Struct tower::ServiceBuilder

pub struct ServiceBuilder<L> { /* private fields */ }

Declaratively construct Service values.

ServiceBuilder provides a builder-like interface for composing layers to be applied to a Service.

Service

A Service is a trait representing an asynchronous function of a request to a response. It is similar to async fn(Request) -> Result<Response, Error>.

A Service is typically bound to a single transport, such as a TCP connection. It defines how all inbound or outbound requests are handled by that connection.

Order

The order in which layers are added impacts how requests are handled. Layers that are added first will be called with the request first. The argument to service will be last to see the request.

ServiceBuilder::new()
    .buffer(100)
    .concurrency_limit(10)
    .service(svc)

In the above example, the buffer layer receives the request first followed by concurrency_limit. buffer enables up to 100 request to be in-flight on top of the requests that have already been forwarded to the next layer. Combined with concurrency_limit, this allows up to 110 requests to be in-flight.

ServiceBuilder::new()
    .concurrency_limit(10)
    .buffer(100)
    .service(svc)

The above example is similar, but the order of layers is reversed. Now, concurrency_limit applies first and only allows 10 requests to be in-flight total.

Examples

A Service stack with a single layer:

ServiceBuilder::new()
    .concurrency_limit(5)
    .service(svc);

A Service stack with multiple layers that contain rate limiting, in-flight request limits, and a channel-backed, clonable Service:

ServiceBuilder::new()
    .buffer(5)
    .concurrency_limit(5)
    .rate_limit(5, Duration::from_secs(1))
    .service(svc);

Implementations

impl ServiceBuilder<Identity>

pub const fn new() -> Self

Create a new ServiceBuilder.

impl<L> ServiceBuilder<L>

pub fn layer<T>(self, layer: T) -> ServiceBuilder<Stack<T, L>>

Add a new layer T into the ServiceBuilder.

This wraps the inner service with the service provided by a user-defined Layer. The provided layer must implement the Layer trait.

pub fn option_layer<T>( self, layer: Option<T>, ) -> ServiceBuilder<Stack<Either<T, Identity>, L>>

Optionally add a new layer T into the ServiceBuilder.

// Apply a timeout if configured
ServiceBuilder::new()
    .option_layer(timeout.map(TimeoutLayer::new))
    .service(svc)

pub fn layer_fn<F>(self, f: F) -> ServiceBuilder<Stack<LayerFn<F>, L>>

Add a Layer built from a function that accepts a service and returns another service.

See the documentation for layer_fn for more details.

pub fn map_request<F, R1, R2>( self, f: F, ) -> ServiceBuilder<Stack<MapRequestLayer<F>, L>>

where F: FnMut(R1) -> R2 + Clone,

Map one request type to another.

This wraps the inner service with an instance of the MapRequest middleware.

Examples

Changing the type of a request:

use tower::ServiceBuilder;
use tower::ServiceExt;

// Suppose we have some `Service` whose request type is `String`:
let string_svc = tower::service_fn(|request: String| async move {
    println!("request: {}", request);
    Ok(())
});

// ...but we want to call that service with a `usize`. What do we do?

let usize_svc = ServiceBuilder::new()
     // Add a middleware that converts the request type to a `String`:
    .map_request(|request: usize| format!("{}", request))
    // ...and wrap the string service with that middleware:
    .service(string_svc);

// Now, we can call that service with a `usize`:
usize_svc.oneshot(42).await?;

Modifying the request value:

use tower::ServiceBuilder;
use tower::ServiceExt;

// A service that takes a number and returns it:
let svc = tower::service_fn(|request: usize| async move {
   Ok(request)
});

let svc = ServiceBuilder::new()
     // Add a middleware that adds 1 to each request
    .map_request(|request: usize| request + 1)
    .service(svc);

let response = svc.oneshot(1).await?;
assert_eq!(response, 2);

pub fn map_response<F>( self, f: F, ) -> ServiceBuilder<Stack<MapResponseLayer<F>, L>>

Map one response type to another.

This wraps the inner service with an instance of the MapResponse middleware.

See the documentation for the map_response combinator for details.

pub fn map_err<F>(self, f: F) -> ServiceBuilder<Stack<MapErrLayer<F>, L>>

Map one error type to another.

This wraps the inner service with an instance of the MapErr middleware.

See the documentation for the map_err combinator for details.

pub fn map_future<F>(self, f: F) -> ServiceBuilder<Stack<MapFutureLayer<F>, L>>

Composes a function that transforms futures produced by the service.

This wraps the inner service with an instance of the MapFutureLayer middleware.

See the documentation for the map_future combinator for details.

pub fn then<F>(self, f: F) -> ServiceBuilder<Stack<ThenLayer<F>, L>>

Apply an asynchronous function after the service, regardless of whether the future succeeds or fails.

This wraps the inner service with an instance of the Then middleware.

This is similar to the map_response and map_err functions, except that the same function is invoked when the service’s future completes, whether it completes successfully or fails. This function takes the Result returned by the service’s future, and returns a Result.

See the documentation for the then combinator for details.

pub fn and_then<F>(self, f: F) -> ServiceBuilder<Stack<AndThenLayer<F>, L>>

Executes a new future after this service’s future resolves. This does not alter the behaviour of the poll_ready method.

This method can be used to change the Response type of the service into a different type. You can use this method to chain along a computation once the service’s response has been resolved.

This wraps the inner service with an instance of the AndThen middleware.

See the documentation for the and_then combinator for details.

pub fn map_result<F>(self, f: F) -> ServiceBuilder<Stack<MapResultLayer<F>, L>>

Maps this service’s result type (Result<Self::Response, Self::Error>) to a different value, regardless of whether the future succeeds or fails.

This wraps the inner service with an instance of the MapResult middleware.

See the documentation for the map_result combinator for details.

pub fn into_inner(self) -> L

Returns the underlying Layer implementation.

pub fn service<S>(&self, service: S) -> L::Service

where L: Layer<S>,

Wrap the service S with the middleware provided by this ServiceBuilder’s Layer’s, returning a new Service.

pub fn service_fn<F>(self, f: F) -> L::Service

where L: Layer<ServiceFn<F>>,

Wrap the async function F with the middleware provided by this ServiceBuilder’s Layers, returning a new Service.

This is a convenience method which is equivalent to calling ServiceBuilder::service with a service_fn, like this:

ServiceBuilder::new()
    // ...
    .service(service_fn(handler_fn))

Example

use std::time::Duration;
use tower::{ServiceBuilder, ServiceExt, BoxError, service_fn};

async fn handle(request: &'static str) -> Result<&'static str, BoxError> {
   Ok(request)
}

let svc = ServiceBuilder::new()
    .buffer(1024)
    .timeout(Duration::from_secs(10))
    .service_fn(handle);

let response = svc.oneshot("foo").await?;

assert_eq!(response, "foo");

pub fn check_clone(self) -> Self

where Self: Clone,

Check that the builder implements Clone.

This can be useful when debugging type errors in ServiceBuilders with lots of layers.

Doesn’t actually change the builder but serves as a type check.

Example

use tower::ServiceBuilder;

let builder = ServiceBuilder::new()
    // Do something before processing the request
    .map_request(|request: String| {
        println!("got request!");
        request
    })
    // Ensure our `ServiceBuilder` can be cloned
    .check_clone()
    // Do something after processing the request
    .map_response(|response: String| {
        println!("got response!");
        response
    });

pub fn check_service_clone<S>(self) -> Self

where L: Layer<S>, L::Service: Clone,

Check that the builder when given a service of type S produces a service that implements Clone.

This can be useful when debugging type errors in ServiceBuilders with lots of layers.

Doesn’t actually change the builder but serves as a type check.

Example

use tower::ServiceBuilder;

let builder = ServiceBuilder::new()
    // Do something before processing the request
    .map_request(|request: String| {
        println!("got request!");
        request
    })
    // Ensure that the service produced when given a `MyService` implements
    .check_service_clone::<MyService>()
    // Do something after processing the request
    .map_response(|response: String| {
        println!("got response!");
        response
    });

pub fn check_service<S, T, U, E>(self) -> Self

where L: Layer<S>, L::Service: Service<T, Response = U, Error = E>,

Check that the builder when given a service of type S produces a service with the given request, response, and error types.

This can be useful when debugging type errors in ServiceBuilders with lots of layers.

Doesn’t actually change the builder but serves as a type check.

Example

use tower::ServiceBuilder;
use std::task::{Poll, Context};
use tower::{Service, ServiceExt};

// An example service
struct MyService;

impl Service<Request> for MyService {
  type Response = Response;
  type Error = Error;
  type Future = futures_util::future::Ready<Result<Response, Error>>;

  fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
      // ...
  }

  fn call(&mut self, request: Request) -> Self::Future {
      // ...
  }
}

struct Request;
struct Response;
struct Error;

struct WrappedResponse(Response);

let builder = ServiceBuilder::new()
    // At this point in the builder if given a `MyService` it produces a service that
    // accepts `Request`s, produces `Response`s, and fails with `Error`s
    .check_service::<MyService, Request, Response, Error>()
    // Wrap responses in `WrappedResponse`
    .map_response(|response: Response| WrappedResponse(response))
    // Now the response type will be `WrappedResponse`
    .check_service::<MyService, _, WrappedResponse, _>();

pub fn boxed<S, R>( self, ) -> ServiceBuilder<Stack<LayerFn<fn(_: L::Service) -> BoxService<R, <L::Service as Service<R>>::Response, <L::Service as Service<R>>::Error>>, L>>

where L: Layer<S>, L::Service: Service<R> + Send + 'static, <L::Service as Service<R>>::Future: Send + 'static,

This wraps the inner service with the Layer returned by BoxService::layer().

See that method for more details.

Example

use tower::{Service, ServiceBuilder, BoxError, util::BoxService};
use std::time::Duration;

let service: BoxService<Request, Response, BoxError> = ServiceBuilder::new()
    .boxed()
    .load_shed()
    .concurrency_limit(64)
    .timeout(Duration::from_secs(10))
    .service_fn(|req: Request| async {
        Ok::<_, BoxError>(Response::new())
    });

pub fn boxed_clone<S, R>( self, ) -> ServiceBuilder<Stack<LayerFn<fn(_: L::Service) -> BoxCloneService<R, <L::Service as Service<R>>::Response, <L::Service as Service<R>>::Error>>, L>>

where L: Layer<S>, L::Service: Service<R> + Clone + Send + 'static, <L::Service as Service<R>>::Future: Send + 'static,

This wraps the inner service with the Layer returned by BoxCloneService::layer().

This is similar to the boxed method, but it requires that Self implement Clone, and the returned boxed service implements Clone.

See BoxCloneService for more details.

Example

use tower::{Service, ServiceBuilder, BoxError, util::BoxCloneService};
use std::time::Duration;

let service: BoxCloneService<Request, Response, BoxError> = ServiceBuilder::new()
    .boxed_clone()
    .load_shed()
    .concurrency_limit(64)
    .timeout(Duration::from_secs(10))
    .service_fn(|req: Request| async {
        Ok::<_, BoxError>(Response::new())
    });

// The boxed service can still be cloned.
service.clone();

Trait Implementations

impl<L: Clone> Clone for ServiceBuilder<L>

fn clone(&self) -> ServiceBuilder<L>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<L: Debug> Debug for ServiceBuilder<L>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for ServiceBuilder<Identity>

fn default() -> Self

Returns the “default value” for a type.

impl<S, L> Layer<S> for ServiceBuilder<L>

where L: Layer<S>,

type Service = <L as Layer<S>>::Service

The wrapped service

fn layer(&self, inner: S) -> Self::Service

Wrap the given service with the middleware, returning a new service that has been decorated with the middleware.