Ring + reitit Clojure

(ns pixel.core
  (:require [reitit.ring :as ring]
            [ring.adapter.jetty :as jetty])
  (:import (java.util Base64)))

(def gif (.decode (Base64/getDecoder) "R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw=="))

(def app
  (ring/ring-handler
   (ring/router
    ["/pixel.gif" {:get (fn [_]
                        {:status 200
                         :headers {"Content-Type" "image/gif"
                                   "Content-Length" (str (alength gif))
                                   "Cache-Control" "no-store"}
                         :body gif})}])
   (ring/create-default-handler)))

(defn -main []
  (jetty/run-jetty app {:port 8080 :join? true}))