ring-cors []2

simple ring middleware for easy cross-origin resource sharing

[jumblerg/ring-cors "3.0.0"] ;; latest release

rationale

for most developers, cors is a nuisance. there's little reason to expose the many options enumerated by the w3c to an application developed with an awareness of cross-origin requests from the outset. the vast majority of these settings exist to provide an upgrade path to servers that predate the specification; by assuming that it will be used in a modern cors-aware environment, these minutiae can be abstracted away to expose a straightforward api.

application

to make your cors problems go away, simply pass the wrapper a predicate that authorizes the request's origin or, for convenience, one or more regular expressions.

(require '[jumblerg.middleware.cors :refer [wrap-cors]])

(defn handle [req] {:status 200 :body "hello cors"})

;; accept everything
(wrap-cors handle #".*")
(wrap-cors handle identity)

;; accept some things
(wrap-cors handle #".*localhost.*" #".*mydomain.org$")
(wrap-cors handle #(= (:allowed-origin db) %))

;; accept one thing
(wrap-cors handle #"^http://myapp.mydomain.org$")

;; accept nothing
(wrap-cors handle)

notice that the regular expressions are anchored in order to prevent partial matches from being accepted.

implementation

if the request satisfies the predicate, and is determined to be a preflight (as indicated by an access-control-request-method header), wrap-cors will respond with the access control headers expected by the browser, bypassing any downstream handlers. wrap-cors will decorate any other requests from allowed origins with the required headers on their way out, and pass through all others without modifications.

considerations

complexities such as the cors preflight exist to ensure that legacy servers predating the cors specification (and, for example, never anticipated a cross-domain DELETE) can upgrade to a new world where browsers permit cross-domain requests in a backwards-compatible way. the preflight allows these legacy servers to selectively expose individual methods and headers to cross-origin requests without having to support all of them. since this scenario doesn't apply to a newly-developed service, this wrapper permissively authorizes the user agent of an allowed origin to use all requested methods, headers, and cookies.

this middleware instructs the browser to cache the preflight for 24 hours based on a w3c discussion of the subject. mozilla has adapted 24 hours as the max cache time for the preflight, while webkit will max out in just five minutes (some chrome documentation states the default is 1/2 hour). the cache can be cleared manually from the browser if necessary during development. in chrome, this is achieved by setting the "disable cache while devtools is open" flag in the development tools settings panel. sometimes maybe.

this middleware works with all desktop and mobile browsers except the usual suspects: internet explorers 8 and 9 (full cors support was added by "the fourth platform" of ie 10). more information can be found regarding ie's partial cors support at caniuse and microsoft's website.

license

copyright (c) jumblerg. all rights reserved.

distributed with clojure under the eclipse public license