actix-web/guide/src/qs_3.md

Overview

Actix web provides some primitives to build web servers and applications with Rust. It provides routing, middlewares, pre-processing of requests, and post-processing of responses, websocket protcol handling, multipart streams, etc.

Application

All actix web server is built around Application instance. It is used for registering handlers for routes and resources, middlewares. Also it stores applicationspecific state that is shared accross all handlers within same application.

Application acts as namespace for all routes, i.e all routes for specific application has same url path prefix:

   let app = Application::default("/prefix")
       .resource("/index.html", |r| r.handler(Method::GET, index)
       .finish()

In this example application with /prefix prefix and index.html resource get created. This resource is available as on /prefix/index.html url.

Application state

Application state is shared with all routes within same application. State could be accessed with HttpRequest::state() method. It is read-only but interior mutability pattern with RefCell could be used to archive state mutability. State could be accessed with HttpRequest::state() method or HttpContext::state() in case of http actor.

Let's write simple application that uses shared state. We are going to store requests count in the state:

extern crate actix;
extern crate actix_web;

use std::cell::Cell;
use actix_web::prelude::*;

// This struct represents state
struct AppState {
    counter: Cell<usize>,
}

fn index(req: HttpRequest<AppState>) -> String {
    let count = req.state().counter.get() + 1; // <- get count
    req.state().counter.set(count);            // <- store new count in state

    format!("Request number: {}", count)      // <- response with count
}

fn main() {
    let sys = actix::System::new("example");

    HttpServer::new(
        Application::build("/", AppState{counter: Cell::new(0)})
            .resource("/", |r| r.handler(Method::GET, index)))
        .serve::<_, ()>("127.0.0.1:8088").unwrap();

    println!("Started http server: 127.0.0.1:8088");
    actix::Arbiter::system().send(actix::msgs::SystemExit(0)); // <- remove this line, this code stops system during testing

    let _ = sys.run();
}

Handler

A request handler can have several different forms. Simple function that accepts HttpRequest and returns HttpResponse or any type that can be converted into HttpResponse. Function that that accepts HttpRequest and returns Stream<Item=Frame, Error=Error>. Or http actor, i.e. actor that has HttpContext<A> as a context.