title: Middleware menu: docs_advanced weight: 220

Middleware

Actix-web’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 App, scope, or Resource and executed in opposite order as registration. In general, a middleware is a type that implements the Service trait and Transform trait. Each method in the traits has a default implementation. Each method can return a result immediately or a future object.

The following demonstrates creating a simple middleware:

{{< include-example example=”middleware” file=”main.rs” section=”simple” >}}

Alternatively, for simple use cases, you can use wrap_fn to create small, ad-hoc middleware:

{{< include-example example=”middleware” file=”wrap_fn.rs” section=”wrap-fn” >}}

Actix-web provides several useful middleware, such as logging, user sessions, compress, etc.

Logging

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).

Usage

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

{{< include-example example=”middleware” file=”logger.rs” section=”logger” >}}

The following is an example of the default logging format:

INFO:actix_web::middleware::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::middleware::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

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.

{{< include-example example=”middleware” file=”default_headers.rs” section=”default-headers” >}}

User sessions

Actix-web provides a general solution for session management. The actix-session middleware can use multiple backend types to store session data.

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

CookieSession 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 CookieSession 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 CookieSession. To access session data the Session extractor must be used. This method returns a Session object, which allows us to get or set session data.

{{< include-example example=”middleware” file=”user_sessions.rs” section=”user-session” >}}

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.

{{< include-example example=”middleware” file=”errorhandler.rs” section=”error-handler” >}}