On Tue, Sep 15, 2009 at 3:23 PM, Mark Volkmann <r.mark.volkm...@gmail.com> wrote: > I think the problem is that you only ensure one of the Refs. If you > want to make sure that a condition between multiple Refs isn't > violated, you need to sure
I meant "ensure" instead of "sure". > all of them. Otherwise other transactions > will be free to modify the Refs that aren't ensured. > > On Tue, Sep 15, 2009 at 3:14 PM, Krukow <karl.kru...@gmail.com> wrote: >> >> After a discussion in dcug about the write-skew anomaly, I wanted to >> write a program exhibiting the anomaly, together with a similar >> program using ensure to eliminate it. >> >> My program doesn't work: it exhibits the anomaly, but ensure doesn't >> fix it, and neither does adding a validator to the refs. I'd like to >> know if it is a bug in my program, in my understanding of 'ensure' and/ >> or validators, or, finally, in Clojure. >> >> The program simulates the write-skew example from wikipedia: there are >> two accounts a1 and a2 initially holding $100 each, and the bank >> requires that @a1 + @a2 >= 0 always. A user concurrently tries two >> transactions: one withdrawing $200 from a1 and one withdrawing $200 >> from a2. The bank code checks that @a1 + @a2 >= 200 before >> transferring any money. With serializable transaction-semantics, the >> bank's constraint is never violated; with snapshot-isolation >> semantics, it may be violated. >> >> I have four programs. The first is intended to exhibit the write-skew >> anomaly, and it works as intended: after running a few times, a state >> with @a1 = @a2 = -100 may be observed. >> >> http://paste.lisp.org/display/87117 >> >> The second is intended to elibinate the write-skew anomaly, by letting >> the changing transactions use ensure: >> >> http://paste.lisp.org/display/87117#1 >> >> The third program tries to eliminate the write-skew by using >> ref :validator instead (also ensures both refs!) >> >> http://paste.lisp.org/display/87117#2 >> >> Finally, I wrote a serialized version using an agent. As expected >> there is no write-skew. >> >> http://paste.lisp.org/display/87117#3 >> >> Help? -- R. Mark Volkmann Object Computing, Inc. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
