---

id: example title: Practical Example App sidebar_label: Example App slug: /example-app

More practical example

You can't do much with static nodes. GX supports components, a component is a node with custom signal handlers.

Lets create an app config

In app config need:

• HTTP server options
• routes (example only, better to define routes inside code and add them as a component)
• router component
• handler component
• server component

{:http/options {:port 8080}

 :http/routes ["/users"
               {:get
                ;; existing functions should be referenced with fully
                ;; qualified name
                {:handler app/get-users-handler}}]

 :http/ring-router {;; component is a fully qualified name of defined component
                    :gx/component components/ring-router
                    ;; props of a component
                    :gx/props {:routes (gx/ref :http/routes)}}

 :http/ring-handler {:gx/component components/ring-handler
                     :gx/props {:router (gx/ref :http/router)}}

 :http/server {:gx/component components/http-server
               :gx/props {:handler (gx/ref :http/ring-handler)
                          :options (gx/ref :http/options)}}

 ;; pseudo logger component, logs port by taking it from :http/options
 :app (println "Application is starting on port " (get (gx/ref :http/options)
                                                       :port))
 ;; independend component
 :another {:gx/start (println "another starting")
           :gx/stop (println "another stopping")}}

Our app is a simple web server with one route /users. You may noticed a new keyword :gx/component which is used for including predefined components. There are two different types of gx references available in config:

type	description	example
gx/ref	gets node's value by key	(gx/ref :a) => {:foo 1}
gx/ref-keys	runs select-keys on graph	(gx/ref-keys [:a :b]) => {:a {:foo 1} :b {:bar 1}}

Alright, let's write app code:

(ns app)

;; /users route handler
(defn get-users-handler
  [_args]
  {:status 200
   :content-type "text/plain"
   :body (pr-str [{:user "John"}
                  {:user "Peter"}
                  {:user "Donald"}])})

(ns components
  (:require [reitit.ring :as reitit-ring]
            [org.httpkit.server :as http-kit]))

(def ring-router
  {:gx/start
   {;; :gx/processor contains signal function
    ;; processor function must accept map with two keys
    ;; :props - resolved props
    ;; :value - current node's value
    ;; data returned by handler is a node's new value
    :gx/processor (fn start-router [{{:keys [routes]} :props}]
                    (reitit-ring/router routes))}})

(def ring-handler
  {:gx/start
   {:gx/processor (fn start-router [{{:keys [router]} :props}]
                    (reitit-ring/ring-handler router))}})

(def http-server
  {:gx/start
   {;; incoming props will be validated against malli chema
    ;; during signal application
    :gx/props-schema [:map
                      [:handler fn?]
                      [:options :map]]
    :gx/processor (fn [{{:keys [handler options]} :props}]
                    (http-kit/run-server handler options))}

   :gx/stop {:gx/processor (fn [{server :value}]
                             (server))}})

We defined handler function, routes, ring router and server components and they are all glued by our config.edn file.

Sweet, our app is ready. Next we add some boring system routines:

(ns main
  (:require [app]
            [clojure.edn :as edn]
            [k16.gx.beta.core :as gx]
            ;; this ns contains helpers for managing systems
            [k16.gx.beta.system :as gx.system]))

(defn load-system! []
  ;; register our system, so later we can get it by name
  (gx.system/register!
   ::system
   ;; :context key is optional, fallbacks to gx/default-context
   {:context gx/default-context
    :graph (edn/read-string (slurp "resources/config.edn"))}))

(defn start! []
  (gx.system/signal! ::system :gx/start))

(defn stop! []
  (gx.system/signal! ::system :gx/stop))

(defn reset! []
  (stop!)
  (load-system!)
  (start!))

(defn failures []
  (gx.system/failures-humanized ::system))

(defn -main [& _args]
  (load-system!)

  (doto (Runtime/getRuntime)
    (.addShutdownHook
     (Thread. #(stop!))))

  (start!)

  ;; we don't want semi-started system on production
  ;; check for failures, print them and exit if any
  (when-let [failures (seq (failures))]
    (doseq [failure failures]
      (println failure))
    (System/exit 1))

  @(promise))

Cool, run the app from terminal

clj -M:main

Open your browser: http://localhost:8080/users. Viola! In the next step, we will add a database component.

Full source code is available on github.