On Sunday, October 5, 2014 5:33:16 PM UTC+2, Nahuel Greco wrote: > > Picture the following: > > producer ---> go-loop ---> external service > > 1- The producer puts a value to a unbuffered (chan) by doing (>! c v) > 2- The go-loop consumes the value with a take operation, **unblocking** > the producer > 3- The go-loop contacts the external-service but the external service > answers it can't process the value yet > 4- The go-loop waits some timeout to retry the request to the external > service > > After step 2 the producer continues to compute (suppose an expensive > computing) a new value but the previous one wasn't effectively consumed by > the external service. > I don't want that, I want to enforce an end-to-end flow-control setup > where the producer blocks on (>! c v) (the step 1) until the value is > consumed by all parties, > If producing the next value is expensive, why would you want to delay it from point 2 to after point 4? Once the external service has processed the current value, you want the next value available as soon as possible, wouldn't you?
> Sure, this flow control can be solved adding an ack channel and sending an > ack from the go-loop to the producer when the external service effectively > consumes the value, previously blocking the producer after step 1 waiting > that ack. > But I think a peek operation in step 2 will be more elegant. Also, I was > curious if the implementation of core.async channels limits in some way > adding a peek operation. > > A take-if with a pure predicate can't solve this, because you need to > contact the external service to decide to consume the value or not. > > > Saludos, > Nahuel Greco. > > On Sun, Oct 5, 2014 at 9:52 AM, Fluid Dynamics <a209...@trbvm.com > <javascript:>> wrote: > >> On Sunday, October 5, 2014 12:51:04 AM UTC-4, Nahuel Greco wrote: >>> >>> I was thinking in a single-consumer scenario with a buffered chan, in >>> which you want to check if you can consume the value before effectively >>> consuming it. As you said, a peek operation has no sense if the channel has >>> multiple consumers. >>> >> >> And if you can't consume the value, then what? Nothing ever does, and >> that channel becomes useless? >> >> Actually the only "peek" operation that to me makes much sense would be a >> (take-if pred chan) or something similar, which atomically tests the next >> value with pred and consumes it or not, so, it can't be consumed elsewhere >> between the pred test and optional consumption here. And if not consumed, >> two behaviors both occur to me as possible -- return nil or some other >> sentinel value for "do not want" or block until the unwanted object is >> consumed by someone else and then test the next item, etc.; at which point >> you've got four versions of take-if you'd want, the inside-go and >> outside-go versions cross product with the two when-not-wanted behaviors. >> >> At that point, you'd probably be better off just writing a consumer that >> splits off the pred-matching items into one out channel and feeds >> everything else into a second channel, with your original consumer taking >> from the first of these and the others taking from the second. That gets >> you the block until version of the behavior. The other version can be had >> by making the pred-using consumer the sole consumer of the in channel, >> which takes a value, applies pred, and branches, on the "want" branch doing >> whatever and on the "do not want" branch putting the value onto an out >> channel that feeds the other consumers before taking its own "do not want" >> actions. >> >> -- >> 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 >> <javascript:> >> 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 <javascript:> >> For more options, visit this group at >> 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 <javascript:>. >> For more options, visit https://groups.google.com/d/optout. >> > > -- 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 --- 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+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
