Add a `ReadyService` trait (#25)

src/lib.rs

@@ -197,6 +197,47 @@ pub trait Service {
     fn call(&mut self, req: Self::Request) -> Self::Future;
 }
 
+/// An asynchronous function from `Request` to a `Response` that is always ready
+/// to process a request.
+///
+/// `ReadyService` is similar to `Service`, except that it is always able to
+/// accept a request. This request may either complete successfully or resolve
+/// to an error, i.e., `ReadyService` implementations may handle out of capacity
+/// situations by returning a response future that immediately resolves to an
+/// error.
+///
+/// The `Service` trait should be prefered over this one. `ReadyService` should
+/// only be used in situations where there is no way to handle back pressure.
+/// When usin a `ReadyService` implementation, back pressure needs to be handled
+/// via some other strategy, such as limiting the total number of in flight
+/// requests.
+///
+/// One situation in which there is no way to handle back pressure is routing.
+/// A router service receives inbound requests and dispatches them to one of N
+/// inner services. In this case, one of the inner services may become "not
+/// ready" while the others remain ready. It would not be ideal for the router
+/// service to flag itself as "not ready" when only one of the inner services is
+/// not ready, but there is no way for the router to communicate to the caller
+/// which requests will be accepted and which will be rejected. The router
+/// service will implement `ReadyService`, indicating to the user that they are
+/// responsible for handling back pressure via some other strategy.
+pub trait ReadyService {
+    /// Requests handled by the service.
+    type Request;
+
+    /// Responses returned by the service.
+    type Response;
+
+    /// Errors produced by the service.
+    type Error;
+
+    /// The future response value.
+    type Future: Future<Item = Self::Response, Error = Self::Error>;
+
+    /// Process the request and return the response asynchronously.
+    fn call(&mut self, req: Self::Request) -> Self::Future;
+}
+
 /// Future yielding a `Service` once the service is ready to process a request
 pub struct Ready<T> {
     inner: Option<T>,