work load balancing in servant: http://marcopolo.io/2013/10/01/servant-cljs.html

servant-load.clj

(go 
  ;; Get a worker from the buffered channel
  (let [worker (<! servant-channel)]
    ;; execute the post message for the web worker
    (post-message-fn worker (pr-str fn-key) args)
    ;; Add an event listener for the worker's return message
    (.addEventListener worker "message"
       #(go 
          ;; Save the data
          (>! out-channel (.-data %1))
          ;; return the worker back to the servant-channel
          (>! servant-channel worker)))))

When there are no more workers available in the pool, the code will "block" until a worker is free.