A lightweight Clojure/ClojureScript library designed to bridge the gap between the triad of CLJ/CLJS, web-sockets and core.async.

Include the following in your project.clj:

[jarohen/chord "0.8.1"]

Chord now supports EDN, JSON, Transit and Fressian out of the box - please remove dependencies to ‘chord-fressian’ and ‘chord-transit’ if you have them. Thanks to Luke Snape, Jeff Rose and Thomas Getgood for their help supporting these formats!

There is a simple example server/client project under the example-project directory. The client sends websocket messages to the server, that get echoed back to the client and written on the page.

You can run it with lein dev - an alias that starts up an http-kit server using Frodo and automatically re-compiles the CLJS.

Once it is running - navigate to http://localhost:3000/ and you should see Send a message to the server:

Chord only has one function, chord.client/ws-ch, which takes a web-socket URL and returns a map, containing either :ws-channel or :error. When the connection opens successfully, this channel then returns a two-way channel that you can use to communicate with the web-socket server:

(:require [chord.client :refer [ws-ch]]
          [cljs.core.async :refer [<! >! put! close!]])
(:require-macros [cljs.core.async.macros :refer [go]])

(go
  (let [{:keys [ws-channel error]} (<! (ws-ch "ws://localhost:3000/ws"))]
    (if-not error
      (>! ws-channel "Hello server from client!")
      (js/console.log "Error:" (pr-str error)))))

Messages that come from the server are received as a map with a :message key:

(go
  (let [{:keys [ws-channel]} (<! (ws-ch "ws://localhost:3000/ws"))
        {:keys [message]} (<! ws-channel)]
    (js/console.log "Got message from server:" (pr-str message))))

Errors in the web-socket channel (i.e. if the server goes away) are returned as a map with an :error key:

(go
  (let [{:keys [ws-channel]} (<! (ws-ch "ws://localhost:3000/ws"))
        {:keys [message error]} (<! ws-channel)]
    (if error
      (js/console.log "Uh oh:" error)
      (js/console.log "Hooray! Message:" (pr-str message)))))

As of 0.3.0, you can pass a :format option, to pass messages over the channel as EDN (default), as raw strings, or JSON (0.3.1). Valid formats are #{:edn :json :json-kw :str :fressian :transit-json}, defaulting to :edn.

(If you do use fressian, you’ll need to require chord.format.fressian, in addition to the usual Chord namespaces)

(:require [cljs.core.async :as a])
(ws-ch "ws://localhost:3000/ws"
       {:format :json-kw})

As of 0.2.1, you can configure the buffering of the channel by (optionally) passing custom read/write channels, as follows:

(:require [cljs.core.async :as a])
(ws-ch "ws://localhost:3000/ws"
       {:read-ch (a/chan (a/sliding-buffer 10))
        :write-ch (a/chan 5)})

By default, Chord uses unbuffered channels, like core.async itself.

Chord wraps the websocket support provided by http-kit, a fast Clojure web server compatible with Ring.

N.B. Currently, Ring’s standard Jetty adapter ~does not~ support Websockets. http-kit is a Ring-compatible alternative.

Again, there’s only one entry point to remember here: a wrapper around http-kit’s with-channel macro. The only difference is that, rather than using http-kit’s functions to interface with the channel, you can use core.async’s primitives.

Chord’s with-channel is used as follows:

(:require [chord.http-kit :refer [with-channel]]
          [org.httpkit.server :refer [run-server]]
          [clojure.core.async :refer [<! >! put! close! go]])

(defn your-handler [req]
  (with-channel req ws-ch
    (go
      (let [{:keys [message]} (<! ws-ch)]
        (prn "Message received:" message)
        (>! ws-ch "Hello client from server!")
        (close! ws-ch)))))

This can take a :format option, and custom buffered read/write channels as well:

(require '[clojure.core.async :as a])

(defn your-handler [req]
  (with-channel req ws-ch
    {:read-ch (a/chan (a/dropping-buffer 10))
         :format :str} ; again, :edn is default
    (go
      (let [{:keys [message]} (<! ws-ch)]
        (prn "Message received:" message)
        (>! ws-ch "Hello client from server!")
        (close! ws-ch)))))

You can also use the wrap-websocket-handler middleware, which will put a :ws-channel key in the request map:

(require '[chord.http-kit :refer [wrap-websocket-handler]]
         '[org.httpkit.server :refer [run-server]]
         '[clojure.core.async :as a])

(defn your-handler [{:keys [ws-channel] :as req}]
  (go
    (let [{:keys [message]} (<! ws-channel)]
      (println "Message received:" message)
      (>! ws-channel "Hello client from server!")
      (close! ws-channel))))

(run-server (-> #'your-handler wrap-websocket-handler) {:port 3000})

You can pass custom channels to wrap-websocket-handler as a second (optional) parameter:

(run-server (-> #'your-handler
              (wrap-websocket-handler {:read-ch ...}))
            {:port 3000})

Yes please! Please submit these in the traditional GitHub manner.

Chord’s contributors are listed in the ChangeLog - thank you all for your help!

Copyright © 2013-2015 James Henderson

Distributed under the Eclipse Public License, the same as Clojure.