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106 lines
6.1 KiB
Markdown
106 lines
6.1 KiB
Markdown
---
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title: Application
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---
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import CodeBlock from "@site/src/components/code_block";
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# Writing an Application
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`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.
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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.
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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.
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> 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.
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<CodeBlock example="application" file="app.rs" section="setup" />
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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.
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> For more information, check the [URL Dispatch][usingappprefix] section.
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## State
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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.
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Let's write a simple application and store the application name in the state:
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<CodeBlock example="application" file="state.rs" section="setup" />
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Next, pass in the state when initializing the App and start the application:
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<CodeBlock example="application" file="state.rs" section="start_app" />
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Any number of state types could be registered within the application.
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## Shared Mutable State
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`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`.
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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].
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In the following example, we will write an application with mutable, shared state. First, we define our state and create our handler:
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<CodeBlock example="application" file="mutable_state.rs" section="setup_mutable" />
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and register the data in an `App`:
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<CodeBlock example="application" file="mutable_state.rs" section="make_app_mutable" />
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Key takeaways:
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- State initialized _inside_ the closure passed to `HttpServer::new` is local to the worker thread and may become de-synced if modified.
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- To achieve _globally shared state_, it must be created **outside** of the closure passed to `HttpServer::new` and moved/cloned in.
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## Using an Application Scope to Compose Applications
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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.
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For example:
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<CodeBlock example="application" file="scope.rs" section="scope" />
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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.
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## Application guards and virtual hosting
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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.
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One of the provided guards is [`Host`][guardhost]. It can be used as a filter based on request header information.
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<CodeBlock example="application" file="vh.rs" section="vh" />
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## Configure
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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.
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<CodeBlock example="application" file="config.rs" section="config" />
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The result of the above example would be:
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```
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/ -> "/"
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/app -> "app"
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/api/test -> "test"
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```
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Each [`ServiceConfig`][serviceconfig] can have its own `data`, `routes`, and `services`.
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<!-- LINKS -->
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[usingappprefix]: /docs/url-dispatch#using-an-application-prefix-to-compose-applications
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[stateexample]: https://github.com/actix/examples/blob/master/basics/state/src/main.rs
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[guardtrait]: https://docs.rs/actix-web/4/actix_web/guard/trait.Guard.html
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[guardfuncs]: https://docs.rs/actix-web/4/actix_web/guard/index.html#functions
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[guardhost]: https://docs.rs/actix-web/4/actix_web/guard/fn.Host.html
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[data]: https://docs.rs/actix-web/4/actix_web/web/struct.Data.html
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[app]: https://docs.rs/actix-web/4/actix_web/struct.App.html
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[appconfig]: https://docs.rs/actix-web/4/actix_web/struct.App.html#method.configure
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[appdata]: https://docs.rs/actix-web/4/actix_web/struct.App.html#method.app_data
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[scope]: https://docs.rs/actix-web/4/actix_web/struct.Scope.html
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[webscopeconfig]: https://docs.rs/actix-web/4/actix_web/struct.Scope.html#method.configure
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[webscope]: https://docs.rs/actix-web/4/actix_web/web/fn.scope.html
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[urlfor]: https://docs.rs/actix-web/4/actix_web/struct.HttpRequest.html#method.url_for
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[serviceconfig]: https://docs.rs/actix-web/4/actix_web/web/struct.ServiceConfig.html
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