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75 lines
3.5 KiB
Markdown
75 lines
3.5 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 responses,
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websocket protocol handling, multipart streams, 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 application.
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Applications act as a namespace for all routes, i.e all routes for a specific application
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have the same url path prefix. The application prefix always contains a leading "/" slash.
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If a supplied prefix does not contain leading slash, it is automatically inserted.
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The prefix should consist of value path segments.
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> For an application with prefix `/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" section="make_app" >}}
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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
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> [URL Dispatch](./sec-6-url-dispatch.html#using-a-application-prefix-to-compose-applications) section.
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Multiple applications can be served with one server:
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{{< include-example example="application" section="run_server" >}}
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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 application.
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When using an http actor,state can be accessed with the `HttpRequest::state()` as read-only,
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but interior mutability with `RefCell` can be used to achieve state mutability.
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State is also available for route matching predicates 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**: http server accepts an application factory rather than an application
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> instance. Http server constructs an application instance for each thread, thus application state
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> must be constructed multiple times. If you want to share state between different threads, a
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> shared object should be used, e.g. `Arc`. Application state does not need to be `Send` and `Sync`,
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> but the application factory must be `Send` + `Sync`.
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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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[server::new](https://docs.rs/actix-web/*/actix_web/server/fn.new.html) requires the handler to have a single type.
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This limitation can easily be overcome with the [App::boxed](https://docs.rs/actix-web/*/actix_web/struct.App.html#method.boxed) method, which converts an App into a boxed trait object.
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{{< include-example example="application" file="state.rs" section="combine" >}}
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