Use Clojure agents to concurrently filter out elements from a collection which requires calling a remote API to know if the element should be filtered or not.

agent_concurrently_filter_api_call.clj

(defn api-pred [e]
  (Thread/sleep 100)
  (even? e))

(let [coll-to-process (range 1000)
      concurrency 100
      agents (repeatedly concurrency #(agent []))]
  (doseq [[i agnt] (map vector coll-to-process (cycle agents))]
    (send-off agnt
      #(if (api-pred i)
        (conj % i)
        %)))
  (apply await agents)
  (->> (mapv deref agents)
    (reduce into)))

;; We spawn concurrency number of agents (so 100 in this example). And then we round-robin
;; sending them the task of calling the api-pred function for each collection item and if
;; true we conj the item over the batch the agent is handling. Then we wait for all of
;; them to be done, and we reduce over their results.