I may have misunderstood what you're trying to do but I think you can
probably use expand for this. In builder pseudocode, something like:
Flow ~> Unzip ~> <the processing you need producing n items> ~> Zip
~> Flow.expand(Iterator.continually(_)) ~>
The expand step will duplicate the last element it received which means
it's always available for the zip step.
Julian
On Wednesday, May 10, 2017 at 12:43:41 AM UTC+1, ben fleis wrote:
>
> Hi,
>
> I am new to akka streams, and working on a project having both real-time
> and batch needs. In the batch cases, I need to take context (e.g.,
> requestID), from the initial request, and apply it at the end of batch-item
> processing. The batch-item subflow is by itself rather simple: (parse,
> mapConcat, transform). The item transforms comes from the real-time
> context.
>
> It is the folding into an output format that is less obvious. That code
> needs the request ID and other context from the original request to
> generate an output byte stream.
>
> What I want to do is treat the context and input bytes as a tuple, unzip
> them in the beginning, zip them at the fold stage. I need to understand
> how to map each input byte stream into a sequence of stream elements in a
> sub-flow, and fold each subsequence in its natural grouping.
>
> mapConcat flattens, thus dropping the boundaries between adjacent batch
> requests.
>
> I could use map, and materialize a new stream within each batch handler
> for its items, and send the folded result as the output of the flow... but
> that seems strange, and likely to be already available. (Especially since
> GroupBy and SubFlow seem closely related...)
>
> Does this exist?
>
> Thanks,
>
> b
>
>
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