ref-count only increment when one of the following occurs:
* min-history > 0
* ref-read-faults > 0 && current-ref-history-count < max-history-count.
;; ref-count increment because min-history > 0
(let [r (ref 1 :min-history 1)
f1 (future (dosync (Thread/sleep 1000)
(println "ref-count: " (ref-history-count r))
@r))
f2 (future (dosync (ref-set r 2)))]
[@f1 @f2])
;; ref-count increment because there's read faults
(let [r (ref 1)
f1 (future (dosync (Thread/sleep 1000)
(println "ref-count: " (ref-history-count r))
@r))
f2 (future (dosync (ref-set r 2)))
f3 (future (dosync (Thread/sleep 1500) (ref-set r 2)))]
[@f1 @f2])
Checkout related clojure source here:
https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/LockingTransaction.java#L321-L326
On 2013-5-30, at 下午10:05, Josh Kamau <[email protected]> wrote:
> (def my-ref (ref 1))
>
>
>
> (defn update-my-ref
> [new-value]
> (dosync
> (alter my-ref #(+ % new-value))
> (ref-history-count my-ref)))
>
> Hi guys ;
>
> I am trying to understand (ref-history-count ref) . I thought it counts the
> number of "old" values in the ref. But in all my tests, its always returning
> zero. How is it used?
>
> Josh
>
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