Clojure Routing Libraries

There are a lot of different routing libraries available through clojure, and it’s hard understand which one best fits your needs. I find what I really wanted when I was researching them was an article comparing them.

There are a lot of libraries in the Clojure Toolbox, so to limit this article I’m only going to talk about libraries that have had a commit within the past 6 months.

Ring

First, I need to point out that everything’s a layer on top of Ring. As such, I’m going to establish some basic terminology:

• Request: Clojure map representing the HTTP request.
• Response: Clojure map representing the HTTP response.
• Handlers: Function translating requests into responses.
• Middleware: Functions that wrap around handlers. I’m not going to replicate Ring’s documentation, but keep in mind that all of these frameworks are fundamentally just functions and maps.

As a simple example, suppose you want your server to respond to GET requests for /foo and /bar. It would look something like this:

(defn handler [request]
  (cond (and (= (request :uri) "/foo") (= (request :request-method) :get)) "foo"
        (and (= (request :uri) "/bar") (= (request :request-method) :get)) "bar"))

Compojure

As you can see in the previous example, a common pattern in ring handlers is to condition on the :request-method and :uri. Compojure standardizes this process, allowing you to instead say this:

(defroutes handler
  (GET "/foo" [] "foo")
  (GET "/bar" [] "bar"))

Pedestal

Instead of using functions to define routing, pedestal prefers a data structure called a routing table. It’s still built on top of Ring, but ends up looking a little different.

(defroutes routes
  [[["/foo" {:get "foo"}]
    ["/bar" {:get "bar"}]]])

This will actually end up being expanded into a much more complicated data structure, but you get the idea. I haven’t had a huge amount of exposure to Pedestal, but a general principle seems to be to prefer data to functions wherever possible.

Twixt

One of the cool things about how Ring is designed is it allows for a fairly arbitrary amount of layering in the middleware. Twixt is an asset pipeline (css, javascript, etc) that’s designed to complement other routing by intercepting requests to /assets/.

Conclusion

When I started writing this article I thought the landscape was larger, but actually there are only a few active projects. I think Compojure is more broadly used, but Pedestal has a lot of power I don’t understand yet.