STM demo

stm-demo.clj

(ns stm-demo 
     (:import (java.util.concurrent TimeUnit Executors ScheduledExecutorService)))

;; concurrency basics

(def scheduler (Executors/newScheduledThreadPool 3))

scheduler

(class scheduler)

(defn say-hello [] 
	(println (str "Hello from " (Thread/currentThread))))

(def hello-task
	(.scheduleWithFixedDelay scheduler say-hello (long 0) (long 1) TimeUnit/SECONDS))

(.cancel hello-task true)


;; getting into STM

(def account1 (ref 100))
(def account2 (ref 500))

(deref account1)

@account1
@account2

; uncomment the following to see what happens 
; when attempting ref-set outside a transaction
;(ref-set account1 200)

(dosync (ref-set account1 200))

@account1

(defn print-balance [ref name]
	 (println (str (Thread/currentThread) " says " name " has balance of " (deref ref))))

(defn print-acct1-balance []
	(print-balance account1 "account1"))

(def acct1-balance-task 
	(.scheduleWithFixedDelay scheduler print-acct1-balance (long 0) (long 1) TimeUnit/SECONDS))

(defn kill-annoying-balance-task []
	(.cancel acct1-balance-task true)
	(println "Killed annoying task"))

; kills that inquiry task after 20 seconds
(.schedule scheduler kill-annoying-balance-task (long 10) TimeUnit/SECONDS)

(dosync
	(ref-set account1 1000000)
	(print-balance account1 "account1")
	(Thread/sleep 3000)
	(print-balance account1 "account1")
	(throw (RuntimeException. "Insufficient funds")))

Sample output is

user=> (load-file "stm.clj")
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[Thread-3,5,main] says account1 has balance of 1000000
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[Thread-3,5,main] says account1 has balance of 1000000
java.lang.RuntimeException: Insufficient funds (stm.clj:5)
user=>
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Thread[pool-4-thread-2,5,main] says account1 has balance of 200
Killed annoying task
user=>