On Mar 15, 1:43 pm, Michał Marczyk <michal.marc...@gmail.com> wrote: > On 15 March 2010 21:08, Meikel Brandmeyer <m...@kotka.de> wrote: > > > Now I'm confused. Calling ensure on r shouldn't have an effect since we > > call alter on r anyway, no? > > ensure "protects the ref from modification by other transactions" > (from the docs). alter does not. > > Reading into the Java code, ensure puts a lock on the ref, which, once > in place, guarantees that the transaction doing the ensuring has an > exclusive right to modify the ref until it commits / retries... or > something, my Java-fu is still nothing to boast about, regrettably. > > At any rate, my current understanding is that, in Garth's example, the > ensure gives (alter r f) all the time it needs to modify r's value > while putting all other transactions which attempt to modify r on > hold. alter, by itself, never interferes with background transactions; > should something disappear from under its feet, it expects to be > retried. > > Ok, back to improving my Java chops in the hope of grasping all the > intricasies of Rich's code sometime... *sigh* > > Sincerely, > Michał
I'm inclined to say this is incorrect as I'm on my iphone so I can't look at the source. The concurrency functions (e.g., ref-set, alter, ensure) only lock their refs during the commit process. The ensure function is provided to add a *non-changing* ref to the set of refs that need to be locked; ref-set, etc., do this implicitly. To lock the refs upon first use would largely obviate the point of the STM. The issue Garth describes is a case of live-locking, an extant failure mode of the STM. Some solutions would be to break up the work from just a single transaction (though sacrificing consistency), or use the locking construct: http://richhickey.github.com/clojure/clojure.core-api.html#clojure.core/locking -- 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
