Executing code at regularly timed intervals in Clojure

Question

What's the best way to make code run at regular intervals in Clojure ? I'm currently using java.util.concurrent.ScheduledExecutorService, but that's Java - is there a Clojure way of scheduling code to run at regular intervals, after a delay, cancellably ? All the Clojure code examples I've seen use Thread/sleep, which also seems too Java.

Answer 1

Well worth looking at the source code for Overtone, in particular the code for scheduling events at a particular time.

It's a music synthesis system so you have to hope they have the timing code right!!

Also they have helpfully separated the timing code out into a separate project (overtone/at-at) so that you can easily import it if you want. This provides a nice Clojure wrapper to the underlying Java libraries (i.e. ScheduledThreadPoolExecutor and friends). The syntax is like this:

;; run some-function every 500ms
(every 500 some-function)

You can also shedule events at specific times:

;; run some-other-function 10 seconds from now
(at (+ 10000 (now)) some-other-function)

Answer 2

From the clojure website http://clojure.org/concurrent_programming:

In all cases, Clojure does not replace the Java thread system, rather it works with it. Clojure functions are java.util.concurrent.Callable, therefore they work with the Executor framework etc.

It sounds like you are already doing it the right way.

(import 'java.util.concurrent.Executors)
(import 'java.util.concurrent.TimeUnit) 
(.scheduleAtFixedRate (Executors/newScheduledThreadPool 1) 
  #(println "Hello") 0 5 TimeUnit/SECONDS)

Answer 3

I answered my own question @ Implementing a cron type scheduler in clojure

maybe cronj might help?