## Using Message Passing to Transfer Data Between Threads

One increasingly popular approach to ensuring safe concurrency is *message
passing*, where threads or actors communicate by sending each other messages
containing data. Here’s the idea in a slogan from [the Go language
documentation](http://golang.org/doc/effective_go.html): “Do not communicate by
sharing memory; instead, share memory by communicating.”

One major tool Rust has for accomplishing message-sending concurrency is the
*channel*, a programming concept that Rust’s standard library provides an
implementation of. You can imagine a channel in programming as being like a
channel of water, such as a stream or a river. If you put something like a
rubber duck or boat into a stream, it will travel downstream to the end of the
waterway.

A channel in programming has two halves: a transmitter and a receiver. The
transmitter half is the upstream location where you put rubber ducks into the
river, and the receiver half is where the rubber duck ends up downstream. One
part of your code calls methods on the transmitter with the data you want to
send, and another part checks the receiving end for arriving messages. A
channel is said to be *closed* if either the transmitter or receiver half is
dropped.

Here, we’ll work up to a program that has one thread to generate values and
send them down a channel, and another thread that will receive the values and
print them out. We’ll be sending simple values between threads using a channel
to illustrate the feature. Once you’re familiar with the technique, you could
use channels to implement a chat system or a system where many threads perform
parts of a calculation and send the parts to one thread that aggregates the
results.

First, in Listing 16-6, we’ll create a channel but not do anything with it.
Note that this won’t compile yet because Rust can’t tell what type of values we
want to send over the channel.

<span class="filename">Filename: src/main.rs</span>

```rust
use std::sync::mpsc;

fn main() {
    let (tx, rx) = mpsc::channel();
#     tx.send(()).unwrap();
}
```

<span class="caption">Listing 16-6: Creating a channel and assigning the two
halves to `tx` and `rx`</span>

We create a new channel using the `mpsc::channel` function; `mpsc` stands for
*multiple producer, single consumer*. In short, the way Rust’s standard library
implements channels means a channel can have multiple *sending* ends that
produce values but only one *receiving* end that consumes those values. Imagine
multiple streams flowing together into one big river: everything sent down any
of the streams will end up in one river at the end. We’ll start with a single
producer for now, but we’ll add multiple producers when we get this example
working.

<!-- NEXT PARAGRAPH WRAPPED WEIRD INTENTIONALLY SEE #199 -->

The `mpsc::channel` function returns a tuple, the first element of which is the
sending end and the second element is the receiving end. The abbreviations `tx`
and `rx` are traditionally used in many fields for *transmitter* and *receiver*
respectively, so we name our variables as such to indicate each end. We’re
using a `let` statement with a pattern that destructures the tuples; we’ll
discuss the use of patterns in `let` statements and destructuring in
Chapter 18. Using a `let` statement this way is a convenient approach to
extract the pieces of the tuple returned by `mpsc::channel`.

Let’s move the transmitting end into a spawned thread and have it send one
string so the spawned thread is communicating with the main thread, as shown in
Listing 16-7. This is like putting a rubber duck in the river upstream or
sending a chat message from one thread to another.

<span class="filename">Filename: src/main.rs</span>

```rust
use std::thread;
use std::sync::mpsc;

fn main() {
    let (tx, rx) = mpsc::channel();

    thread::spawn(move || {
        let val = String::from("hi");
        tx.send(val).unwrap();
    });
}
```

<span class="caption">Listing 16-7: Moving `tx` to a spawned thread and sending
“hi”</span>

Again, we’re using `thread::spawn` to create a new thread and then using `move`
to move `tx` into the closure so the spawned thread owns `tx`. The spawned
thread needs to own the transmitting end of the channel to be able to send
messages through the channel.

The transmitting end has a `send` method that takes the value we want to send.
The `send` method returns a `Result<T, E>` type, so if the receiving end has
already been dropped and there’s nowhere to send a value, the send operation
will return an error. In this example, we’re calling `unwrap` to panic in case
of an error. But in a real application, we would handle it properly: return to
Chapter 9 to review strategies for proper error handling.

In Listing 16-8, we’ll get the value from the receiving end of the channel in
the main thread. This is like retrieving the rubber duck from the water at the
end of the river or like getting a chat message.

<span class="filename">Filename: src/main.rs</span>

```rust
use std::thread;
use std::sync::mpsc;

fn main() {
    let (tx, rx) = mpsc::channel();

    thread::spawn(move || {
        let val = String::from("hi");
        tx.send(val).unwrap();
    });

    let received = rx.recv().unwrap();
    println!("Got: {}", received);
}
```

<span class="caption">Listing 16-8: Receiving the value “hi” in the main thread
and printing it</span>

The receiving end of a channel has two useful methods: `recv` and `try_recv`.
We’re using `recv`, short for *receive*, which will block the main thread’s
execution and wait until a value is sent down the channel. Once a value is
sent, `recv` will return it in a `Result<T, E>`. When the sending end of the
channel closes, `recv` will return an error to signal that no more values will
be coming.

The `try_recv` method doesn’t block, but will instead return a `Result<T, E>`
immediately: an `Ok` value holding a message if one is available and an `Err`
value if there aren’t any messages this time. Using `try_recv` is useful if
this thread has other work to do while waiting for messages: we could write a
loop that calls `try_recv` every so often, handles a message if one is
available, and otherwise does other work for a little while until checking
again.

We’ve used `recv` in this example for simplicity; we don’t have any other work
for the main thread to do other than wait for messages, so blocking the main
thread is appropriate.

When we run the code in Listing 16-8, we’ll see the value printed from the main
thread:

```text
Got: hi
```

Perfect!

### Channels and Ownership Transference

The ownership rules play a vital role in message sending because they help you
write safe, concurrent code. Preventing errors in concurrent programming is the
advantage of thinking about ownership throughout your Rust programs. Let’s do
an experiment to show how channels and ownership work together to prevent
problems: we’ll try to use a `val` value in the spawned thread *after* we’ve
sent it down the channel. Try compiling the code in Listing 16-9 to see why
this code isn’t allowed:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,does_not_compile
use std::thread;
use std::sync::mpsc;

fn main() {
    let (tx, rx) = mpsc::channel();

    thread::spawn(move || {
        let val = String::from("hi");
        tx.send(val).unwrap();
        println!("val is {}", val);
    });

    let received = rx.recv().unwrap();
    println!("Got: {}", received);
}
```

<span class="caption">Listing 16-9: Attempting to use `val` after we’ve sent it
down the channel</span>

Here, we try to print `val` after we’ve sent it down the channel via `tx.send`.
Allowing this would be a bad idea: once the value has been sent to another
thread, that thread could modify or drop it before we try to use the value
again. Potentially, the other thread’s modifications could cause errors or
unexpected results due to inconsistent or nonexistent data. However, Rust gives
us an error if we try to compile the code in Listing 16-9:

```text
error[E0382]: use of moved value: `val`
  --> src/main.rs:10:31
   |
9  |         tx.send(val).unwrap();
   |                 --- value moved here
10 |         println!("val is {}", val);
   |                               ^^^ value used here after move
   |
   = note: move occurs because `val` has type `std::string::String`, which does
not implement the `Copy` trait
```

Our concurrency mistake has caused a compile time error. The `send` function
takes ownership of its parameter, and when the value is moved, the receiver
takes ownership of it. This stops us from accidentally using the value again
after sending it; the ownership system checks that everything is okay.

### Sending Multiple Values and Seeing the Receiver Waiting

The code in Listing 16-8 compiled and ran, but it didn’t clearly show us that
two separate threads were talking to each other over the channel. In Listing
16-10 we’ve made some modifications that will prove the code in Listing 16-8 is
running concurrently: the spawned thread will now send multiple messages and
pause for a second between each message.

<span class="filename">Filename: src/main.rs</span>

```rust
use std::thread;
use std::sync::mpsc;
use std::time::Duration;

fn main() {
    let (tx, rx) = mpsc::channel();

    thread::spawn(move || {
        let vals = vec![
            String::from("hi"),
            String::from("from"),
            String::from("the"),
            String::from("thread"),
        ];

        for val in vals {
            tx.send(val).unwrap();
            thread::sleep(Duration::from_secs(1));
        }
    });

    for received in rx {
        println!("Got: {}", received);
    }
}
```

<span class="caption">Listing 16-10: Sending multiple messages and pausing
between each</span>

This time, the spawned thread has a vector of strings that we want to send to
the main thread. We iterate over them, sending each individually, and pause
between each by calling the `thread::sleep` function with a `Duration` value of
1 second.

In the main thread, we’re not calling the `recv` function explicitly anymore:
instead, we’re treating `rx` as an iterator. For each value received, we’re
printing it. When the channel is closed, iteration will end.

When running the code in Listing 16-10, you should see the following output
with a 1-second pause in between each line:

```text
Got: hi
Got: from
Got: the
Got: thread
```

Because we don’t have any code that pauses or delays in the `for` loop in the
main thread, we can tell that the main thread is waiting to receive values from
the spawned thread.

### Creating Multiple Producers by Cloning the Transmitter

Earlier we mentioned that `mpsc` was an acronym for *multiple producer,
single consumer*. Let’s put `mpsc` to use and expand the code in Listing 16-10
to create multiple threads that all send values to the same receiver. We can do
so by cloning the transmitting half of the channel, as shown in Listing 16-11:

<span class="filename">Filename: src/main.rs</span>

```rust
# use std::thread;
# use std::sync::mpsc;
# use std::time::Duration;
#
# fn main() {
// --snip--

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

let tx1 = mpsc::Sender::clone(&tx);
thread::spawn(move || {
    let vals = vec![
        String::from("hi"),
        String::from("from"),
        String::from("the"),
        String::from("thread"),
    ];

    for val in vals {
        tx1.send(val).unwrap();
        thread::sleep(Duration::from_secs(1));
    }
});

thread::spawn(move || {
    let vals = vec![
        String::from("more"),
        String::from("messages"),
        String::from("for"),
        String::from("you"),
    ];

    for val in vals {
        tx.send(val).unwrap();
        thread::sleep(Duration::from_secs(1));
    }
});

for received in rx {
    println!("Got: {}", received);
}

// --snip--
# }
```

<span class="caption">Listing 16-11: Sending multiple messages from multiple
producers</span>

This time, before we create the first spawned thread, we call `clone` on the
sending end of the channel. This will give us a new sending handle we can pass
to the first spawned thread. We pass the original sending end of the channel to
a second spawned thread. This gives us two threads, each sending different
messages to the receiving end of the channel.

When you run the code, your output should look something like this:

```text
Got: hi
Got: more
Got: from
Got: messages
Got: for
Got: the
Got: thread
Got: you
```

You might see the values in another order; it depends on your system. This is
what makes concurrency interesting as well as difficult. If you experiment with
`thread::sleep`, giving it various values in the different threads, each run
will be more nondeterministic and create different output each time.

Now that we’ve looked at how channels work, let’s look at a different method of
concurrency.
