ref-history-stress.clj

; STM history stress-test

(defn stress [hmin hmax] 
  (let [r (ref 0 :min-history hmin :max-history hmax)
        slow-tries (atom 0)]
    (future
      (dosync
        (swap! slow-tries inc)
        (Thread/sleep 200)
        @r)
      (println "a is:" @r "history:" (.getHistoryCount r)
               "after:" @slow-tries "tries"))
    (dotimes [i 500]
      (Thread/sleep 10)
      (dosync (alter r inc)))
    :done))


(stress 0 10) ; default ref settings
;=> :done
;   a is: 500 history: 10 after: 26 tries
; This indicates that the slow transaction never succeeded until the dotimes
; loop was complete (and returned :done)

(stress 0 30)
;   a is: 415 history: 19 after: 21 tries
;=> :done
; Here the slow transaction completed first, after growing the history to 19.
; Success!  But it took 21 tries to grow the history from 0, so we can help out
; by starting with a larger min-history:

(stress 15 30)
;   a is: 119 history: 20 after: 6 tries
;=> :done
; Only 6 retries of the slow transaction this time, enough to grow the history
; from 15 to 20.