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Homework 7: A Multithreaded Chat System

Due Wednesday, 2016-03-23, 11:59pm.

Okay, you admit it - a BBS isn't exactly the most modern web technology of all time. Inspired by the rising success of this newfangled "AOL Instant Messenger" thing all the kids are using, you've decided to make your own chat service!

In this homework, you'll write your own multithreaded, multi-user, IRC-esque chat service! And as a bonus, it'll run in the browser using WebSockets!

Classroom for GitHub

We're using Classroom for GitHub, which manages private homework repositories for students. To create your very own private homework repository (owned by us), click this link:

Background

This assignment should work on any modern browser; tested on Firefox and Chrome.

WebSockets

WebSockets are a web technology for allowing JavaScript in webpages to connect back to servers using a bidirectional data stream. WebSockets behave very similarly to TCP streams (but are message-based).

We will be using the rust-websocket library. You'll definitely need to take a look at some of the documentation and examples:

Multithreaded Networking

In this assignment, we'll be using threads to easily handle several network clients at once. One way to do this is to create one thread for each client. As clients connect, a thread is spawned to manage that particular connection:

// fn listen
for connection in server {
    // Spawn a client_thread.
}

In this paradigm, each client thread will block as it waits for data to come in from its respective client. This can be expressed very elegantly using iterators: every time a message comes in, the loop runs once. When the channel closes, the loop terminates.

// fn client_thread
for message in client_recv.incoming_messages() {
    // Handle message; relay via MPSC channel.
}

In the relay thread, each message received via the MPSC channel should be sent to all of the clients (including the originator). Once again, iterators allow us to use the MPSC receiver very nicely:

// fn relay_thread
for action in relay_mpsc_recv {
    // Send message to all clients.
}

Aside: For many applications, the one-thread-per-client model does not work well. For systems which will have many (thousands or more) clients, spawning thousands of threads is very inefficient. In these cases, a more complex system will typically be used; for example, several threads (usually about one per core) might run in a thread pool, where each asynchronously handles many clients. That is, each thread will periodically poll for incoming data from each client (non-blocking), rather than waiting for incoming data from a single client (blocking). For rust-websocket, there is a discussion here.

IRC Principals

IRC (Internet Relay Chat) operates on very simple principals:

  • Maintain a list of all of the currently connected clients.
  • For each message that comes in, relay it back out to all of the clients.

We won't be implementing IRC precisely, but we will use the "relay" idea.

Instructions

main.rs and webpage.rs are provided for you; they just serve a static HTML webpage over port 1980. To access this, just run the server (cargo run) and open localhost:1980 in a web browser. The static webpage is also written for you.

Your job is to write chatserver.rs. In chatserver::start, you should spawn a thread to listen for incoming WebSockets connections on port 1981. We've also given you a (de)serializable enum ChatAction - don't modify this; the JavaScript code depends on it!

Note: The messages received-from and sent-to the client should be JSON object strings with the same form as rustc_serialize's serialization of the ChatAction type. This means you can json::encode ChatActions to create text to send to the web browser, and json::decode to turn the web browser's messages into ChatActions.

The example server will be an important resource - you'll use a lot of the same boilerplate code. (Note: we aren't using a protocol; the protocol response isn't necessary.)

Now, go check out the comments left in chatserver.rs!

Submission

Commit and push your work to the master branch of your Classroom for Github repository for this HW. Make sure it is visible on Github! This is your submission. (Work must be in the master branch at the deadline.)

If you have any comments or feedback on this assignment, include them in the README of your submission or post on Piazza or the Google Group.