My understanding of commute is that it would not restart the transaction, it
would just apply the function. So I wrote this little test program:
(defmacro in-thread [& body]
`(.start
(Thread.
(fn []
(println "Thread" (.getId (Thread/currentThread)) " started")
~...@body
(println "Thread" (.getId (Thread/currentThread)) " finished")))))
(def numbers (ref []))
(defn conj-count []
(dosync (commute numbers conj (inc (count @numbers)))))
(defn conj-count-later []
(dosync (println "Starting Transaction...")
(Thread/sleep 10000)
(conj-count)))
The in-thread macro just runs what you give it in a thread, printing at the
beginning and the end so you know when the thread finishes. The conj-count
function just conjoins a number onto the numbers vector, the number being
one more than the current size of the vector. Here's what happens when I
run it at the repl:
user=> #'user/numbers
user=> #'user/conj-count
user=> #'user/conj-count-later
user=> (in-thread (conj-count-later))
nil
user=> Thread 21 started
Starting Transaction...
user=> (conj-count)
[1]
user=> Starting Transaction...
Thread 21 finished
user=> @numbers
[1 2]
Once I call conj-count-later in in-thread, that starts a transaction in a
thread what will wait 10 seconds and call conj-count. While that thread is
running, in the meantime I call conj-count directly, so the ref is modified.
What happens next is that you can see "Starting Transaction..." print again
for the second time, which means the transaction is executing for a second
time. Next I don't call conj-count again, I just wait for the thread to
finish, which it does without restarting.
But since all conj-count-later does is call commute (by calling conj-count),
why does the transaction restart? If conj-count called alter instead of
commute, that would be the behavior I would expect, but I guess I don't
quite understand how dosync and/or commute works.
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