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Ring-Websocket-Async

A Clojure library for using core.async with Ring's websocket API (currently in beta testing).

Installation​

Add the following dependency to your deps.edn file:

org.ring-clojure/ring-websocket-async {:mvn {"0.1.0-beta2"}}

Or to your Leiningen project file:

[org.ring-clojure/ring-websocket-async "0.1.0-beta2"]

Usage​

The most convenient way to get started is to use the go-websocket macro. Here's an example that echos any message received back to the client:

(require '[clojure.core.async :as a :refer [<! >!]]
         '[ring.websocket.async :as wsa])

(defn echo-websocket-handler [request]
  (wsa/go-websocket [in out err]
    (loop []
      (when-let [mesg (<! in)]
        (>! out mesg)
        (recur)))))

The macro sets three binding variables:

  • in - the input channel
  • out - the output channel
  • err - the error channel

Then executes its body in a core.async go block. When the block completes, the websocket is closed by the server. If the client closes the websocket, the associated channels are also closed.

A Ring websocket response will be returned by the macro, so you can directly return it from the handler.

The error channel may be omitted. In this case, any errors with the websocket protocol will close the connection with a 1011 unexpected server error message.

To close the connection from the server with a specific error code of your choice, you can use the closed function to send a special message to the output channel:

(defn closes-with-error [request]
  (wsa/go-websocket [in out err]
    (>! out (wsa/closed 1001 "Gone Away"))))

If you want more control, you can use the lower-level websocket-listener function. The following handler example is equivalent to the one using the go-websocket macro:

(defn echo-websocket-handler [request]
  (let [in  (a/chan)
        out (a/chan)
        err (a/chan)]
    (go (try
          (loop []
            (when-let [mesg (<! in)]
              (>! out mesg)
              (recur)))
          (finally
            (a/close! out))))  ;; closes the websocket
    {:ring.websocket/listener (websocket-listener in out err)}))

License​

Copyright © 2023 James Reeves

Released under the MIT license.