Readme.md

Problem 2: Actors

Setup

Use the following commands to make a fresh clone of your repository:

git clone -b concpar21final02 git@gitlab.epfl.ch:lamp/student-repositories-s22/cs206-GASPAR.git concpar21final02

If you have issues with the IDE, try reimporting the build, if you still have problems, use compile in sbt instead.

Useful links

• A guide to the Scala parallel collections
• The API documentation of the Scala parallel collections
• The API documentation of the Scala standard library
• The API documentation of the Java standard library

Exercise

Your task is to implement a couple of actors in Discord like service. You will only focus on two actors in the system: (1) the NotificationService and (2) and a DiscordChannel.

A user will be able to post messages on the channel and retrieve the messages from the channel. A user also has the ability to (un)register from the notifications. Before it can receive requests, each channel is initialized with a notification service. When a message is posted to the channel, it tells its notification service to notify the users.

The following diagram shows how the communication between actors works. Requests are within () and responses are within [].

           (Post)                                     [Active]
       (GetLastPosts)       ┏━━━━━━━━━━━━━━━━━━┓  [AlreadyActive]  ┏━━━━━━━━━┓
    ┌──────────────────────>┃                  ┃──────────────────►┃         ┃
    │                       ┃  DiscordChannel  ┃                   ┃ System  ┃
    │                       ┃                  ┃◄──────────────────┨         ┃
    │          [Posts]      ┗━┯━━━━━━━━━━━━━━━┯┛      (Init)       ┗━━━━━━━━━┛
 ┏━━┷━━━┓    [NotActive]      │               │
 ┃      ┃◄────────────────────┘               │
 ┃ User ┃                                     │
 ┃      ┃◄────────────────┐                   │
 ┗━━┯━━━┛  [Registered]   │                   │ (NotifyAll)
    │     [Notification]  │                   │
    │                     │                   │
    │                ┏━━━━┷━━━━━━━━━━━━━━━━┓  │
    └───────────────►┃                     ┃  │
     (Register)      ┃ NotificationService ┃◄─┘
    (UnRegister)     ┃                     ┃
                     ┗━━━━━━━━━━━━━━━━━━━━━┛

Your tasks in the exercise will be to:

TASK 1: Complete the implementation of the NotificationService by implementing method receive to handle messages from NotificationService.Protocol:

def receive: Receive = ???

TASK 2: Complete the implementation of the DiscordChannel by implementing methods nonActive and active to handle messages from DiscordChannel.Protocol:

def nonActive: Receive = ???
def active(notificationService: ActorRef): Receive = ???

NotificationService protocol

• Register: Registers the user for notifications and responds with a Registered(true).
• UnRegister: Un-registers the user for notifications and responds with a Registered(false).
• NotifyAll: Sends a Notification to all registered users.

DiscordService protocol

The channel can be in one of two states: non-active or active.

• Init: When non-active, responds with Active and the state becomes active. Otherwise responds AlreadyActive.
• Post: When active, stores the message and sends NotifyAll to its NotificationService. Otherwise responds NotActive.
• GetLastPosts: When active, responds with Posts containing the latest messages (ordered from most to least recent). Otherwise responds NotActive.

Running the code

Apart from the grading tests, we include a @main def debug method, where you can write your own tests to help you debug your implementation. This method is not graded. It can be executed using run within SBT.