|
| Question | How do we validate that the whole system works, once we have built our Docker image? |
| We check first that the auditor listens correctly to the musicians, and adds them to his map (by connecting to the auditor and checking the musicians that he hears). Then we make sure that if we remove a musician the auditor detects it 5 sec later and removes it from the map. We can check this by connecting to the auditor, and by checking the logs |
Constraints
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.