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129 lines
5.6 KiB
Markdown
129 lines
5.6 KiB
Markdown
---
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title: Application
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menu: docs_basics
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weight: 140
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---
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# Writing an Application
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`actix-web` provides various primitives to build web servers and applications with Rust.
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It provides routing, middlewares, pre-processing of requests, post-processing of
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responses, etc.
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All `actix-web` servers are built around the `App` instance. It is used for
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registering routes for resources and middlewares. It also stores application
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state shared across all handlers within same scope.
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An application's `scope` acts as a namespace for all routes, i.e. all routes for a
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specific application scope have the same url path prefix. The application prefix always
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contains a leading "/" slash. If a supplied prefix does not contain leading slash,
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it is automatically inserted. The prefix should consist of value path segments.
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> For an application with scope `/app`,
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> any request with the paths `/app`, `/app/`, or `/app/test` would match;
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> however, the path `/application` would not match.
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{{< include-example example="application" file="app.rs" section="setup" >}}
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In this example, an application with the `/app` prefix and a `index.html` resource
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are 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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Multiple application scopes can be served with one server:
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{{< include-example example="application" file="main.rs" section="multi" >}}
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All `/app1` requests route to the first application, `/app2` to the second, and all other to the third.
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**Applications get matched based on registration order**. If an application with a more generic
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prefix is registered before a less generic one, it would effectively block the less generic
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application matching. For example, if an `App` with the prefix `"/"` was registered
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as the first application, it would match all incoming requests.
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## State
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Application state is shared with all routes and resources within the same scope. State
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can be accessed with the `web::Data<State>` extractor as read-only, but interior mutability with
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`Cell` can be used to achieve state mutability. State is also available for route
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matching guards and middlewares.
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Let's write a simple application that uses shared state. We are going to store request count
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in the state:
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{{< include-example example="application" file="state.rs" section="setup" >}}
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When the app is initialized it needs to be passed the initial state:
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{{< include-example example="application" file="state.rs" section="make_app" >}}
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> **Note**: `HttpServer` accepts an application factory rather than an application
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> instance. `HttpServer` constructs an application instance for each thread, thus
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> application state must be constructed multiple times. If you want to share state between
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> different threads, a shared object should be used, e.g. `Arc`. There is also an
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> [Example][stateexample] using `Arc` for this. Application state does not need to be
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> `Send` and `Sync`, but the application factory must be `Send` + `Sync`.
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To start the previous app, create it into a closure:
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{{< include-example example="application" file="state.rs" section="start_app" >}}
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## Combining applications with different state
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Combining multiple applications with different state is possible as well.
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{{< include-example example="application" file="combine.rs" section="combine" >}}
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## Using an Application Scope to Compose Applications
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The `web::scope()` method allows to set a specific application prefix. This scope represents
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a resource prefix that will be prepended to all resource patterns added by the resource
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configuration. This can be used to help mount a set of routes at a different location
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than the included callable's author intended while still maintaining the same resource names.
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For example:
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{{< include-example 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
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*/users/show* instead of */show* because the application's scope argument will be prepended
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to the pattern. The route will then only match if the URL path is */users/show*,
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and when the `HttpRequest.url_for()` function is called with the route name show_users,
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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
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and returns *true* or *false*. Formally, a guard is any object that implements the
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[`Guard`][guardtrait] trait. Actix-web provides several guards, you can check
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[functions section][guardfuncs] of api docs.
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One of the provided guards is [`Header`][guardheader], it can be used as application's
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filter based on request's header information.
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{{< include-example example="application" file="vh.rs" section="vh" >}}
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# Configure
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For simplicity and reusability both `App` and `web::scope` provide the `configure` method.
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This function is useful for moving parts of configuration to a different module or even
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library. For example, some of the resource's configuration could be moved to different
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module.
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{{< include-example 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` can have it's own `data`, `routes`, and `services`
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[usingappprefix]: /docs/url-dispatch/index.html#using-an-application-prefix-to-compose-applications
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[stateexample]: https://github.com/actix/examples/blob/master/state/src/main.rs
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[guardtrait]: https://docs.rs/actix-web/1.0.2/actix_web/guard/trait.Guard.html
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[guardfuncs]: https://docs.rs/actix-web/1.0.2/actix_web/guard/index.html#functions
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[guardheader]: ((https://docs.rs/actix-web/1.0.2/actix_web/guard/fn.Header.html
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