Implementing a cron type scheduler in clojure

Question

I'm looking for any way for clojure that can trigger an event at a given time,

For example: I want a particular process to start at 9:30am and then I can trigger another process to start running half an hour later etc.

Thanks in advance!

Updated 2:

Thanks @arthur-ulfeoldt and @unknown-person who also suggested using https://github.com/samaaron/at-at before deleting his answer. The documentation is a little out of date but here's how I got going.

(use 'overtone.at-at)

(def my-pool (mk-pool))
;=> make a thread pool

(every 1000 #(println "I am super cool!") my-pool :initial-delay 2000) 
;=> starts print function every 1 sec after a 2 sec delay

(stop *1) 
;=> stops it

So to make it start at exactly 9, with an interval of half an hour, I would do:

(require '[clj-time.core :as t])
(require '[clj-time.coerce :as c])
(use 'overtone.at-at)

;Make Thread Pool
(def my-pool (mk-pool))

(def current-time (t/now))

(def current-date (t/date-time 
                    (t/year current-time)
                    (t/month current-time)
                    (t/day current-time)))

(def next-9-oclock
  (if (> 9 (t/hour current-time))
    (t/plus current-date (t/hours 9))
    (t/plus current-date (t/days 1) (t/hours 9))))

(def initial-delay
   (- (c/to-long next-9-oclock) (c/to-long current-time))

(every 1800000 
       #(println "I am super cool!") 
       my-pool 
       :initial-delay 
       initial-delay)

Updated:

@arthur-ulfeoldt, I am not sure how to translate some of the java code into clojure. http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ScheduledExecutorService.html

like:

final ScheduledFuture<?> beeperHandle = 
        scheduler.scheduleAtFixedRate(beeper, 10, 10, SECONDS);

and:

final Runnable beeper = new Runnable() {
        public void run() { System.out.println("beep"); }
     };

Entire Example

 class BeeperControl {
    private final ScheduledExecutorService scheduler = 
       Executors.newScheduledThreadPool(1);

    public void beepForAnHour() {
        final Runnable beeper = new Runnable() {
                public void run() { System.out.println("beep"); }
            };
        final ScheduledFuture beeperHandle = 
            scheduler.scheduleAtFixedRate(beeper, 10, 10, SECONDS);
        scheduler.schedule(new Runnable() {
                public void run() { beeperHandle.cancel(true); }
            }, 60 * 60, SECONDS);
    }
 }

zcaudate zcaudate · Accepted Answer · 2012-07-28T02:09:09

I needed something that

started scheduled tasks at whole-second intervals as opposed to whole minute intervals having high system-time accuracy.
would spawn as many threads as needed, so that tasks started at earlier intervals could exist along side tasks started at later intervals.

After doing a bit of source code reading for at-at, Monotony and Quartzite, I felt that they did not fit the requirements that I was after (which was really something more bare bone) so I wrote my own - cronj

An example of the usage.

(require '[cronj.core :as cj])

(cj/schedule-task! {:id 0   :desc 0 
              :handler #(println "job 0:" %) 
              :tab "/5 * * * * * *"}) ;; every 5 seconds
(cj/schedule-task! {:id 1   :desc 1 
              :handler #(println "job 1:" %) 
              :tab "/3 * * * * * *"}) ;; every 3 seconds

(cj/start!) 

;; wait for scheduler to do its thing ......

(cj/stop!)

Implementing a cron type scheduler in clojure

Updated 2:

Updated:

Entire Example

3 Answers