Middleware

A middleware is a function with this signature:

fn(Conn(body), fn(Conn(body)) -> Response(ResponseData)) -> Response(ResponseData)

It receives the current conn, calls next(conn) to continue the pipeline, and can inspect or modify the conn before and the response after.

Built-in middleware

Middleware	Description
rescue.middleware	Catches crashes, returns 500
middleware.logger	Logs method, path, status, and duration
middleware.body_limit(n)	Reads and limits the request body
middleware.static_files(under:, from:)	Serves files from disk
auth.load(secret)	Populates conn.auth from session
auth.require(secret, to:)	Redirects to to if unauthenticated

Writing middleware

fn request_id_middleware(
  c: Conn(body),
  next: fn(Conn(body)) -> Response(ResponseData),
) -> Response(ResponseData) {
  let id = generate_id()
  let c2 = Conn(..c, assigns: dict.insert(c.assigns, "request_id", id))
  next(c2)
  |> response.set_header("x-request-id", id)
}

Pass it to a scope:

router.scope("/", [request_id_middleware], fn(r) { ... })

Order

Middleware in a scope list runs outermost-first on the way in, innermost-first on the way out — like plug in Phoenix. Put rescue.middleware first so it wraps everything else.

[rescue.middleware, auth.load(secret), logger_middleware]
//  ↑ wraps all of ↓