Web applications in Clojure all the way (with Compojure & Om)

Today we are going to build a web application that primarily uses Clojure programming language. On the backend there will be a simple API written in Compojure that produces data in JSON format; on the frontend we will use Om: a ClojureScript interface for Facebook’s React.

You can automatically generate the application from this tutorial using a Leiningen template called atw-om.

Update 2014-12 Updated dependencies: Om 0.7.3, React 0.11.1, Clojure 1.6.0 and Compojure 1.2.2.

Update 2015-03 Updated dependencies: Om 0.8.8, React 0.12.2, Clojure 1.7.0-alpha5 and Compojure 1.3.2. Removed deprecated compojure.handler namespace in favor of ring-defaults (sensible Ring middleware defaults).


Let’s start by generating a Compojure application.

lein new compojure acme

We need to adjust dependencies in project.clj.

:dependencies [[org.clojure/clojure "1.7.0-alpha5"]
               [compojure "1.3.2"]
               [ring/ring-core "1.3.2"]
               [ring/ring-json "0.3.1”]
               [ring/ring-defaults "0.1.4"]]

Next, extend the namespace declaration in src/acme/handler.clj.

(ns acme.handler
  (:require [compojure.route :as route]
            [compojure.core :refer [GET defroutes]]
            [ring.util.response :refer [resource-response response]]
            [ring.middleware.json :as middleware]
            [ring.middleware.defaults :refer [wrap-defaults api-defaults]]))

Now, we can define two routes: / and /widgets.

(defroutes app-routes
  (GET  "/" [] (resource-response "index.html" {:root "public"}))
  (GET  "/widgets" [] (response [{:name "Widget 1"} {:name "Widget 2"}]))
  (route/resources "/")
  (route/not-found "Page not found"))

The first will serve index.html located in resources/public/; the second route will produce a list of two widgets and return it in JSON format.

For the JSON serialization to work we have to adjust app declaration by wrapping wrap-json-response middleware around the route handler.

(def app
  (-> app-routes
      (wrap-defaults api-defaults)))

wrap-json-body is an equivalent of wrap-json-response but it’s for requests.. I’m adding it here for convenience. It comes handy once you need to automatically deserialize the incoming data i.e. from POST request.

Finally, we need to put index.html in resources/public.

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8"/>
    <meta http-equiv="X-UA-Compatible" content="IE=edge"/>
    <link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/normalize/3.0.1/normalize.min.css" />
    <div class="row">
      <div class="large-12 columns">
        <h1>Acme Corp.</h1>
        <div id="content"></div>

Try to run the application.

lein ring server-headless

Then open localhost:3000 in your browser and check if both / and /widgets work.


We want to have Clojure backend and frontend live side-by-side in one project. For that we need to slightly adjust the directory structure. Inside src/, create two directories clj/ (for backend code) and cljs/ (for frontend code), then move src/acme to src/clj so it’s under src/clj/acme. Under cljs/ create an empty directory acme/ and put there an empty core.cljs file.

In project.clj add :source-paths.

(defproject acme "0.1.0-SNAPSHOT”
  :source-paths ["src/clj”])

Now we need to update project.clj with things related to the frontend development. Let’s start with dependencies.

:dependencies [[org.clojure/clojure "1.6.0"]
               [org.clojure/tools.reader "0.8.4"]
               [ring/ring-core "1.3.2"]
               [ring/ring-json "0.3.1"]
               [compojure "1.2.2"]
               [org.clojure/clojurescript "0.0-2371"]
               [org.clojure/core.async "0.1.346.0-17112a-alpha"]
               [cljs-http "0.1.21"]
               [om "0.7.3"]]

Then, :plugins

:plugins [[lein-cljsbuild "1.0.3"]
          [lein-ring "0.8.13"]
          [lein-pdo "0.1.1"]]

lein-cljsbuild for on-the-fly ClojureScript compilation and lein-pdo to simplify the process of running the app by combining frontend files compilation with the backend server launch. This is defined using :alias.

:aliases {"up" ["pdo" "cljsbuild" "auto" "dev," "ring" "server-headless"]}

Lastly, we have to specify ClojureScript compilation parameters using :cljsbuild.

:cljsbuild {:builds [{:id "dev"
                      :source-paths ["src/cljs"]
                      :compiler {:output-to "resources/public/js/app.js"
                                 :output-dir "resources/public/js/out"
                                 :optimizations :none
                                 :source-map true}}]}

Now, we are ready to build the frontend logic that displays the list of widgets. From now on, all modifications concern src/cljs/core.cljs.

Let’s start with namespace declaration.

(ns acme.core
  (:require-macros [cljs.core.async.macros :refer [go alt!]])
  (:require [goog.events :as events]
            [cljs.core.async :refer [put! <! >! chan timeout]]
            [om.core :as om :include-macros true]
            [om.dom :as dom :include-macros true]
            [cljs-http.client :as http]))

Then, we enable printing on the console with:


Let’s try to communicate with the API by fetching available widgets from /widgets. The code uses core.async to asynchronously fetch data from a given URL and then, when available, it puts it it into a channel.

(defn fetch-widgets
  (let [c (chan)]
    (go (let [{widgets :body} (<! (http/get url))]
          (>! c (vec widgets))))

Now we are ready to start with Om. Let’s define the root of the application (attached to #content).

(om/root app-state om-app (.getElementById js/document "content"))

We define Om application (om-app) as a component that wraps a widget box inside a div. This should be declared before (om/root ...).

(defn om-app [app owner]
   (dom/div nil
            (om/build widget-box app
                      {:opts {:url "/widgets"
                              :poll-interval 2000}}))))

widget-box is a more complicated component. It has a state created by consuming /widgets route on the backend. It renders a title and then a list of available widgets. This should be declared before (defn om-app …).

(defn widget-box [app owner opts]
    (will-mount [_]
                (om/transact! app [:widgets] (fn [] []))
                (go (while true
                      (let [widgets (<! (fetch-widgets (:url opts)))]
                        (.log js/console (pr-str widgets))
                        (om/update! app #(assoc % :widgets widgets)))
                      (<! (timeout (:poll-interval opts))))))
    (render [_]
            (dom/h1 nil "Widgets")
            (om/build widget-list app))))

widget-list is again a simple component that defines a list using ul tag and then asks widget component to display a name for each of them from the list using li tag. Those two components must be declared before (defn widget-box …).

(defn widget [{:keys [name]} owner opts]
   (dom/li nil name)))

(defn widget-list [{:keys [widgets]}]
   (apply dom/ul nil
          (om/build-all widget widgets))))

Finally, we initialize app-state as an atom with an empty map; put it before (om/root …) declaration.

(def app-state
  (atom {}))

The very last thing is to specify in index.html our script app.js (compiled to JavaScript) along with necessary dependencies.

<script src="http://fb.me/react-0.11.1.js"></script>
<script src="js/out/goog/base.js" type="text/javascript"></script>
<script src="js/app.js" type="text/javascript"></script>
<script type="text/javascript">goog.require("acme.core");</script>

Those <script> declarations should be put just before closing </body>.

Run lein up, wait till ClojureScript is successfully compiled and open localhost:3000 in your browser. You should see a list of available widgets from Acme Corp.


We have just built a very simple web application entirely in Clojure. Our backend uses Compojure, which makes it small and easy to understand. The frontend consumes /widgets in JSON format. core.async simplifies the communication, it’s either available data or timeout after 2000 milliseconds. Finally, Om helps us to build user interfaces in flexible way and using functional approach

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