- 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? |
| Musicians every second | |
| Question | Who is going to listen for UDP datagrams and what should happen when a datagram is received? |
| Only Auditor is listening for UDP datagrams. Once one is received the auditor will keep track of it to monitor the current musicians alive | |
| Question | What payload should we put in the UDP datagrams? |
| An UUID corresponding to the musician, the sound and 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? |
| Receiver: a dictionary to store all active musicians Sender: an object to map the instruments with the sounds and a musician object which is the payload we send |
| # | Topic |
|---|---|
| Question | In a JavaScript program, if we have an object, how can we serialize it in JSON? |
| JSON.stringify(object) | |
| Question | What is npm? |
| npm means Node package manager. The name is self explainable | |
| Question | What is the npm install command and what is the purpose of the --save flag? |
npm install will install all dependencies in the package.json if no package specified. The purpose of --save flag was to add a new dependency to package.json but this flag is deprecated for latest version of npm npm install <my_package> will do the same |
|
| Question | How can we use the https://www.npmjs.com/ web site? |
| We can use this web site to search for node packages | |
| Question | In JavaScript, how can we generate a UUID compliant with RFC4122? |
| By using a package named uuid | |
| Question | In Node.js, how can we execute a function on a periodic basis? |
using the javascript function setInterval() |
|
| Question | In Node.js, how can we emit UDP datagrams? |
By using the module dgram |
|
| Question | In Node.js, how can we access the command line arguments? |
we can access command lines with process.argv in a similar way as C which means that argv is an array containing all params from cmd line |
| # | Topic |
|---|---|
| Question | How do we define and build our own Docker image? |
To define a docker image we need to write a dockerfile. To build it we use the following command docker build -t <image-name> <dockerfile> |
|
| Question | How can we use the ENTRYPOINT statement in our Dockerfile? |
by putting as a first param the cmd to execute and as a second param the script tu run i.e ENTRYPOINT ["node", "app.js"] |
|
| Question | After building our Docker image, how do we use it to run containers? |
docker run <image-name> we can also use the flag -d to run the container in background and the flag -p to do port mapping |
|
| Question | How do we get the list of all running containers? |
docker ps |
|
| Question | How do we stop/kill one running container? |
docker kill <name/id_of_running_container> |
|
| Question | How can we check that our running containers are effectively sending UDP datagrams? |
by running tcpdump listening the docker interface sudo tcpdump -i docker0 udp |
| # | Topic |
|---|---|
| Question | With Node.js, how can we listen for UDP datagrams in a multicast group? |
| By subscribing to the multicast group | |
| Question | How can we use the Map built-in object introduced in ECMAScript 6 to implement a dictionary? |
by doing a new Map() after that .set(key, value) to add or modify a value .delete(key) to delete a pair from the map, etc. |
|
| Question | How can we use the Moment.js npm module to help us with date manipulations and formatting? |
moment() gives us the current datetime.toISOString() gives us the iso format 2016-04-27T05:39:03.211Z.diff(other_date_time, '<time_unit>') gives us the time diff between two date times |
|
| Question | When and how do we get rid of inactive players? |
| The most efficient way to do it is to clean the dictionary of musicians each time a client ask for the list of active musicians. We could have done that each time a musician makes a sound or each second but those two ways are not very efficient. to get rid of them we simply compare the last_active time with current time and test if it's greater than 5 seconds if it's true we remove the musician from the dictionary |
|
| Question | How do I implement a simple TCP server in Node.js? |
We can use the net module |
| # | Topic |
|---|---|
| Question | How do we validate that the whole system works, once we have built our Docker image? |
by running the script validate.sh or by hand starting and killing containers and communicating with the auditor using telnet and for monitoring we could use tcpdump as explained previously |
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.