Add Runtimes & Tasks

src/SUMMARY.md

@@ -232,7 +232,9 @@
 - [Async](async.md)
   - [async/await](async/async-await.md)
   - [Async Blocks](async/async-blocks.md)
-  - [Futures](async/futures.md)
+- [Futures](async/futures.md)
+- [Runtimes](async/runtimes.md)
+- [Tasks](async/tasks.md)
 - [Exercises](exercises/day-4/async.md)
 
 # Final Words

src/async/futures.md

@@ -47,9 +47,6 @@ own types. For example, the `JoinHandle` returned from `tokio::spawn` implements
 The `.await` keyword, applied to a Future, causes the current async function or
 block to pause until that Future is ready, and then evaluates to its output.
 
-An important difference from other languages is that a Future is inert: it does
-not do anything until it is polled.
-
 <details>
 
 * Run the example and look at the error message. `_: () = ..` is a common

src/async/runtimes.md

@@ -0,0 +1,29 @@
+# Runtimes and Tasks
+
+A *runtime* provides support for performing operations asynchronously (a
+*reactor*) and is responsible for executing futures (an *executor*). Rust does not have a
+"built-in" runtime, but several options are available:
+
+ * [Tokio](https://tokio.rs/) - performant, with a well-developed ecosystem of
+   functionality like [Hyper](https://hyper.rs/) for HTTP or
+   [Tonic](https://github.com/hyperium/tonic) for gRPC.
+ * [async-std](https://async.rs/) - aims to be a "std for async", and includes a
+   basic runtime in `async::task`.
+ * [smol](https://docs.rs/smol/latest/smol/) - simple and lightweight
+
+Several larger applications have their own runtimes. For example,
+[Fuchsia](https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/src/lib/fuchsia-async/src/lib.rs)
+already has one.
+
+<details>
+
+* Note that of the listed runtimes, only Tokio is supported in the Rust
+  playground. The playground also does not permit any I/O, so most interesting
+  async things can't run in the playground.
+
+* Futures are "inert" in that they do not do anything (not even start an I/O
+  operation) unless there is an executor polling them. This differs from JS
+  Promises, for example, which will run to completion even if they are never
+  used.
+
+</details>

src/async/tasks.md

@@ -0,0 +1,60 @@
+# Tasks
+
+Runtimes have the concept of a "Task", similar to a thread but much
+less resource-intensive.
+
+A Task has a single top-level Future which the executor polls to make progress.
+That future may have one or more nested futures that its `poll` method polls,
+corresponding loosely to a call stack. Concurrency is possible within a task by
+polling multiple child futures, such as racing a timer and an I/O operation.
+
+```rust,editable,compile_fail
+use tokio::io::{self, AsyncReadExt, AsyncWriteExt};
+use tokio::net::TcpListener;
+
+#[tokio::main]
+async fn main() -> io::Result<()> {
+    let listener = TcpListener::bind("127.0.0.1:6142").await?;
+	println!("listening on port 6142");
+
+    loop {
+        let (mut socket, addr) = listener.accept().await?;
+
+        println!("connection from {addr:?}");
+
+        tokio::spawn(async move {
+            if let Err(e) = socket.write_all(b"Who are you?\n").await {
+                println!("socket error: {e:?}");
+                return;
+            }
+
+            let mut buf = vec![0; 1024];
+            let reply = match socket.read(&mut buf).await {
+                Ok(n) => {
+                    let name = std::str::from_utf8(&buf[..n]).unwrap().trim();
+                    format!("Thanks for dialing in, {name}!\n")
+                }
+                Err(e) => {
+                    println!("socket error: {e:?}");
+                    return;
+                }
+            };
+
+            if let Err(e) = socket.write_all(reply.as_bytes()).await {
+                println!("socket error: {e:?}");
+            }
+        });
+    }
+}
+```
+
+<details>
+
+Copy this example into your prepared `src/main.rs` and run it from there.
+
+* Ask students to visualize what the state of the example server would be with a
+  few connected clients. What tasks exist? What are their Futures?
+
+* Refactor the async block into a function, and improve the error handling using `?`.
+
+</details>

src/running-the-course/day-4.md

@@ -23,5 +23,18 @@ Ensure that `adb sync` works with your emulator or real device and pre-build
 all Android examples using `src/android/build_all.sh`. Read the script to see
 the commands it runs and make sure they work when you run them by hand.
 
+## Async
+
+If you chose Async for Day 4 afternoon, you will need a fresh crate set up and
+the dependencies downloaded and ready to go. You can then copy/paste the
+examples into `src/main.rs` to experiment with them.
+
+```shell
+cargo init day4
+cd day4
+cargo add tokio --features full
+cargo run
+```
+
 [1]: https://source.android.com/docs/setup/download/downloading
 [2]: https://github.com/google/comprehensive-rust