Crate async_compat[−][src]
Compatibility adapter between tokio and futures.
There are two kinds of compatibility issues between tokio and futures:
- Tokio’s types cannot be used outside tokio context, so any attempt to use
them will panic.
- Solution: If you apply the
Compat
adapter to a future, the future will enter the context of a global single-threaded tokio runtime started by this crate. That does not mean the future runs on the tokio runtime - it only means the future sets a thread-local variable pointing to the global tokio runtime so that tokio’s types can be used inside it.
- Solution: If you apply the
- Tokio and futures have similar but different I/O traits
AsyncRead
,AsyncWrite
,AsyncBufRead
, andAsyncSeek
.- Solution: When the
Compat
adapter is applied to an I/O type, it will implement traits of the opposite kind. That’s how you can use tokio-based types wherever futures-based types are expected, and the other way around.
- Solution: When the
You can apply the Compat
adapter using the Compat::new()
constructor or using any
method from the CompatExt
trait.
Examples
This program reads lines from stdin and echoes them into stdout, except it’s not going to work:
fn main() -> std::io::Result<()> { futures::executor::block_on(async { let stdin = tokio::io::stdin(); let mut stdout = tokio::io::stdout(); // The following line will not work for two reasons: // 1. Runtime error because stdin and stdout are used outside tokio context. // 2. Compilation error due to mismatched `AsyncRead` and `AsyncWrite` traits. futures::io::copy(stdin, &mut stdout).await?; Ok(()) }) }
To get around the compatibility issues, apply the Compat
adapter to stdin
, stdout
, and
futures::io::copy()
:
use async_compat::CompatExt; fn main() -> std::io::Result<()> { futures::executor::block_on(async { let stdin = tokio::io::stdin(); let mut stdout = tokio::io::stdout(); futures::io::copy(stdin.compat(), &mut stdout.compat_mut()).compat().await?; Ok(()) }) }
It is also possible to apply Compat
to the outer future passed to
futures::executor::block_on()
rather than futures::io::copy()
itself.
When applied to the outer future, individual inner futures don’t need the adapter because
they’re all now inside tokio context:
use async_compat::{Compat, CompatExt}; fn main() -> std::io::Result<()> { futures::executor::block_on(Compat::new(async { let stdin = tokio::io::stdin(); let mut stdout = tokio::io::stdout(); futures::io::copy(stdin.compat(), &mut stdout.compat_mut()).await?; Ok(()) })) }
The compatibility adapter converts between tokio-based and futures-based I/O types in any direction. Here’s how we can write the same program by using futures-based I/O types inside tokio:
use async_compat::CompatExt; use blocking::Unblock; #[tokio::main] async fn main() -> std::io::Result<()> { let mut stdin = Unblock::new(std::io::stdin()); let mut stdout = Unblock::new(std::io::stdout()); tokio::io::copy(&mut stdin.compat_mut(), &mut stdout.compat_mut()).await?; Ok(()) }
Finally, we can use any tokio-based crate from any other async runtime. Here are reqwest and warp as an example:
use async_compat::{Compat, CompatExt}; use warp::Filter; fn main() { futures::executor::block_on(Compat::new(async { // Make an HTTP GET request. let response = reqwest::get("https://www.rust-lang.org").await.unwrap(); println!("{}", response.text().await.unwrap()); // Start an HTTP server. let routes = warp::any().map(|| "Hello from warp!"); warp::serve(routes).run(([127, 0, 0, 1], 8080)).await; })) }
Structs
Compat | Compatibility adapter for futures and I/O types. |
Traits
CompatExt | Applies the |