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Middleware

Middleware is a logic chain between the client and a Vapor route handler. It allows you to perform operations on incoming requests before they get to the route handler and on outgoing responses before they go to the client.

Configuration

Middleware can be registered globally (on every route) in configure(_:) using app.middleware.

app.middleware.use(MyMiddleware())

You can also add middleware to individual routes using route groups.

let group = app.grouped(MyMiddleware())
group.get("foo") { req in
    // This request has passed through MyMiddleware.
}

Order

The order in which middleware are added is important. Requests coming into your application will go through the middleware in the order they are added. Responses leaving your application will go back through the middleware in reverse order. Route-specific middleware always runs after application middleware. Take the following example:

app.middleware.use(MiddlewareA())
app.middleware.use(MiddlewareB())

app.group(MiddlewareC()) {
    $0.get("hello") { req in 
        "Hello, middleware."
    }
}

A request to GET /hello will visit middleware in the following order:

Request → A → B → C → Handler → C → B → A → Response

File Middleware

FileMiddleware enables the serving of assets from the Public folder of your project to the client. You might include static files like stylesheets or bitmap images here.

let file = FileMiddleware(publicDirectory: app.directory.publicDirectory)
app.middleware.use(file)

Once FileMiddleware is registered, a file like Public/images/logo.png can be linked from a Leaf template as <img src="/images/logo.png"/>.

CORS Middleware

Cross-origin resource sharing (CORS) is a mechanism that allows restricted resources on a web page to be requested from another domain outside the domain from which the first resource was served. REST APIs built in Vapor will require a CORS policy in order to safely return requests to modern web browsers.

An example configuration could look something like this:

let corsConfiguration = CORSMiddleware.Configuration(
    allowedOrigin: .all,
    allowedMethods: [.GET, .POST, .PUT, .OPTIONS, .DELETE, .PATCH],
    allowedHeaders: [.accept, .authorization, .contentType, .origin, .xRequestedWith, .userAgent, .accessControlAllowOrigin]
)
let cors = CORSMiddleware(configuration: corsConfiguration)
let error = ErrorMiddleware.default(environment: app.environment)
// Clear any existing middleware.
app.middleware = .init()
app.middleware.use(cors)
app.middleware.use(error)

Given that thrown errors are immediately returned to the client, the CORSMiddleware must be listed before the ErrorMiddleware. Otherwise, the HTTP error response will be returned without CORS headers, and cannot be read by the browser.