actix-extras/guide/src/qs_10.md

# Middlewares

## Logging

Logging is implemented as middleware. Middlewares get executed in same order as registraton order.
It is common to register logging middleware as first middleware for application. 
Logging middleware has to be registered for each application.

### Usage

Create `Logger` middlewares with the specified `format`.
Default `Logger` could be created with `default` method, it uses the default format:

```ignore
  %a %t "%r" %s %b "%{Referrer}i" "%{User-Agent}i" %T
```
```rust
extern crate actix_web;
use actix_web::Application;
use actix_web::middlewares::Logger;

fn main() {
    Application::default("/")
       .middleware(Logger::default())
       .middleware(Logger::new("%a %{User-Agent}i"))
       .finish();
}
```

Here is example of default logging format:

```
INFO:actix_web::middlewares::logger: 127.0.0.1:59934 [02/Dec/2017:00:21:43 -0800] "GET / HTTP/1.1" 302 0 "-" "curl/7.54.0" 0.000397
INFO:actix_web::middlewares::logger: 127.0.0.1:59947 [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
```

### Format

 `%%`  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

Tto set default response headers `DefaultHeaders` middleware could be used.
*DefaultHeaders* middleware does not set header if response headers already contains it.

```rust
extern crate actix_web;
use actix_web::*;

fn main() {
    let app = Application::default("/")
        .middleware(
            middlewares::DefaultHeaders::build()
                .header("X-Version", "0.2")
                .finish())
        .resource("/test", |r| {
             r.method(Method::GET).handler(|req| httpcodes::HTTPOk);
             r.method(Method::HEAD).handler(|req| httpcodes::HTTPMethodNotAllowed);
        })
       .finish();
}
```

## User sessions