• 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.

• 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?
	The one sending UDP diagram is musician.js, the musicians when they play music (every secs). The sound must reach the auditor. As it is a sound made to reach the auditor's ears we don't need no answer from the auditor.
Question	Who is going to listen to UDP datagrams and what should happen when a datagram is received?
	The one listening to the UDP diagrams is auditor.js, the auditor that will receive the datagram. He should do nothing to respond to the musicians but he should tell the client via TCP what is the instrument that is playing
Question	What payload should we put in the UDP datagrams?
	The payload we should put in the UDP datagrams are the sounds of the instrument that is playing.
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 data structure we can use for this is a map with JSON.

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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.