Technically I'm not sure this is correct: since f is only called from within a 
single operator (and assuming only a single materialization) the shown code 
should work just fine. Of course it is necessary to know what you're doing when 
coding like this, and as Konrad says there are better alternatives available, 
but guaranteeing ordering for example for db writes would certainly be 
desirable.

Regards, Roland 

Sent from my iPhone

> On 21 Sep 2016, at 01:33, Konrad Malawski <konrad.malaw...@lightbend.com> 
> wrote:
> 
>> 
>>     final Source<Integer, NotUsed> source = // ...
>>     final MutableInt max = new MutableInt(Integer.MIN_VALUE);
>>     final Procedure<Integer> f = i -> {
>>       if (i > max.intValue()) {
>>         max.setValue(i);
>>       }
>>     };
>>     final CompletionStage<Integer> result = source
>>         .runForeach(f, materializer)
>>         .thenApply(__ -> max.intValue());
>> 
>> Is the above code correct in the sense that it always commutes the maximum 
>> (or MIN_VALUE in case the stream is empty)?
>> 
>> According to Akka Stream documentation on Stream Ordering, there is a Java 
>> happens-before relation between invocations of f. Do I still need to use 
>> AtomicInteger so that the effects are seen by all threads?
> 
> You're side-effecting outside the stream, so nothing is guaranteed.
> 
> Yes, it would have to be an atomic integer.
> 
> 
> 
> Instead consider using a better operator for this kind of thing than foreach 
> - like fold (runFold) which is enough for such operation you're doing here.
> 
> 
> 
> Happy hakking.
> 
> -- konrad
> 
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