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A Clojure library to help build HTTP compliant servers.

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README.md

playnice

playnice contains a URL router and helpful Ring middleware for creating HTTP-compliant web servers.

Quick Start

Add this to your dependencies:

[playnice "1.0.1"]

Then you can define a simple server like this.

(ns my.namespace
  (:require [playnice.core  :as pn])
  (:require [ring.adapter.jetty :as jetty]))

(defn homepage [req]
   {:status 200
    :body "Home page!"
    :headers {}})

(defn greeting [{:keys [greeting greetee]}]
   {:status 200
    :body (str greeting ", " greetee)
    :headers {}})

(def routes (-> nil
              (pn/dassoc "/"                   homepage)
              (pn/dassoc "/:greeting/:greetee" greeting)))

(defn app [req] (pn/dispatch routes req))

(jetty/run-jetty app 8080)

Philosophy

  1. Make it easier to serve correct HTTP responses.
  2. Avoid confounding HTTP logic with business logic.
  3. Make it easier for limited or non-standard clients to access resources.
  4. As much as possible separate orthogonal functionality into composable units.

Usage

playnice tries to be good about the HTTP Spec. It also tries to separate out functionality into composable units. playnice works really well with Liberator.

Router

The router only does one thing: determine which handler to run for a given URL. It is most readily compared to Compojure. It differs from Compojure in the following ways:

  1. Compojure matches the HTTP Method before it matches the URL. This violates the HTTP Spec. Compojure will respond 404 when the URL matches and a POST was sent.

    The correct response is 405 Method Not Allowed. playnice does not route differently based on the request method. Instead, it is left to the handler to determine how to handle the request. I typically use Liberator to properly handle the resource.

  2. Compojure routes are dependent on the order in which they are defined. While this works well when there are a small number of non-overlapping routes, it becomes difficult to reason about when the number of routes increases.

    playnice is not order-dependent. As you add more routes, the new routes are checked against existing routes. If there is a conflict, an exception is thrown. This means that the operation of adding a new route is commutative. Storing the routes in an atom or ref gives you nice guarantees.

  3. Compojure routes are tried in order using a linear search.

    playnice routes are stored in a tree, meaning log(n) complexity.

  4. Compojure routes are built using handy macros.

    playnice routes are stored in a Clojure map and are built using functions. This means that a routing tree is first-class. This is especially helpful when aggregating routes from different files. The order of file includes is hard to predict and can change easily, so order independence is very important.

Using the router is quite easy. There are two functions to know: dassoc and dispatch in the playnice.core namespace.

dassoc takes three arguments: the routing tree, a path pattern, and a handler. It returns a new routing tree.

The path pattern is quite simple. The path pattern is a string that is broken down into path segments. If the path segment starts with a :, then it is considered a variable segment. Otherwise, it is an exact-match segment.

Examples:

"/" ;;-> Will match only the root  
"/some/path/to/something" ;;-> All segments much match exactly  
"/user/:userid" ;;-> Will match all paths of two segments that start with "/user/"

Exact-match segments are self-explanatory. Variable segments always succed when the segment exists. They also store the entire segment that is matched in the request. For instance:

"/purchase/:purchaseid" matched on "/purchase/123" will assoc :purchaseid "123" in the Ring request object passed to the handler. You can then access it as (:purchaseid req)

The other function you need to know is called dispatch. It takes a routing tree and a Ring request and returns a Ring response. dispatch performs the routing logic required to choose a handler and either runs the handler or returns a generic 404 response. Thus concludes the routing section.

Middleware

wrap-fake-methods

Forms submitted in browsers can only perform GET and POST requests. This is unfortunate since sometimes the operation that form submission is supposed to perform maps better to DELETE or another HTTP method. But you don't want to bake in the limitations of each client into the logic of your server.

wrap-fake-methods lets you code your handler to use the correct method for the operation at hand. This function is Ring middleware which looks for the "__method" parameter (query or form post parameter). If it exists, the value of that parameter is used as the request method instead. This happens transparently so that your handler does not have to know the difference. You can now POST to /purchase/123?__method=DELETE to cancel the purchase order.

Reminder: this middleware depends on parameters from the query string and/or the form post body. This middleware should be wrapped by the middleware which parses those parameters.

wrap-fake-accept

The HTTP standard defines the Accept header to notify the server what mime-types it can handle in the response. This is great. However, it can be quite inconvenient when headers cannot be set or if setting them is a chore. For instance, how can you set headers in the href of a link?

Let's say there is a resource at /user/1138 that returns information about a user. Normally, it will use HTTP content negotiation to determine what content-type to use. For this example, we assume we can return results in JSON, HTML, and XML. Because of the standard Accept header of most browsers, HTML will always be preferred.

But in this case, we don't want HTML, we want JSON. We could alter the Accept header. But we can't do that in a link. Or we could add a new route just for JSON (/user/1138/json), but that is not scalable (3 handlers per resource?) and does not take full advantage of our content negotiation machinery. What wrap-fake-accept does is to fake an Accept header using the filename extension of the path.

If the last path segment contains a "." and doesn't end in a slash, then this middleware considers it a filename with an extension. The mime-type of the extension is looked up and set as the Accept header for the handler. Also, the extension is removed from the path so that the generic filename is used. If there is no extension, the request is passed as-is.

Reminder: this middleware should happen before content negotiation.

Example:

{:uri "/user/1138.json"
 :headers {"accept" "text/html, text/plain, image/jpeg, */*"
           ...}
 ...}

becomes

{:uri "/user/1138"
 :headers {"accept" "application/json"}
           ...}
 ...}

wrap-ip-forwarding

Many servers put a forwarding proxy in front of them. These servers add security, load balancing, and performance features. However, they often transform the requests. On our development machines, we often will hit the server directly. So the requests look different on dev machines and production. This middleware un-transforms a particular part of the request, namely, it restores the :remote-addr portion of the Ring request to the value of the X-Forwarded-For header, if it exists. This way, we ensure that the behavior is the same regardless of whether it is deployed or running on our local machine.

Use it!

Please use it and send your patches. I'd love to hear your feedback. Just create an issue in Github.

License

Copyright (C) 2013 Eric Normand

Distributed under the Eclipse Public License, the same as Clojure.

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