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Actix's middleware system allows us to add additional behavior to request/response processing. Middleware can hook into an incoming request process, enabling us to modify requests as well as halt request processing to return a response early.

Middleware can also hook into response processing.

Typically, middleware is involved in the following actions:

  • Pre-process the Request
  • Post-process a Response
  • Modify application state
  • Access external services (redis, logging, sessions)

Middleware is registered for each application and executed in same order as registration. In general, a middleware is a type that implements the Middleware trait. Each method in this trait has a default implementation. Each method can return a result immediately or a future object.

The following demonstrates using middleware to add request and response headers:

use http::{header, HttpTryFrom};
use actix_web::{App, HttpRequest, HttpResponse, Result};
use actix_web::middleware::{Middleware, Started, Response};

struct Headers;  // <- Our middleware

/// Middleware implementation, middlewares are generic over application state,
/// so you can access state with `HttpRequest::state()` method.
impl<S> Middleware<S> for Headers {

    /// Method is called when request is ready. It may return
    /// future, which should resolve before next middleware get called.
    fn start(&self, req: &HttpRequest<S>) -> Result<Started> {

    /// Method is called when handler returns response,
    /// but before sending http message to peer.
    fn response(&self, req: &HttpRequest<S>, mut resp: HttpResponse)
        -> Result<Response>

fn main() {
       // Register middleware, this method can be called multiple times
       .resource("/", |r| r.f(|_| HttpResponse::Ok()));

Actix provides several useful middlewares, such as logging, user sessions, etc.


Logging is implemented as a middleware. It is common to register a logging middleware as the first middleware for the application. Logging middleware must be registered for each application.

The Logger middleware uses the standard log crate to log information. You should enable logger for actix_web package to see access log (env_logger or similar).


Create Logger middleware with the specified format. Default Logger can be created with default method, it uses the default format:

  %a %t "%r" %s %b "%{Referer}i" "%{User-Agent}i" %T
extern crate env_logger;
use actix_web::App;
use actix_web::middleware::Logger;

fn main() {
    std::env::set_var("RUST_LOG", "actix_web=info");

       .middleware(Logger::new("%a %{User-Agent}i"))

The following is an example of the default logging format:

INFO:actix_web::middleware::logger: [02/Dec/2017:00:21:43 -0800] "GET / HTTP/1.1" 302 0 "-" "curl/7.54.0" 0.000397
INFO:actix_web::middleware::logger: [02/Dec/2017:00:22:40 -0800] "GET /index.html HTTP/1.1" 200 0 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:57.0) Gecko/20100101 Firefox/57.0" 0.000646


%% The percent sign

%a Remote IP-address (IP-address of proxy if using reverse proxy)

%t Time when the request was started to process

%P The process ID of the child that serviced the request

%r First line of request

%s Response status code

%b Size of response in bytes, including HTTP headers

%T Time taken to serve the request, in seconds with floating fraction in .06f format

%D Time taken to serve the request, in milliseconds

%{FOO}i request.headers['FOO']

%{FOO}o response.headers['FOO']

%{FOO}e os.environ['FOO']

Default headers

To set default response headers, the DefaultHeaders middleware can be used. The DefaultHeaders middleware does not set the header if response headers already contain a specified header.

use actix_web::{http, middleware, App, HttpResponse};

fn main() {
    let app = App::new()
                .header("X-Version", "0.2"))
        .resource("/test", |r| {
             r.method(http::Method::GET).f(|req| HttpResponse::Ok());
             r.method(http::Method::HEAD).f(|req| HttpResponse::MethodNotAllowed());

User sessions

Actix provides a general solution for session management. The SessionStorage middleware can be used with different backend types to store session data in different backends.

By default, only cookie session backend is implemented. Other backend implementations can be added.

CookieSessionBackend uses cookies as session storage. CookieSessionBackend creates sessions which are limited to storing fewer than 4000 bytes of data, as the payload must fit into a single cookie. An internal server error is generated if a session contains more than 4000 bytes.

A cookie may have a security policy of signed or private. Each has a respective CookieSessionBackend constructor.

A signed cookie may be viewed but not modified by the client. A private cookie may neither be viewed nor modified by the client.

The constructors take a key as an argument. This is the private key for cookie session - when this value is changed, all session data is lost.

In general, you create a SessionStorage middleware and initialize it with specific backend implementation, such as a CookieSessionBackend. To access session data, HttpRequest::session() must be used. This method returns a Session object, which allows us to get or set session data.

use actix_web::{server, App, HttpRequest, Result};
use actix_web::middleware::session::{RequestSession, SessionStorage, CookieSessionBackend};

fn index(req: &HttpRequest) -> Result<&'static str> {
    // access session data
    if let Some(count) = req.session().get::<i32>("counter")? {
        println!("SESSION value: {}", count);
        req.session().set("counter", count+1)?;
    } else {
        req.session().set("counter", 1)?;


fn main() {
    let sys = actix::System::new("basic-example");
        || App::new().middleware(
             CookieSessionBackend::signed(&[0; 32])
    let _ =;

Error handlers

ErrorHandlers middleware allows us to provide custom handlers for responses.

You can use the ErrorHandlers::handler() method to register a custom error handler for a specific status code. You can modify an existing response or create a completly new one. The error handler can return a response immediately or return a future that resolves into a response.

use actix_web::{
    App, HttpRequest, HttpResponse, Result,
    http, middleware::Response, middleware::ErrorHandlers};

fn render_500<S>(_: &HttpRequest<S>, resp: HttpResponse) -> Result<Response> {
   let mut builder = resp.into_builder();
   builder.header(http::header::CONTENT_TYPE, "application/json");

fn main() {
    let app = App::new()
                .handler(http::StatusCode::INTERNAL_SERVER_ERROR, render_500))
        .resource("/test", |r| {
             r.method(http::Method::GET).f(|_| HttpResponse::Ok());
             r.method(http::Method::HEAD).f(|_| HttpResponse::MethodNotAllowed());