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//! A chat server that broadcasts a message to all connections.
//! This example is explicitly more verbose than it has to be. This is to
//! illustrate more concepts.
//! A chat server for telnet clients. After a telnet client connects, the first
//! line should contain the client's name. After that, all lines sent by a
//! client are broadcasted to all other connected clients.
//! Because the client is telnet, lines are delimited by "\r\n".
//! You can test this out by running:
//! cargo run --example chat
//! And then in another terminal run:
//! telnet localhost 6142
//! You can run the `telnet` command in any number of additional windows.
//! You can run the second command in multiple windows and then chat between the
//! two, seeing the messages from the other client as they're received. For all
//! connected clients they'll all join the same room and see everyone else's
//! messages.
extern crate tokio;
extern crate futures;
extern crate bytes;
use tokio::io;
use tokio::net::{TcpListener, TcpStream};
use tokio::prelude::*;
use futures::sync::mpsc;
use futures::future::{self, Either};
use bytes::{BytesMut, Bytes, BufMut};
use std::collections::HashMap;
use std::net::SocketAddr;
use std::sync::{Arc, Mutex};
/// Shorthand for the transmit half of the message channel.
type Tx = mpsc::UnboundedSender<Bytes>;
/// Shorthand for the receive half of the message channel.
type Rx = mpsc::UnboundedReceiver<Bytes>;
/// Data that is shared between all peers in the chat server.
/// This is the set of `Tx` handles for all connected clients. Whenever a
/// message is received from a client, it is broadcasted to all peers by
/// iterating over the `peers` entries and sending a copy of the message on each
/// `Tx`.
struct Shared {
peers: HashMap<SocketAddr, Tx>,
/// The state for each connected client.
struct Peer {
/// Name of the peer.
/// When a client connects, the first line sent is treated as the client's
/// name (like alice or bob). The name is used to preface all messages that
/// arrive from the client so that we can simulate a real chat server:
/// ```text
/// alice: Hello everyone.
/// bob: Welcome to telnet chat!
/// ```
name: BytesMut,
/// The TCP socket wrapped with the `Lines` codec, defined below.
/// This handles sending and receiving data on the socket. When using
/// `Lines`, we can work at the line level instead of having to manage the
/// raw byte operations.
lines: Lines,
/// Handle to the shared chat state.
/// This is used to broadcast messages read off the socket to all connected
/// peers.
state: Arc<Mutex<Shared>>,
/// Receive half of the message channel.
/// This is used to receive messages from peers. When a message is received
/// off of this `Rx`, it will be written to the socket.
rx: Rx,
/// Client socket address.
/// The socket address is used as the key in the `peers` HashMap. The
/// address is saved so that the `Peer` drop implementation can clean up its
/// entry.
addr: SocketAddr,
/// Line based codec
/// This decorates a socket and presents a line based read / write interface.
/// As a user of `Lines`, we can focus on working at the line level. So, we send
/// and receive values that represent entire lines. The `Lines` codec will
/// handle the encoding and decoding as well as reading from and writing to the
/// socket.
struct Lines {
/// The TCP socket.
socket: TcpStream,
/// Buffer used when reading from the socket. Data is not returned from this
/// buffer until an entire line has been read.
rd: BytesMut,
/// Buffer used to stage data before writing it to the socket.
wr: BytesMut,
impl Shared {
/// Create a new, empty, instance of `Shared`.
fn new() -> Self {
Shared {
peers: HashMap::new(),
impl Peer {
/// Create a new instance of `Peer`.
fn new(name: BytesMut,
state: Arc<Mutex<Shared>>,
lines: Lines) -> Peer
// Get the client socket address
let addr = lines.socket.peer_addr().unwrap();
// Create a channel for this peer
let (tx, rx) = mpsc::unbounded();
// Add an entry for this `Peer` in the shared state map.
.peers.insert(addr, tx);
Peer {
/// This is where a connected client is managed.
/// A `Peer` is also a future representing completely processing the client.
/// When a `Peer` is created, the first line (representing the client's name)
/// has already been read. When the socket closes, the `Peer` future completes.
/// While processing, the peer future implementation will:
/// 1) Receive messages on its message channel and write them to the socket.
/// 2) Receive messages from the socket and broadcast them to all peers.
impl Future for Peer {
type Item = ();
type Error = io::Error;
fn poll(&mut self) -> Poll<(), io::Error> {
// Tokio (and futures) use cooperative scheduling without any
// preemption. If a task never yields execution back to the executor,
// then other tasks may be starved.
// To deal with this, robust applications should not have any unbounded
// loops. In this example, we will read at most `LINES_PER_TICK` lines
// from the client on each tick.
// If the limit is hit, the current task is notified, informing the
// executor to schedule the task again asap.
const LINES_PER_TICK: usize = 10;
// Receive all messages from peers.
for i in 0..LINES_PER_TICK {
// Polling an `UnboundedReceiver` cannot fail, so `unwrap` here is
// safe.
match self.rx.poll().unwrap() {
Async::Ready(Some(v)) => {
// Buffer the line. Once all lines are buffered, they will
// be flushed to the socket (right below).
// If this is the last iteration, the loop will break even
// though there could still be lines to read. Because we did
// not reach `Async::NotReady`, we have to notify ourselves
// in order to tell the executor to schedule the task again.
if i+1 == LINES_PER_TICK {
_ => break,
// Flush the write buffer to the socket
let _ = self.lines.poll_flush()?;
// Read new lines from the socket
while let Async::Ready(line) = self.lines.poll()? {
println!("Received line ({:?}) : {:?}",, line);
if let Some(message) = line {
// Append the peer's name to the front of the line:
let mut line =;
line.extend_from_slice(b": ");
// We're using `Bytes`, which allows zero-copy clones (by
// storing the data in an Arc internally).
// However, before cloning, we must freeze the data. This
// converts it from mutable -> immutable, allowing zero copy
// cloning.
let line = line.freeze();
// Now, send the line to all other peers
for (addr, tx) in &self.state.lock().unwrap().peers {
// Don't send the message to ourselves
if *addr != self.addr {
// The send only fails if the rx half has been dropped,
// however this is impossible as the `tx` half will be
// removed from the map before the `rx` is dropped.
} else {
// EOF was reached. The remote client has disconnected. There is
// nothing more to do.
return Ok(Async::Ready(()));
// As always, it is important to not just return `NotReady` without
// ensuring an inner future also returned `NotReady`.
// We know we got a `NotReady` from either `self.rx` or `self.lines`, so
// the contract is respected.
impl Drop for Peer {
fn drop(&mut self) {
impl Lines {
/// Create a new `Lines` codec backed by the socket
fn new(socket: TcpStream) -> Self {
Lines {
rd: BytesMut::new(),
wr: BytesMut::new(),
/// Buffer a line.
/// This writes the line to an internal buffer. Calls to `poll_flush` will
/// attempt to flush this buffer to the socket.
fn buffer(&mut self, line: &[u8]) {
// Ensure the buffer has capacity. Ideally this would not be unbounded,
// but to keep the example simple, we will not limit this.
// Push the line onto the end of the write buffer.
// The `put` function is from the `BufMut` trait.
/// Flush the write buffer to the socket
fn poll_flush(&mut self) -> Poll<(), io::Error> {
// As long as there is buffered data to write, try to write it.
while !self.wr.is_empty() {
// Try to write some bytes to the socket
let n = try_ready!(self.socket.poll_write(&self.wr));
// As long as the wr is not empty, a successful write should
// never write 0 bytes.
assert!(n > 0);
// This discards the first `n` bytes of the buffer.
let _ = self.wr.split_to(n);
/// Read data from the socket.
/// This only returns `Ready` when the socket has closed.
fn fill_read_buf(&mut self) -> Poll<(), io::Error> {
loop {
// Ensure the read buffer has capacity.
// This might result in an internal allocation.
// Read data into the buffer.
let n = try_ready!(self.socket.read_buf(&mut self.rd));
if n == 0 {
return Ok(Async::Ready(()));
impl Stream for Lines {
type Item = BytesMut;
type Error = io::Error;
fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
// First, read any new data that might have been received off the socket
let sock_closed = self.fill_read_buf()?.is_ready();
// Now, try finding lines
let pos =
.find(|&(_, bytes)| bytes == b"\r\n")
.map(|(i, _)| i);
if let Some(pos) = pos {
// Remove the line from the read buffer and set it to `line`.
let mut line = self.rd.split_to(pos + 2);
// Drop the trailing \r\n
// Return the line
return Ok(Async::Ready(Some(line)));
if sock_closed {
} else {
/// Spawn a task to manage the socket.
/// This will read the first line from the socket to identify the client, then
/// add the client to the set of connected peers in the chat service.
fn process(socket: TcpStream, state: Arc<Mutex<Shared>>) {
// Wrap the socket with the `Lines` codec that we wrote above.
// By doing this, we can operate at the line level instead of doing raw byte
// manipulation.
let lines = Lines::new(socket);
// The first line is treated as the client's name. The client is not added
// to the set of connected peers until this line is received.
// We use the `into_future` combinator to extract the first item from the
// lines stream. `into_future` takes a `Stream` and converts it to a future
// of `(first, rest)` where `rest` is the original stream instance.
let connection = lines.into_future()
// `into_future` doesn't have the right error type, so map the error to
// make it work.
.map_err(|(e, _)| e)
// Process the first received line as the client's name.
.and_then(|(name, lines)| {
// If `name` is `None`, then the client disconnected without
// actually sending a line of data.
// Since the connection is closed, there is no further work that we
// need to do. So, we just terminate processing by returning
// `future::ok()`.
// The problem is that only a single future type can be returned
// from a combinator closure, but we want to return both
// `future::ok()` and `Peer` (below).
// This is a common problem, so the `futures` crate solves this by
// providing the `Either` helper enum that allows creating a single
// return type that covers two concrete future types.
let name = match name {
Some(name) => name,
None => {
// The remote client closed the connection without sending
// any data.
return Either::A(future::ok(()));
println!("`{:?}` is joining the chat", name);
// Create the peer.
// This is also a future that processes the connection, only
// completing when the socket closes.
let peer = Peer::new(
// Wrap `peer` with `Either::B` to make the return type fit.
// Task futures have an error of type `()`, this ensures we handle the
// error. We do this by printing the error to STDOUT.
.map_err(|e| {
println!("connection error = {:?}", e);
// Spawn the task. Internally, this submits the task to a thread pool.
pub fn main() -> Result<(), Box<std::error::Error>> {
// Create the shared state. This is how all the peers communicate.
// The server task will hold a handle to this. For every new client, the
// `state` handle is cloned and passed into the task that processes the
// client connection.
let state = Arc::new(Mutex::new(Shared::new()));
let addr = "".parse()?;
// Bind a TCP listener to the socket address.
// Note that this is the Tokio TcpListener, which is fully async.
let listener = TcpListener::bind(&addr)?;
// The server task asynchronously iterates over and processes each
// incoming connection.
let server = listener.incoming().for_each(move |socket| {
// Spawn a task to process the connection
process(socket, state.clone());
.map_err(|err| {
// All tasks must have an `Error` type of `()`. This forces error
// handling and helps avoid silencing failures.
// In our example, we are only going to log the error to STDOUT.
println!("accept error = {:?}", err);
println!("server running on localhost:6142");
// Start the Tokio runtime.
// The Tokio is a pre-configured "out of the box" runtime for building
// asynchronous applications. It includes both a reactor and a task
// scheduler. This means applications are multithreaded by default.
// This function blocks until the runtime reaches an idle state. Idle is
// defined as all spawned tasks have completed and all I/O resources (TCP
// sockets in our case) have been dropped.
// In our example, we have not defined a shutdown strategy, so this will
// block until `ctrl-c` is pressed at the terminal.