Example: An Echo Server

We’re going to use what has been covered so far to build an echo server. This is a Tokio application that incorporates everything we’ve learned so far. The server will simply receive messages from the connected client and send back the same message it received to the client.

We’ll be able to test this echo server using the basic Tcp client we created in the hello world section.

The full code can be found here.


First, generate a new crate.

$ cargo new --bin echo-server
cd echo-server

Next, add the necessary dependencies:

tokio = { version = "0.2", features = ["full"] }
futures = "0.3"

Note that Tokio is split into many features. In this case we simply enable all features for simplicity, but you can speed up compilation by only enabling the features you use. Next, add the crates and types into scope in main.rs:

# #![allow(unused_imports)]
use tokio::net::TcpListener;
use tokio::prelude::*;
use futures::stream::StreamExt;

# fn main() {}

Now, we setup the necessary structure for a server:

  • Bind a TcpListener to a local port.
  • Define a task that accepts inbound connections and processes them.
  • Start running the task using await.

Again, no work actually happens when creating the server variable. You have to actually await it or otherwise spawn it on the executor using e.g. tokio::spawn.

# #![deny(deprecated)]
# use tokio::net::TcpListener;
# use futures::stream::StreamExt;
async fn main() {
    let addr = "";
    let mut listener = TcpListener::bind(addr).await.unwrap();

    // Here we convert the `TcpListener` to a stream of incoming connections
    // with the `incoming` method.
    let server = async move {
        let mut incoming = listener.incoming();
        while let Some(socket_res) = incoming.next().await {
            match socket_res {
                Ok(socket) => {
                    println!("Accepted connection from {:?}", socket.peer_addr());
                    // TODO: Process socket
                Err(err) => {
                    // Handle error by printing to STDOUT.
                    println!("accept error = {:?}", err);

    println!("Server running on localhost:6142");
#    // `select` completes when the first of the two futures completes. Since
#    // future::ready() completes immediately, the server won't hang waiting for
#    // more connections. This is just so the doc test doesn't hang.
#    use futures::future;
#    let server = future::select(Box::pin(server), future::ready(Ok::<_, ()>(())));

    // Start the server and block this async fn until `server` spins down.

Here we’ve created a TcpListener that can listen for incoming TCP connections. On the listener we call incoming which turns the listener into a Stream of inbound client connections. We then call the StreamExt::next() trait method and await on it to get new inbound client connection. For now we’re not doing anything with this inbound connection - that’s our next step.

Once we have our server, we .await on it. Up until this point our server future has done nothing. It’s up to the Tokio runtime to drive our future to completion.

Handling the connections

Now that we have incoming client connections, we should handle them.

We just want to copy all data read from the socket back onto the socket itself (e.g. “echo”). We can use the standard io::copy function to do precisely this.

The copy function takes two arguments: where to read from and where to write to. We only have one argument, though, with socket. Luckily there’s a method, split, which will split a readable and writeable stream into its two halves. This operation allows us to work with each stream independently, such as pass them as two arguments to the copy function.

The copy method then returns a future that represents the task of copying the data. When the data has been transferred, the future will complete to the number of bytes that were copied.

Let’s take a look at the connection accept code again.

# #![deny(warnings)]
# use std::env;
# use futures::prelude::*;
# use tokio::net::TcpListener;
# #[tokio::main]
# async fn main() {
# let addr = env::args().nth(1).unwrap_or("".to_string());
# // Bind the server's socket.
# let mut listener = TcpListener::bind(&addr)
#     .await
#     .expect("unable to bind TCP listener");
let server = {
  async move {
    let mut incoming = listener.incoming();
    while let Some(conn) = incoming.next().await {
      match conn {
        Err(e) => eprintln!("accept failed = {:?}", e),
        Ok(mut sock) => {
          // Spawn the future that echos the data and returns how
          // many bytes were copied as a concurrent task.
          tokio::spawn(async move {
            // Split up the reading and writing parts of the
            // socket.
            let (mut reader, mut writer) = sock.split();

            match tokio::io::copy(&mut reader, &mut writer).await {
              Ok(amt) => {
                println!("wrote {} bytes", amt);
              Err(err) => {
                eprintln!("IO error {:?}", err);
# let server = future::select(Box::pin(server), future::ready(Ok::<_, ()>(())));
# server.await;
# }

As you can see we’ve split the socket stream into readable and writeable parts. We then used io::copy to read from reader and write into writer. We await on the result and inspect it printing some diagnostics.

The call to tokio::spawn is the key here. We crucially want all clients to make progress concurrently, rather than blocking one on completion of another. To achieve this we use the tokio::spawn function to execute the work in the background.

If we did not do this then each invocation of the block in the loop would be resolved at a time meaning we could never have two client connections processed concurrently!

The full code can be found here.

Next up: async fn