Hi Kunai,

On Thursday, September 15, 2016 at 7:49:18 AM UTC+2, Kunal Deshpande wrote:
> Few questions on back pressure
> 1. While using Flows in akka-streams using .via will a downstream flow 
> apply back pressure to a flow upstream or is back pressure only signaled to 
> a Source?

Backpressure is applied everywhere inside of akka-stream and also across 
boundaries using the more generic reactive-streams interfaces (Publisher, 
Subscriber). The details how much backpressure is exactly propagated (or 
which buffers are around) are not so easy to predict, though. Still 
everything in akka-stream should use bounded memory.

> 2. Will exceptions in a Flow trigger back pressure

No, exceptions will kill the complete materialized flow. Note, that an 
exception propagating through the stream in downstream direction may also 
overtake elements which are buffered but may not have been delivered. 
(Maybe, the question was also "can exception propagation be delayed by 
backpressure" and the answer to that is "no".

> 3. Is there a mathematical way to represent back pressure and is it 
> consistent across different reactive streams implementations?

I don't know whether someone has already come up with a mathematical model 
of reactive streams semantics. If you read the spec you can see that 
implementations have a lot of freedom of how to implement backpressure 
concretely. E.g. one detail of the akka-stream implementation is that 
akka-stream will request and buffer a few elements at the upstream side of 
an asynchronous boundary. Other implementations may choose other strategies 
how many elements to request in one go.


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