Trait tower_async_service::Service
source · pub trait Service<Request> {
type Response;
type Error;
// Required method
fn call(
&self,
req: Request
) -> impl Future<Output = Result<Self::Response, Self::Error>>;
}Expand description
An asynchronous function from a Request to a Response.
The Service trait is a simplified interface making it easy to write
network applications in a modular and reusable way, decoupled from the
underlying protocol. It is one of Tower’s fundamental abstractions.
Functional
A Service is a function of a Request. It immediately returns a
Future representing the eventual completion of processing the
request. The actual request processing may happen at any time in the
future, on any thread or executor. The processing may depend on calling
other services. At some point in the future, the processing will complete,
and the Future will resolve to a response or error.
At a high level, the Service::call function represents an RPC request. The
Service value can be a server or a client.
Server
An RPC server implements the Service trait. Requests received by the
server over the network are deserialized and then passed as an argument to the
server value. The returned response is sent back over the network.
As an example, here is how an HTTP request is processed by a server:
use http::{Request, Response, StatusCode};
struct HelloWorld;
impl Service<Request<Vec<u8>>> for HelloWorld {
type Response = Response<Vec<u8>>;
type Error = http::Error;
async fn call(&self, req: Request<Vec<u8>>) -> Result<Self::Response, Self::Error> {
// create the body
let body: Vec<u8> = "hello, world!\n"
.as_bytes()
.to_owned();
// Create the HTTP response
let resp = Response::builder()
.status(StatusCode::OK)
.body(body)
.expect("Unable to create `http::Response`");
// Return the response
Ok(resp)
}
}Client
A client consumes a service by using a Service value. The client may
issue requests by invoking call and passing the request as an argument.
It then receives the response by waiting for the returned future.
As an example, here is how a Redis request would be issued:
let client = redis::Client::new()
.connect("127.0.0.1:6379".parse().unwrap())
.unwrap();
let resp = client.call(Cmd::set("foo", "this is the value of foo")).await?;
// Wait for the future to resolve
println!("Redis response: {:?}", resp);Middleware / Layer
More often than not, all the pieces needed for writing robust, scalable network applications are the same no matter the underlying protocol. By unifying the API for both clients and servers in a protocol agnostic way, it is possible to write middleware that provide these pieces in a reusable way.
Take timeouts as an example:
use tower_async_service::Service;
use tower_async_layer::Layer;
use futures::FutureExt;
use std::future::Future;
use std::task::{Context, Poll};
use std::time::Duration;
use std::pin::Pin;
use std::fmt;
use std::error::Error;
// Our timeout service, which wraps another service and
// adds a timeout to its response future.
pub struct Timeout<T> {
inner: T,
timeout: Duration,
}
impl<T> Timeout<T> {
pub fn new(inner: T, timeout: Duration) -> Timeout<T> {
Timeout {
inner,
timeout
}
}
}
// The error returned if processing a request timed out
#[derive(Debug)]
pub struct Expired;
impl fmt::Display for Expired {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "expired")
}
}
impl Error for Expired {}
// We can implement `Service` for `Timeout<T>` if `T` is a `Service`
impl<T, Request> Service<Request> for Timeout<T>
where
T: Service<Request>,
T::Error: Into<Box<dyn Error + Send + Sync>>,
T::Response: 'static,
{
// `Timeout` doesn't modify the response type, so we use `T`'s response type
type Response = T::Response;
// Errors may be either `Expired` if the timeout expired, or the inner service's
// `Error` type. Therefore, we return a boxed `dyn Error + Send + Sync` trait object to erase
// the error's type.
type Error = Box<dyn Error + Send + Sync>;
async fn call(&self, req: Request) -> Result<Self::Response, Self::Error> {
tokio::select! {
res = self.inner.call(req) => {
res.map_err(|err| err.into())
},
_ = tokio::time::sleep(self.timeout) => {
Err(Box::new(Expired) as Box<dyn Error + Send + Sync>)
},
}
}
}
// A layer for wrapping services in `Timeout`
pub struct TimeoutLayer(Duration);
impl TimeoutLayer {
pub fn new(delay: Duration) -> Self {
TimeoutLayer(delay)
}
}
impl<S> Layer<S> for TimeoutLayer {
type Service = Timeout<S>;
fn layer(&self, service: S) -> Timeout<S> {
Timeout::new(service, self.0)
}
}The above timeout implementation is decoupled from the underlying protocol and is also decoupled from client or server concerns. In other words, the same timeout middleware could be used in either a client or a server.
Backpressure
There is no explicit support for Backpressure in the service. This can be achieved by having middleware at the front which implements backpressure and propagates it through the service
Or one can also implement it in the location where this service gets created.
The original tower library had a poll_ready method which
was used to implement backpressure. This was removed in this fork in
favor of the above approach.