Touch up comments on echo, add connect example

2 files changed, 209 insertions, 24 deletions

diff --git a/examples/connect.rs b/examples/connect.rs
new file mode 100644
index 00000000..eeaba217
--- /dev/null
+++ b/examples/connect.rs
@@ -0,0 +1,119 @@
+//! A simple example of hooking up stdin/stdout to a TCP stream.
+//!
+//! This example will connect to a server specified in the argument list and
+//! then forward all data read on stdin to the server, printing out all data
+//! received on stdout.
+//!
+//! Note that this is not currently optimized for performance, especially around
+//! buffer management. Rather it's intended to show an example of working with a
+//! client.
+
+extern crate futures;
+extern crate tokio_core;
+
+use std::env;
+use std::io::{self, Read, Write};
+use std::net::SocketAddr;
+use std::thread;
+
+use futures::{Sink, Future, Stream};
+use futures::sync::mpsc;
+use tokio_core::reactor::Core;
+use tokio_core::io::{Io, EasyBuf, Codec};
+use tokio_core::net::TcpStream;
+
+fn main() {
+    // Parse what address we're going to connect to
+    let addr = env::args().nth(1).unwrap_or_else(|| {
+        panic!("this program requires at least one argument")
+    });
+    let addr = addr.parse::<SocketAddr>().unwrap();
+
+    // Create the event loop and initiate the connection to the remote server
+    let mut core = Core::new().unwrap();
+    let handle = core.handle();
+    let tcp = TcpStream::connect(&addr, &handle);
+
+    // Right now Tokio doesn't support a handle to stdin running on the event
+    // loop, so we farm out that work to a separate thread. This thread will
+    // read data from stdin and then send it to the event loop over a standard
+    // futures channel.
+    let (stdin_tx, stdin_rx) = mpsc::channel(0);
+    thread::spawn(|| read_stdin(stdin_tx));
+    let stdin_rx = stdin_rx.map_err(|_| panic!()); // errors not possible on rx
+
+    // After the TCP connection has been established, we set up our client to
+    // start forwarding data.
+    //
+    // First we use the `Io::framed` method with a simple implementation of a
+    // `Codec` (listed below) that just ships bytes around. We then split that
+    // in two to work with the stream and sink separately.
+    //
+    // Half of the work we're going to do is to take all data we receive on
+    // stdin (`stdin_rx`) and send that along the TCP stream (`sink`). The
+    // second half is to take all the data we receive (`stream`) and then write
+    // that to stdout. Currently we just write to stdout in a synchronous
+    // fashion.
+    //
+    // Finally we set the client to terminate once either half of this work
+    // finishes. If we don't have any more data to read or we won't receive any
+    // more work from the remote then we can exit.
+    let mut stdout = io::stdout();
+    let client = tcp.and_then(|(sink, stream)| {
+        let (sink, stream) = stream.framed(Bytes).split();
+        let send_stdin = stdin_rx.forward(sink);
+        let write_stdout = stream.for_each(move |buf| {
+            stdout.write_all(buf.as_slice())
+        });
+
+        send_stdin.map(|_| ())
+                  .select(write_stdout.map(|_| ()))
+                  .then(|_| Ok(()))
+    });
+
+    // And now that we've got our client, we execute it in the event loop!
+    core.run(client).unwrap();
+}
+
+/// A simple `Codec` implementation that just ships bytes around.
+///
+/// This type is used for "framing" a TCP stream of bytes but it's really just a
+/// convenient method for us to work with streams/sinks for now. This'll just
+/// take any data read and interpret it as a "frame" and conversely just shove
+/// data into the output location without looking at it.
+struct Bytes;
+
+impl Codec for Bytes {
+    type In = EasyBuf;
+    type Out = Vec<u8>;
+
+    fn decode(&mut self, buf: &mut EasyBuf) -> io::Result<Option<EasyBuf>> {
+        if buf.len() > 0 {
+            let len = buf.len();
+            Ok(Some(buf.drain_to(len)))
+        } else {
+            Ok(None)
+        }
+    }
+
+    fn encode(&mut self, data: Vec<u8>, buf: &mut Vec<u8>) -> io::Result<()> {
+        buf.extend(data);
+        Ok(())
+    }
+}
+
+// Our helper method which will read data from stdin and send it along the
+// sender provided.
+fn read_stdin(mut rx: mpsc::Sender<Vec<u8>>) {
+    let mut stdin = io::stdin();
+    loop {
+        let mut buf = vec![0; 1024];
+        let n = match stdin.read(&mut buf) {
+            Err(_) |
+            Ok(0) => break,
+            Ok(n) => n,
+        };
+        buf.truncate(n);
+        rx = rx.send(buf).wait().unwrap();
+    }
+}
diff --git a/examples/echo.rs b/examples/echo.rs
index eb7aecd9..97e2a927 100644
--- a/examples/echo.rs
+++ b/examples/echo.rs
@@ -1,16 +1,22 @@
-//! An echo server that just writes back everything that's written to it.
+//! An "hello world" echo server with tokio-core
 //!
-//! If you're on unix you can test this out by in one terminal executing:
+//! This server will create a TCP listener, accept connections in a loop, and
+//! simply write back everything that's read off of each TCP connection. Each
+//! TCP connection is processed concurrently with all other TCP connections, and
+//! each connection will have its own buffer that it's reading in/out of.
+//!
+//! To see this server in action, you can run this in one terminal:
 //!
 //!     cargo run --example echo
 //!
 //! and in another terminal you can run:
 //!
-//!     nc -4 localhost 8080
+//!     cargo run --example connect 127.0.0.1:8080
 //!
-//! Each line you type in to the `nc` terminal should be echo'd back to you!
+//! Each line you type in to the `connect` terminal should be echo'd back to
+//! you! If you open up multiple terminals running the `connect` example you
+//! should be able to see them all make progress simultaneously.
 
-extern crate env_logger;
 extern crate futures;
 extern crate tokio_core;
 
@@ -24,40 +30,100 @@ use tokio_core::net::TcpListener;
 use tokio_core::reactor::Core;
 
 fn main() {
-    env_logger::init().unwrap();
+    // Allow passing an address to listen on as the first argument of this
+    // program, but otherwise we'll just set up our TCP listener on
+    // 127.0.0.1:8080 for connections.
     let addr = env::args().nth(1).unwrap_or("127.0.0.1:8080".to_string());
     let addr = addr.parse::<SocketAddr>().unwrap();
 
-    // Create the event loop that will drive this server
-    let mut l = Core::new().unwrap();
-    let handle = l.handle();
+    // First up we'll create the event loop that's going to drive this server.
+    // This is done by creating an instance of the `Core` type, tokio-core's
+    // event loop. Most functions in tokio-core return an `io::Result`, and
+    // `Core::new` is no exception. For this example, though, we're mostly just
+    // ignoring errors, so we unwrap the return value.
+    //
+    // After the event loop is created we acquire a handle to it through the
+    // `handle` method. With this handle we'll then later be able to create I/O
+    // objects and spawn futures.
+    let mut core = Core::new().unwrap();
+    let handle = core.handle();
 
-    // Create a TCP listener which will listen for incoming connections
+    // Next up we create a TCP listener which will listen for incoming
+    // connections. This TCP listener is bound to the address we determined
+    // above and must be associated with an event loop, so we pass in a handle
+    // to our event loop. After the socket's created we inform that we're ready
+    // to go and start accepting connections.
     let socket = TcpListener::bind(&addr, &handle).unwrap();
-
-    // Once we've got the TCP listener, inform that we have it
     println!("Listening on: {}", addr);
 
-    // Pull out the stream of incoming connections and then for each new
-    // one spin up a new task copying data.
+    // Here we convert the `TcpListener` to a stream of incoming connections
+    // with the `incoming` method. We then define how to process each element in
+    // the stream with the `for_each` method.
     //
-    // We use the `io::copy` future to copy all data from the
-    // reading half onto the writing half.
+    // This combinator, defined on the `Stream` trait, will allow us to define a
+    // computation to happen for all items on the stream (in this case TCP
+    // connections made to the server).  The return value of the `for_each`
+    // method is itself a future representing processing the entire stream of
+    // connections, and ends up being our server.
     let done = socket.incoming().for_each(move |(socket, addr)| {
+
+        // Once we're inside this closure this represents an accepted client
+        // from our server. The `socket` is the client connection and `addr` is
+        // the remote address of the client (similar to how the standard library
+        // operates).
+        //
+        // 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`
+        // combinator in the `tokio-core` crate 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, `Io::split`, which will split an Read/Write
+        // 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` function then returns a future, and this future will be
+        // resolved when the copying operation is complete, resolving to the
+        // amount of data that was copied.
         let (reader, writer) = socket.split();
         let amt = copy(reader, writer);
 
-        // Once all that is done we print out how much we wrote, and then
-        // critically we *spawn* this future which allows it to run
-        // concurrently with other connections.
-        let msg = amt.map(move |amt| {
-            println!("wrote {} bytes to {}", amt, addr)
-        }).map_err(|e| {
-            panic!("error: {}", e);
+        // After our copy operation is complete we just print out some helpful
+        // information.
+        let msg = amt.then(move |result| {
+            match result {
+                Ok(amt) => println!("wrote {} bytes to {}", amt, addr),
+                Err(e) => println!("error on {}: {}", addr, e),
+            }
+
+            Ok(())
         });
+
+        // And this is where much of the magic of this server happens. We
+        // crucially want all clients to make progress concurrently, rather than
+        // blocking one on completion of another. To achieve this was use the
+        // `spawn` function on `Handle` to essentially execute some work in the
+        // background.
+        //
+        // This function will transfer ownership of the future (`msg` in this
+        // case) to the event loop that `handle` points to. The event loop will
+        // then drive the future to completion.
+        //
+        // Essentially here we're spawning a new task to run concurrently, which
+        // will allow all of our clients to be processed concurrently.
         handle.spawn(msg);
 
         Ok(())
     });
-    l.run(done).unwrap();
+
+    // And finally now that we've define what our server is, we run it! We
+    // didn't actually do much I/O up to this point and this `Core::run` method
+    // is responsible for driving the entire server to completion.
+    //
+    // The `run` method will return the result of the future that it's running,
+    // but in our case the `done` future won't ever finish because a TCP
+    // listener is never done accepting clients. That basically just means that
+    // we're going to be running the server until it's killed (e.g. ctrl-c).
+    core.run(done).unwrap();
 }