Netchat

A fully decentralized (thus inefficient) chat written in rust, using named pipes (fifo files) for communication.

by Mathis Chenuet, Emilien Fugier, Elias Hariz

Two-instance communication

mkfifo a2b b2a
netchat --input b2a --output a2b --name IamA
# in another terminal:
netchat -i a2b -o b2a -n IamB

Cross-computer communication

# You
mkfifo in out
cat out | netcat -l 1234 > in1 &
netchat -i in -o out
# Someone else
mkfifo in out
cat out | netcat IP 1234 > in & # replace IP with your IP
netchat -i in -o out

Troubleshooting: rm in out and killall netcat on both computers, then redo the aforementioned steps in the exact same order.

Commands

• Enter sends the content of the input field to everyone
• Ctrl+c exit
• Ctrl+s get a snapshot containing every messages sent by every site
• Ctrl+r set the private message recipient id to the content of the input field or, if let empty, to the id which sent you the last private message
• Ctrl+p sends the content of the input field to the current private recipient
• Up scroll messages up
• Down scroll messages down

Dev hints

Arguments

Use -- to mark the end of cargo arguments and the begining of the program's own.

cargo run -- --help

Redirect stderr to get debug output

mkfifo err
tail -f err
# From another terminal
cargo run --color=always  -- -i fifo -o fifo  -l err 2> err

Basic test: one app speaking to itlsef

mkfifo fifo
cargo run -- -i fifo -o fifo

Passing args to the binary through cargo run

cargo run -- -i a -o b

Multiple instances

./launch-network.sh automates the fifo creation and routing for 2 instances, and ./launch.py N does the same for N instances.

Need help ?

netchat --help

---

App architecture

src
├── main.rs
├── app
│  ├── events.rs
│  └── mod.rs
└── server
   ├── events.rs
   ├── messages.rs
   └── mod.rs

There are two modules: app and server.

• app handle all the user-facing processes, user-input, user-interface...
• server handles the routing and all the logic tied the distributed nature of the app.

Each module has his own events submodule which provides an events object that centralizes all the possible input sources of the module (e.g. for the app the server and the user).

Event management

Our events are coded as Rust enums which can very handily hold various types under a single parent one.

pub enum Event {
    /// User public message
    UserPublicMessage(String),
    /// User public message
    UserPrivateMessage(AppId, String),
    /// Input from a distant agent (write in a file)
    DistantInput(String),
    /// Shutdown the server
    Shutdown,
    /// Request snapshot from other apps
    GetSnapshot,
    ...
}

To pass events around we use Multiple Producer Single Consumer (MPSC).

               
producer 1  +--+
               |
               | events       queue
producer 2  +-----------> +------------+
                   |      |            |
                   |      +------------+
producer 3  +------+      |            |
                          +------------+
                          |            |
                          +------------+ +-----> consumer

let (tx, rx) = mpsc::channel();

// the transmitter (tx) can be cloned indefinitely and the clone can be use
// to send data to the receiver (rx)
let tx2 = tx.clone();

Transmitter can be passed around and use in different threads

let _thread_handle = {
    let tx = tx.clone();
    thread::spawn(move || loop {
        if let Ok(event) = server_rx.recv() {
            let data = process(event);
            tx.send(data).unwrap();
        }
    })
};

To process received events we make heavy use Pattern matching which can destructure the events and access the wrapped object.

loop {
    // events.next() is a blocking operation
    // that consumes the events sent by producers
    match events.next()? {
        Event::UserPublicMessage(message) => {
            process(message);
        }
        Event::UserPrivateMessage(app_id, message) => {
            ...
        }
        Event::Shutdown => {
            ...
        }
    }
}

Each service (app and server) runs in its own thread and communicates by sending messages as mentioned above.

App(rx)                             Server(rx)

+-----------+                       +-----------+
|event loop |                       |event loop |
+-----+-----+                       +-----+-----+
      ^                                   ^
      |                                   |
 +----+--+                         +-------------+
 |       |                         |      |      |
 |       |                         |      |      |
 +       +                         +      +      +
User   Server            Distant input   App   Server

Cross site messages

serde is used to serialize and deserialize Rust object to and from strings, the strings are then sent through the pipes for others to read.

Message are serialized to json in order to be human readable, for a production application, we would use a less verbose format (switching is transparent thanks to serde).

User Interface

The interface is built using tui-rs with a termion backend.

---

Snapshot and message history

Snapshots are built when requested by a user. The app will then generate two files :

• snapshot.json : a complete snapshot of the network made of every app currently running
• history.json : an history of every sent message, in a (roughly) chronolgical order

Topology-agnostic protocol

Each site broadcasts every received message to ensure propagation.