As you can read here:
http://martintrojer.github.io/clojure/2013/07/07/coreasync-and-blocking-io
doing blocking IO in a go block should be avoided. So I was thinking that
an alternative to non-blocking IO APIs is using agents and channels.
The following sample program intends to illustrate the technique:
(require '[clojure.core.async :as async :refer :all])
(import '[java.util.concurrent Executors])
(spit "bar" (apply str (shuffle (range 100))))
(def slurper (agent nil))
(def spitter (agent nil))
(def max-concurrency 1)
;; when a go block wants to slurp a file, it communicates it by writing to
thins channel
(def slurping-request (chan max-concurrency))
;; where requested slurped content gets served
(def slurps (chan max-concurrency))
(def rw-pool (Executors/newFixedThreadPool max-concurrency))
(def serving
(future
(while (<!! slurping-request)
(send-via rw-pool slurper (fn [_] (>!! slurps (seq (slurp "bar")))
_)))))
(go (while true
(>! slurping-request :req)
(let [old (<! slurps)
new (apply str (shuffle old))]
(send-via rw-pool spitter (fn [_] (spit "bar" new))))))
It is supposed to constantly modify a file, until you (close!
slurping-request). However, somehow nil-related issues crept in:
- With max-concurrency set to 1, at times an empty string gets written
to the file.
- With max-concurrency set to 4, the spitter agent fails.
What is going on? Maybe files need locking? Or am I misunderstanding some
aspect of core.async itself?
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