Unfortunately, my problem is not one where the operation is communitive. To be concrete, I am dealing with multiple threads plucking elements off the top of many different queues, and putting them into a new queue. So adding element A, then B results in line [A, B], whereas adding element B, then A results in line [B, A], which aren't equal.
Maybe I'm misunderstanding my problem, perhaps it is communitive. I just don't see it though. On Thu, Aug 1, 2013 at 9:13 AM, Timothy Baldridge <tbaldri...@gmail.com>wrote: > " > Try using an agent send from inside a transaction. Such sends are only > dispatched if the transaction commits, and successive sends to the same > agent from a single thread are run by the agent in the same order the > thread sent them." > > That's not the problem. The problem is that agent sends (just like > watchers) are dispatched outside of the transaction. So multiple threads > accessing the same refs can be releasing sends to agents at the same time. > This in turn means that the messages being sent to the agents can arrive > out-of-order since one thread could pause allowing the other to send > messages. > > Something to think about though is that you probably should be dealing > with commutative operations. So instead of sending "set bank account to X" > to refs/agents, instead send "increment bank account by 10". In this way > many of these race-conditions no longer exist. > > Timothy > > > On Thu, Aug 1, 2013 at 6:54 AM, Mike Drogalis <madrush...@gmail.com>wrote: > >> Cedric: Will agents give me the guarantee that if I have something in >> Agent A, and I want to move it to agent B, and I do this inside a >> transaction: >> - perceivers will see it in A at read point >> - perceivers will see it in B at write point >> - perceivers will never see it in neither nor both? >> >> If that's the case, which is effectively synchronous coordination (I >> think?), what do refs buy you that agents don't? >> >> >> On Thu, Aug 1, 2013 at 5:57 AM, Cedric Greevey <cgree...@gmail.com>wrote: >> >>> Try using an agent send from inside a transaction. Such sends are only >>> dispatched if the transaction commits, and successive sends to the same >>> agent from a single thread are run by the agent in the same order the >>> thread sent them. >>> >>> >>> On Wed, Jul 31, 2013 at 1:12 PM, Mike Drogalis <madrush...@gmail.com>wrote: >>> >>>> Good reasoning; that makes a lot of sense -- even if intuitively it >>>> doesn't follow through. >>>> >>>> I'll do some more thinking about my concurrency needs and come back >>>> with a follow up question later. Thanks man! >>>> >>>> >>>> On Wed, Jul 31, 2013 at 12:51 PM, Timothy Baldridge < >>>> tbaldri...@gmail.com> wrote: >>>> >>>>> Deadlocks? Imagine a watch that ended up running a transaction against >>>>> the ref it was triggered from. Sounds insane, but it could be fairly easy >>>>> to do in a situation like this: >>>>> >>>>> ref -> calls watchers -> calls fn1 -> calls fn 2 -> calls fn3 -> >>>>> starts transaction on ref >>>>> >>>>> Since watchers are notified outside of a ref transaction deadlocking >>>>> is impossible. Inside a transaction you'd have a instant deadlock. >>>>> >>>>> Timothy Baldridge >>>>> >>>>> >>>>> On Wed, Jul 31, 2013 at 10:47 AM, Mike Drogalis >>>>> <madrush...@gmail.com>wrote: >>>>> >>>>>> I'll play around with agents for this when I get some more free time. >>>>>> >>>>>> I find it odd that, as the sole perceiver of an indentity, events are >>>>>> capable of being perceived out of order. >>>>>> If I were watching a line queue up in person, events are obviously >>>>>> "dispatched" in order through my visual perception. >>>>>> I trust there's a good reason it's designed this way, at any rate. >>>>>> >>>>>> >>>>>> On Wed, Jul 31, 2013 at 12:25 PM, Timothy Baldridge < >>>>>> tbaldri...@gmail.com> wrote: >>>>>> >>>>>>> You might want to consider switching to agents (or something else) I >>>>>>> don't think it's possible to do what you want with refs. >>>>>>> >>>>>>> Timothy Baldridge >>>>>>> >>>>>>> >>>>>>> On Wed, Jul 31, 2013 at 10:08 AM, Mike Drogalis < >>>>>>> madrush...@gmail.com> wrote: >>>>>>> >>>>>>>> Thanks for the link. :) I understand that the behavior I'm seeing >>>>>>>> is correct. Any idea how to achieve the desired behavior, though? >>>>>>>> >>>>>>>> >>>>>>>> On Wed, Jul 31, 2013 at 12:06 PM, Timothy Baldridge < >>>>>>>> tbaldri...@gmail.com> wrote: >>>>>>>> >>>>>>>>> The answer is here: >>>>>>>>> >>>>>>>>> >>>>>>>>> https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/LockingTransaction.java#L361 >>>>>>>>> >>>>>>>>> Agents and watches are dispatched outside of the transaction >>>>>>>>> locking. This means that multiple transactions could be executing >>>>>>>>> watches >>>>>>>>> in parallel, and hence execute out of order. >>>>>>>>> >>>>>>>>> Timothy Baldridge >>>>>>>>> >>>>>>>>> >>>>>>>>> On Wed, Jul 31, 2013 at 9:43 AM, Michael Drogalis < >>>>>>>>> madrush...@gmail.com> wrote: >>>>>>>>> >>>>>>>>>> I can precisely exemplify the behavior here: >>>>>>>>>> >>>>>>>>>> https://gist.github.com/MichaelDrogalis/6123177 >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> On Wednesday, July 31, 2013 11:13:17 AM UTC-4, Michael Drogalis >>>>>>>>>> wrote: >>>>>>>>>>> >>>>>>>>>>> Aaron: Yep, I'm aware - and am using the value provided by the >>>>>>>>>>> last parameter. >>>>>>>>>>> >>>>>>>>>>> This is going to be tough to show the problem without bringing >>>>>>>>>>> more details of my concurrency set up. >>>>>>>>>>> I'm not sure if this will exhibit the problem, but this is what >>>>>>>>>>> it boils down to: >>>>>>>>>>> >>>>>>>>>>> https://gist.github.com/**MichaelDrogalis/6122834<https://gist.github.com/MichaelDrogalis/6122834> >>>>>>>>>>> >>>>>>>>>>> When enough threads are trying to write to the queue, the watches >>>>>>>>>>> triggered later can finish before watches triggered earlier. >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> On Wednesday, July 31, 2013 11:05:16 AM UTC-4, Aaron Cohen wrote: >>>>>>>>>>>> >>>>>>>>>>>> A watcher fn has 4 parameters: key, reference, old-state, >>>>>>>>>>>> new-state >>>>>>>>>>>> >>>>>>>>>>>> If you use old-state and new-state rather than the reference, >>>>>>>>>>>> you should not see your problem. >>>>>>>>>>>> >>>>>>>>>>>> --Aaron >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> On Wed, Jul 31, 2013 at 11:00 AM, Michael Drogalis < >>>>>>>>>>>> madru...@gmail.com> wrote: >>>>>>>>>>>> >>>>>>>>>>>>> Problem: >>>>>>>>>>>>> >>>>>>>>>>>>> I have a ref representing a queue of people in line. >>>>>>>>>>>>> I add a watch to the ref to print out the contents of the >>>>>>>>>>>>> queue whenever it changes. >>>>>>>>>>>>> Naturally, and expected, the following can happen if queuing >>>>>>>>>>>>> happens in rapid succession: >>>>>>>>>>>>> >>>>>>>>>>>>> Queue: [] >>>>>>>>>>>>> <add "Mike" to queue> >>>>>>>>>>>>> <add "John" to queue> >>>>>>>>>>>>> >>>>>>>>>>>>> console: "Queue is Mike, John" >>>>>>>>>>>>> console: "Queue is Mike" >>>>>>>>>>>>> >>>>>>>>>>>>> I'd like to write a UI for this program, but I clearly can't >>>>>>>>>>>>> reliably render based on the result delivered >>>>>>>>>>>>> by the add-watch hook. What's the solution for this problem? >>>>>>>>>>>>> >>>>>>>>>>>>> -- >>>>>>>>>>>>> -- >>>>>>>>>>>>> You received this message because you are subscribed to the >>>>>>>>>>>>> Google >>>>>>>>>>>>> Groups "Clojure" group. >>>>>>>>>>>>> To post to this group, send email to clo...@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+u...@googlegroups.com >>>>>>>>>>>>> For more options, visit this group at >>>>>>>>>>>>> http://groups.google.com/**group/clojure?hl=en<http://groups.google.com/group/clojure?hl=en> >>>>>>>>>>>>> --- >>>>>>>>>>>>> You received this message because you are subscribed to the >>>>>>>>>>>>> Google Groups "Clojure" group. >>>>>>>>>>>>> To unsubscribe from this group and stop receiving emails from >>>>>>>>>>>>> it, send an email to clojure+u...@googlegroups.com. >>>>>>>>>>>>> For more options, visit https://groups.google.com/** >>>>>>>>>>>>> groups/opt_out <https://groups.google.com/groups/opt_out>. >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> -- >>>>>>>>>> -- >>>>>>>>>> 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 - 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