--- title: Application --- import CodeBlock from "@site/src/components/code_block"; # Writing an Application `actix-web` provides various primitives to build web servers and applications with Rust. It provides routing, middleware, pre-processing of requests, post-processing of responses, etc. All `actix-web` servers are built around the [`App`][app] instance. It is used for registering routes for resources and middleware. It also stores application state shared across all handlers within the same scope. An application's [`scope`][scope] acts as a namespace for all routes, i.e. all routes for a specific application scope have the same url path prefix. The application prefix always contains a leading "/" slash. If a supplied prefix does not contain leading slash, it is automatically inserted. The prefix should consist of value path segments. > For an application with scope `/app`, any request with the paths `/app`, `/app/`, or `/app/test` would match; however, the path `/application` would not match. <CodeBlock example="application" file="app.rs" section="setup" /> In this example, an application with the `/app` prefix and an `index.html` resource is created. This resource is available through the `/app/index.html` url. > For more information, check the [URL Dispatch][usingappprefix] section. ## State Application state is shared with all routes and resources within the same scope. State can be accessed with the [`web::Data<T>`][data] extractor where `T` is the type of the state. State is also accessible for middleware. Let's write a simple application and store the application name in the state: <CodeBlock example="application" file="state.rs" section="setup" /> Next, pass in the state when initializing the App and start the application: <CodeBlock example="application" file="state.rs" section="start_app" /> Any number of state types could be registered within the application. ## Shared Mutable State `HttpServer` accepts an application factory rather than an application instance. An `HttpServer` constructs an application instance for each thread. Therefore, application data must be constructed multiple times. If you want to share data between different threads, a shareable object should be used, e.g. `Send` + `Sync`. Internally, [`web::Data`][data] uses `Arc`. So in order to avoid creating two `Arc`s, we should create our Data before registering it using [`App::app_data()`][appdata]. In the following example, we will write an application with mutable, shared state. First, we define our state and create our handler: <CodeBlock example="application" file="mutable_state.rs" section="setup_mutable" /> and register the data in an `App`: <CodeBlock example="application" file="mutable_state.rs" section="make_app_mutable" /> Key takeaways: - State initialized _inside_ the closure passed to `HttpServer::new` is local to the worker thread and may become de-synced if modified. - To achieve _globally shared state_, it must be created **outside** of the closure passed to `HttpServer::new` and moved/cloned in. ## Using an Application Scope to Compose Applications The [`web::scope()`][webscope] method allows setting a resource group prefix. This scope represents a resource prefix that will be prepended to all resource patterns added by the resource configuration. This can be used to help mount a set of routes at a different location than the original author intended while still maintaining the same resource names. For example: <CodeBlock example="application" file="scope.rs" section="scope" /> In the above example, the `show_users` route will have an effective route pattern of `/users/show` instead of `/show` because the application's scope argument will be prepended to the pattern. The route will then only match if the URL path is `/users/show`, and when the [`HttpRequest.url_for()`][urlfor] function is called with the route name `show_users`, it will generate a URL with that same path. ## Application guards and virtual hosting You can think of a guard as a simple function that accepts a _request_ object reference and returns _true_ or _false_. Formally, a guard is any object that implements the [`Guard`][guardtrait] trait. Actix Web provides several guards. You can check the [functions section][guardfuncs] of the API docs. One of the provided guards is [`Host`][guardhost]. It can be used as a filter based on request header information. <CodeBlock example="application" file="vh.rs" section="vh" /> ## Configure For simplicity and reusability both [`App`][appconfig] and [`web::Scope`][webscopeconfig] provide the `configure` method. This function is useful for moving parts of the configuration to a different module or even library. For example, some of the resource's configuration could be moved to a different module. <CodeBlock example="application" file="config.rs" section="config" /> The result of the above example would be: ``` / -> "/" /app -> "app" /api/test -> "test" ``` Each [`ServiceConfig`][serviceconfig] can have its own `data`, `routes`, and `services`. <!-- LINKS --> [usingappprefix]: /docs/url-dispatch#using-an-application-prefix-to-compose-applications [stateexample]: https://github.com/actix/examples/blob/master/basics/state/src/main.rs [guardtrait]: https://docs.rs/actix-web/4/actix_web/guard/trait.Guard.html [guardfuncs]: https://docs.rs/actix-web/4/actix_web/guard/index.html#functions [guardhost]: https://docs.rs/actix-web/4/actix_web/guard/fn.Host.html [data]: https://docs.rs/actix-web/4/actix_web/web/struct.Data.html [app]: https://docs.rs/actix-web/4/actix_web/struct.App.html [appconfig]: https://docs.rs/actix-web/4/actix_web/struct.App.html#method.configure [appdata]: https://docs.rs/actix-web/4/actix_web/struct.App.html#method.app_data [scope]: https://docs.rs/actix-web/4/actix_web/struct.Scope.html [webscopeconfig]: https://docs.rs/actix-web/4/actix_web/struct.Scope.html#method.configure [webscope]: https://docs.rs/actix-web/4/actix_web/web/fn.scope.html [urlfor]: https://docs.rs/actix-web/4/actix_web/struct.HttpRequest.html#method.url_for [serviceconfig]: https://docs.rs/actix-web/4/actix_web/web/struct.ServiceConfig.html