Question

I put together a simple socket server (see below). Currently, it is not capable of handling multiple/concurrent requests. How can I make the socket server more efficient — i.e. capable of handling concurrent requests? Are there any clojure constructs I can leverage? Thus far, I’ve thought of using either java’s NIO (instead of IO) or netty (as pointed out here).

(ns server.policy
    (:import 
        (java.net ServerSocket SocketException)
        java.io.PrintWriter))

    (defn create-socket
        "Creates a socket on given port."
        [port]
        (ServerSocket. port))

    (defn get-writer
        "Create a socket file writer."
        [client]
        (PrintWriter. (.getOutputStream client)))

    (defn listen-and-respond
        "Accepts connection and responds."
        [server-socket service]
        (let [client (.accept server-socket)
              socket-writer (get-writer client)]
            (service socket-writer)))

    (defn policy-provider
        "Returns domain policy content."
        [socket-writer]
        (.print socket-writer "<content>This is a test</content>")
        (.flush socket-writer)
        (.close socket-writer))

    (defn run-server
        [port]
        (let [server-socket (create-socket port)]
            (while (not (.isClosed server-socket))
                (listen-and-respond server-socket policy-provider))))

Answer 1

I have had success using Netty directly. However, if you want something that feels a little more like idiomatic Clojure code, take a look at the aleph library. It uses Netty internally, but results in much more simple code:

(use 'lamina.core 'aleph.tcp)

(defn echo-handler [channel client-info]
  (siphon channel channel))

(start-tcp-server echo-handler {:port 1234})

Also, keep in mind that sometimes you need to reference the lamina documentation in addition to the aleph documentation.