Answer by Jarlax for Race condition in Clojure stm?

Let's look at example:

(defn test-trans []  (let [x (ref 1)        t-inc #(dosync (alter x inc))        t-mul #(dosync (alter x (partial * 2)))        fns (flatten (repeat 10 [t-mul t-inc]))]    (last (pmap (fn [f] (f)) fns))    @x))

Here we have 2 transactional functions - increase x by 1 and multiply x by 2. We apply 20 such functions (10 of each kind) in parallel and observe final value of ref. Indeed results are different for each run:

=> (test-trans)2418=> (test-trans)2380=> (test-trans)1804=> (test-trans)4210

Actually this is correct behaviour. STM guarantees that code will be executed without locks and changes are applied atomically (they cannot be applied only partially). However it does not guarantee that we will have the same result for different order of transactions.

Clojure provides great parallel programming instruments which simplify writing of correct code a lot. But avoiding this kind of race conditions is responsibility of developer (in fact, such cases are clear sign of bad system design).

Another example from SQL:

DELETE FROM tbl WHERE col=1UPDATE tbl SET col=2 WHERE col=1

If these queries are executed in parallel, then no matter what isolation level will be used for transactions - result will depend on the order.