Off the top of my head: not sure how you are getting the parameters, if
any, to getNextItem, but since xxAsync methods like Source.unfoldAsync and
mapAsync expect a future-returning function, I would use getNextItem
(rather than identity) in one of those to give you downstream elements of
type Option[A] and then use filter or map or collect to deal with the
Option-ness.
On Mon, Sep 11, 2017 at 1:44 PM, Ori Popowski <ori....@gmail.com> wrote:
> Hi,
>
> What is the correct way to create a source from a function which returns
> Future[Option[A]], besides a custom GraphStage?
>
> The source should emit A elements (not wrapped in Future[Option[]]).
>
> I considered a few options, none of them worked
>
> *First: using RestartSource*
>
> RestartSource.withBackoff(1.second, 1.second, 0.2) { () =>
> Source(Stream.continually(getNextItem.map(_.getOrElse(throw new
> NoSuchElementException)))).mapAsync(4)(identity)
> }
>
>
>
> This method drops elements
>
>
> *Second: filtering*
>
> Source(Stream.continually(getNextItem)).mapAsync(4)(identity).collect {
> case Some(e) => a
> }
>
>
>
> This method will create too many unnecessary calls to the resource
> (imagine a database) and cause DOS.
>
>
> So what is the idiomatic / preferable way?
>
>
>
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