Actually found a much simpler solution using Source.unfoldAsync. 

On Thursday, February 22, 2018 at 6:54:54 PM UTC-8, Jeff wrote:
>
> My particular use case is a long poll where the id to be sent on the next 
> request is returned from the previous request.
>
> On Thursday, February 22, 2018 at 6:40:48 PM UTC-8, Konrad Malawski wrote:
>>
>> In general relying on exact timing of pulls is rather seen as an anti 
>> pattern…
>> Even if you do a custom stage, it’s very brittle, as any element put 
>> before your stage could cause pulls, simply because that’s how things work.
>> The same can be said about simply changed buffer sizes between stages — 
>> introducing an async boundary anywhere in the pipeline can lead 
>> to more buffer space being allocated, and thus, more pulls being issued.
>>
>> In other words — due to the fact that backpressure is connected through 
>> an entire pipeline (and has to, otherwise it would not do what it should),
>> you can not rely on exact timing of pulls, because changes in the 
>> pipeline may affect this.
>>
>> In your very specific case perhaps it’s doable to make a stage or 
>> pipeline that’ll do the right thing, with enforcing it with zips and loops,
>> but that’ll be specific and take some work and very precise wording of 
>> what you want to achieve (sample code?)
>>
>> -- 
>> Cheers,
>> Konrad 'ktoso <http://kto.so>' Malawski
>> Akka <http://akka.io/> @ Lightbend <http://lightbend.com/>
>>
>> On February 23, 2018 at 7:05:18, Jeff (jknigh...@gmail.com) wrote:
>>
>> Currently, Flow.mapAsync and Flow.mapAsyncUnordered always pull upstream 
>> even if there is no demand from downstream and then buffer. However, there 
>> are situations where one might want to way for explicit downstream demand 
>> before pulling. For example, let's say that the next upstream item depends 
>> on the results of the previous item and that you need to feed that back 
>> into the upstream before pulling again. In its current form, Flow.mapAsync 
>> will pull before that feedback loop has been completed.  
>>
>> Is there any way to work around that other than writing a custom 
>> GraphStage? 
>>
>> Thanks
>> Jeff
>> --
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