A few thoughts on ways to improve this code:
Using agents and go blocks in the same code is a bit odd. If you need a
queue plus a process, just do something like this:
(defn faux-agent [c]
(go (loop []
(when-let [msg (<! c)]
...do something...
(recur)))))
This go block will automatically terminate once the input channel is
closed. Now you have code that is almost like an agent, but works well with
channels (including back-pressure).
Also, instead of having a public req channel and a response channel,
consider using higher-order channels. Yes, you can send channels via
channels. So instead of having a request look like this:
(>! req-chan :req)
do something like this:
(>! req-chan [file-name my-response-channel])
(<! my-response-channel)
This allows many processes to make requests and your code will end up a
little easier to reason about.
Just some thoughts.
Timothy Baldridge
On Mon, Jul 8, 2013 at 12:05 AM, vemv <v...@vemv.net> wrote:
> As you can read here:
> http://martintrojer.github.io/clojure/2013/07/07/coreasync-and-blocking-iodoing
> 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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