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: “Do not communicate by sharing memory; instead, share memory by communicating.”
To accomplish message-sending concurrency, Rust's standard library provides an implementation of channels. A channel is a general programming concept by which data is sent from one thread to another.
You can imagine a channel in programming as being like a directional channel of water, such as a stream or a river. If you put something like a rubber duck into a river, it will travel downstream to the end of the waterway.
A channel 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 for any threads that needs to communicate between each other, such as 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.
Filename: src/main.rs
use std::sync::mpsc;
fn main() {
let (tx, rx) = mpsc::channel();
}
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.
The mpsc::channel
function returns a tuple, the first element of which is the
sending end--the transmitter--and the second element is the receiving end--the
receiver. 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. For now, know that 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.
Filename: src/main.rs
use std::sync::mpsc; use std::thread; fn main() { let (tx, rx) = mpsc::channel(); thread::spawn(move || { let val = String::from("hi"); tx.send(val).unwrap(); }); }
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 transmitter to be able to send messages through the
channel. The transmitter has a send
method that takes the value we want to
send. The send
method returns a Result<T, E>
type, so if the receiver 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 receiver in the main thread. This is like retrieving the rubber duck from the water at the end of the river or receiving a chat message.
Filename: src/main.rs
use std::sync::mpsc; use std::thread; 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); }
The receiver 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 transmitter 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:
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:
Filename: src/main.rs
use std::sync::mpsc;
use std::thread;
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);
}
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:
$ cargo run
Compiling message-passing v0.1.0 (file:///projects/message-passing)
error[E0382]: borrow of moved value: `val`
--> src/main.rs:10:31
|
8 | let val = String::from("hi");
| --- move occurs because `val` has type `String`, which does not implement the `Copy` trait
9 | tx.send(val).unwrap();
| --- value moved here
10 | println!("val is {}", val);
| ^^^ value borrowed here after move
|
= note: this error originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
For more information about this error, try `rustc --explain E0382`.
error: could not compile `message-passing` due to previous error
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.
Filename: src/main.rs
use std::sync::mpsc;
use std::thread;
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);
}
}
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:
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 transmitter, as shown in Listing 16-11:
Filename: src/main.rs
use std::sync::mpsc;
use std::thread;
use std::time::Duration;
fn main() {
// --snip--
let (tx, rx) = mpsc::channel();
let tx1 = tx.clone();
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--
}
This time, before we create the first spawned thread, we call clone
on the
transmitter. This will give us a new transmitter we can pass to the first
spawned thread. We pass the original transmitter to a second spawned thread.
This gives us two threads, each sending different messages to the one receiver.
When you run the code, your output should look something like this:
Got: hi
Got: more
Got: from
Got: messages
Got: for
Got: the
Got: thread
Got: you
You might see the values in another order, depending 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.