Compojure

Middleware

Middleware are higher-order functions that provide common functionaly to handlers. A middleware function takes a handler function as an argument, and returns a modified handler function.

In essense, middleware acts as a wrapper around your handlers.

Compojure uses middleware to provide common functionality without making it mandatory. For example, many websites use HTTP sessions, but many web services do not. In Compojure, sessions are handled with middleware, so you can choose whether or not to include sessions in your application.

Another benefit is that middleware can be applied to only parts of an application. For instance, you might wish to limit file uploads to logged in users only.

To apply middleware to a handler, use the decorate macro:

(decorate your-handler
  (with-multipart)
  (with-session :memory))

This macro will wrap a pre-defined function in middleware, whilst retaining the metadata. In the above example, middleware is used to add support for HTTP sessions and file uploads via multipart forms.

Middleware is not only simple to use; it is also simple to write. A useful middleware function can be written in a few lines of Clojure:

(defn with-header [handler header value]
  (fn [request]
    (let [response (handler request)]
      (assoc-in response [:headers header] value)))) 

This middleware adds a custom header to the response.

Note that by convention, the first argument of Compojure middleware is always the handler. This is required for your middleware to work with decorate:

(decorate your-handler
  (with-multipart)
  (with-session :memory)
  (with-header "X-Framework" "Compojure"))

Middleware is used extensively in Compojure, and is a valuable tool for abstracting common functionality.