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docs: tweak middleware authoring docs

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Rob Ede 2023-07-19 03:35:05 +01:00
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actix-web/src/middleware/mod.rs
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@ -1,24 +1,26 @@
//! A collection of common middleware.
//!
//! # Introduction
//! # What Is Middleware?
//!
//! Actix Web's middleware system allows us to add additional behavior to request/response processing.
//! Middleware can hook into incoming request and outgoing response processes,
//! enabling us to modify requests and responses as well as halt request processing to return a response early.
//! Actix Web's middleware system allows us to add additional behavior to request/response
//! processing. Middleware can hook into incoming request and outgoing response processes, enabling
//! us to modify requests and responses as well as halt request processing to return a response
//! early.
//!
//! Typically, middleware is involved in the following actions:
//!
//! * Pre-process the request
//! * Post-process a response
//! * Modify application state (through [`ServiceRequest`][crate::dev::ServiceRequest])
//! * Access external services ([redis](https://docs.rs/actix-redis), [logging][Logger], [sessions](https://docs.rs/actix-session))
//! - Pre-process the request (e.g., [normalizing paths](NormalizePath))
//! - Post-process a response (e.g., [logging][Logger])
//! - Modify application state (through [`ServiceRequest`][crate::dev::ServiceRequest])
//! - Access external services (e.g., [sessions](https://docs.rs/actix-session), etc.)
//!
//! Middleware is registered for each [`App`][crate::App], [`Scope`][crate::Scope], or [`Resource`][crate::Resource]
//! and executed in opposite order as registration.
//! In general, a middleware is a type that implements the [`Service`][Service] trait and [`Transform`][Transform] trait.
//! Each method in the traits has a default implementation. Each method can return a result immediately or a [`Future`][std::future::Future].
//! Middleware is registered for each [`App`], [`Scope`](crate::Scope), or
//! [`Resource`](crate::Resource) and executed in opposite order as registration. In general, a
//! middleware is a pair of types that implements the [`Service`] trait and [`Transform`] trait,
//! respectively. The [`new_transform`] and [`call`] methods must return a [`Future`], though it
//! can often be [an immediately-ready one](actix_utils::future::Ready).
//!
//! ## Order
//! # Ordering
//!
//! ```
//! # use actix_web::{web, middleware, get, App, Responder};
@ -42,65 +44,78 @@
//! # }
//! ```
//!
//! ```text
//! ```plain
//! Request
//! ⭣
//! ╭────────────────────┼───╮
//! ╭────────────────────┼────╮
//! │ MiddlewareC │ │
//! │ ╭──────────────────┼──╮│
//! │ ╭──────────────────┼───╮│
//! │ │ MiddlewareB │ ││
//! │ │ ╭────────────────┼─╮││
//! │ │ ╭────────────────┼──╮││
//! │ │ │ MiddlewareA │ │││
//! │ │ │ ╭──────────────┼╮│││
//! │ │ │ │ ExtractorA │││││
//! │ │ │ ├┈┈┈┈┈┈┈┈┈┈┈┈┈┈┼┤│││
//! │ │ │ │ ExtractorB │││││
//! │ │ │ ├┈┈┈┈┈┈┈┈┈┈┈┈┈┈┼┤│││
//! │ │ │ │ service │││││
//! │ │ │ ╰──────────────┼╯│││
//! │ │ ╰────────────────┼─╯││
//! │ ╰──────────────────┼──╯│
//! ╰────────────────────┼───╯
//! │ │ │ ╭──────────────┼─╮│││
//! │ │ │ │ ExtractorA │ ││││
//! │ │ │ ├┈┈┈┈┈┈┈┈┈┈┈┈┈┈┼┈┤│││
//! │ │ │ │ ExtractorB │ ││││
//! │ │ │ ├┈┈┈┈┈┈┈┈┈┈┈┈┈┈┼┈┤│││
//! │ │ │ │ service │ ││││
//! │ │ │ ╰──────────────┼─╯│││
//! │ │ ╰────────────────┼──╯││
//! │ ╰──────────────────┼───╯│
//! ╰────────────────────┼────╯
//! ⭣
//! Response
//! ```
//! The request _first_ gets processed by the middleware specified _last_ - `MiddlewareC`.
//! It passes the request (or a modified one) to the next middleware - `MiddlewareB` -
//! _or_ directly responds to the request (e.g. when the request was invalid or an error occurred).
//! `MiddlewareB` processes the request as well and passes it to `MiddlewareA`, which then passes it to the [`Service`][Service].
//! In the [`Service`][Service], the extractors will run first. They don't pass the request on, but only view it (see [`FromRequest`][crate::FromRequest]).
//! After the [`Service`][Service] responds to the request, the response it passed back through `MiddlewareA`, `MiddlewareB`, and `MiddlewareC`.
//! The request _first_ gets processed by the middleware specified _last_ - `MiddlewareC`. It passes
//! the request (modified a modified one) to the next middleware - `MiddlewareB` - _or_ directly
//! responds to the request (e.g. when the request was invalid or an error occurred). `MiddlewareB`
//! processes the request as well and passes it to `MiddlewareA`, which then passes it to the
//! [`Service`]. In the [`Service`], the extractors will run first. They don't pass the request on,
//! but only view it (see [`FromRequest`]). After the [`Service`] responds to the request, the
//! response it passed back through `MiddlewareA`, `MiddlewareB`, and `MiddlewareC`.
//!
//! As you register middleware using [`wrap`][crate::App::wrap] and [`wrap_fn`][crate::App::wrap_fn] in the [`App`][crate::App] builder,
//! imagine wrapping layers around an inner [`App`][crate::App].
//! The first middleware layer exposed to a Request is the outermost layer (i.e., the *last* registered in
//! the builder chain, in the example above: `MiddlewareC`). Consequently, the *first* middleware registered in the builder chain is
//! the *last* to start executing during request processing (`MiddlewareA`).
//! As you register middleware using [`wrap`][crate::App::wrap] and [`wrap_fn`][crate::App::wrap_fn]
//! in the [`App`] builder, imagine wrapping layers around an inner [`App`]. The first middleware
//! layer exposed to a Request is the outermost layer (i.e., the _last_ registered in the builder
//! chain, in the example above: `MiddlewareC`). Consequently, the _first_ middleware registered in
//! the builder chain is the _last_ to start executing during request processing (`MiddlewareA`).
//! Ordering is less obvious when wrapped services also have middleware applied. In this case,
//! middlewares are run in reverse order for [`App`][crate::App] _and then_ in reverse order for the
//! wrapped service.
//! middleware are run in reverse order for [`App`] _and then_ in reverse order for the wrapped
//! service.
//!
//! # Middleware Traits
//!
//! ## `Transform<S, Req>`
//!
//! The [`Transform`][Transform] trait is the factory for the actual [`Service`][crate::dev::Service]s that handle the requests.
//! All the middleware you pass to the `wrap` methods implement this trait.
//! During construction, each thread assembles a chain of [`Service`][Service]s
//! by calling [`new_transform`][crate::dev::Transform::new_transform] and passing the next [`Service`][Service] (`S`) in the chain.
//! The created [`Service`][Service] handles requests of type `Req`.
//! The [`Transform`] trait is the builder for the actual [`Service`]s that handle the requests. All
//! the middleware you pass to the `wrap` methods implement this trait. During construction, each
//! thread assembles a chain of [`Service`]s by calling [`new_transform`] and passing the next
//! [`Service`] (`S`) in the chain. The created [`Service`] handles requests of type `Req`.
//!
//! In the example from the [Order](#Order) section, the chain would be: `MiddlewareCService { next: MiddlewareBService { next: MiddlewareAService {..} } } }`.
//! In the example from the [ordering](#ordering) section, the chain would be:
//!
//! ```plain
//! MiddlewareCService {
//! next: MiddlewareBService {
//! next: MiddlewareAService { ... }
//! }
//! }
//! ```
//!
//! ## `Service<Req>`
//!
//! A [`Service`][Service] `S` represents an asynchronous operation
//! that turns a request of type `Req` into a response of type [`S::Response`][crate::dev::Service::Response]
//! or an error of type [`S::Error`][crate::dev::Service::Error]. You can think of the service of being a `async fn (&self, req: Req) -> Result<S::Response, S::Error>`.
//! A [`Service`] `S` represents an asynchronous operation that turns a request of type `Req` into a
//! response of type [`S::Response`](crate::dev::Service::Response) or an error of type
//! [`S::Error`](crate::dev::Service::Error). You can think of the service of being roughly:
//!
//! In most cases the [`Service`][Service] implementation will call the next [`Service`][Service] in its [`Future`][std::future::Future] returned by [`call`][crate::dev::Service::call].
//! ```ignore
//! async fn(&self, req: Req) -> Result<S::Response, S::Error>
//! ```
//!
//! Note that the [`Service`][Service]s created by [`new_transform`][crate::dev::Transform::new_transform] don't need to be [`Send`][Send] nor [`Sync`][Sync].
//! In most cases the [`Service`] implementation will, at some point, call the wrapped [`Service`]
//! in its [`call`] implementation.
//!
//! Note that the [`Service`]s created by [`new_transform`] don't need to be [`Send`] or [`Sync`].
//!
//! # Example
//!
@ -140,11 +155,11 @@
//! }
//!
//! // This future doesn't have the requirement of being `Send`.
//! // See futures_util::future::LocalBoxFuture
//! // See: futures_util::future::LocalBoxFuture
//! type LocalBoxFuture<T> = Pin<Box<dyn Future<Output = T> + 'static>>;
//!
//! // `S` - type of the next service
//! // `B` - type of the body - try to be generic over the body where possible
//! // `S`: type of the wrapped service
//! // `B`: type of the body - try to be generic over the body where possible
//! impl<S, B> Service<ServiceRequest> for SayHiMiddleware<S>
//! where
//! S: Service<ServiceRequest, Response = ServiceResponse<B>, Error = Error>,
@ -181,36 +196,26 @@
//! # }
//! ```
//!
//! # Simple Middleware
//! # Simpler Middleware
//!
//! In simple cases, you can use a function instead.
//! You can register these in [`App::wrap_fn`][crate::App::wrap_fn], [`Scope::wrap_fn`][crate::Scope::wrap_fn], and [`Resource::wrap_fn`][crate::Resource::wrap_fn].
//! The [order](#order) remains the same.
//! In many cases, you _can_ actually use an async function via a helper that will provide a more
//! natural flow for your behavior.
//!
//! The middleware from [above](#example) can be written using `wrap_fn`:
//! The experimental `actix_web_lab` crate provides a [`from_fn`][lab_from_fn] utility which allows
//! an async fn to be wrapped and used in the same way as other middleware. See the [`from_fn`] docs
//! for more info and examples of it's use.
//!
//! ```
//! use actix_web::{dev::Service, web, App};
//! While [`from_fn`] is experimental currently, it's likely this helper will graduate to Actix Web
//! at some stage.
//!
//! # fn main() {
//! let app = App::new()
//! .wrap_fn(|req, srv| {
//! println!("Hi from start. You requested: {}", req.path());
//! let fut = srv.call(req);
//! async {
//! let res = fut.await?;
//!
//! println!("Hi from response");
//!
//! Ok(res)
//! }
//! })
//! .route("/", web::get().to(|| async { "Hello, middleware!" }));
//! # }
//! ```
//!
//! [Service]: crate::dev::Service
//! [Transform]: crate::dev::Transform
//! [`Future`]: std::future::Future
//! [`App`]: crate::App
//! [`FromRequest`]: crate::FromRequest
//! [`Service`]: crate::dev::Service
//! [`Transform`]: crate::dev::Transform
//! [`call`]: crate::dev::Service::call()
//! [`new_transform`]: crate::dev::Transform::new_transform()
//! [lab_from_fn]: https://docs.rs/actix-web-lab/latest/actix_web_lab/middleware/fn.from_fn.html
mod compat;
mod condition;