Simplifying sequential() / parallel()

Remi Forax forax at
Fri Mar 22 06:43:29 PDT 2013

On 03/22/2013 02:33 PM, Brian Goetz wrote:
> The problem with stateful lambdas is that, unless one block of code 
> has control over the entire pipeline, it is an accident waiting to 
> happen.
> Let's say you receive a stream as a parameter:
>   void foo(Stream s) { ... }
> and you want to do something that requires a stateful mapper:
>   void foo(Stream s) {
> stateful ...)...
>   }
> That's a bug already.  Because you don't know that the stream you were 
> passed in is sequential.  But I doubt that people will remember, even 
> most of the time, they need to do:
>       s.sequential().map(... stateful ...)...
> instead.  Won't happen.
> Stateful lambdas introduce the need for non-modular reasoning about 
> stream pipelines (who created this? who will consume this? in what 
> state was it created?).  And, it has all the same self-deception 
> problems as thread-safety.  People convince themselves "I don't need 
> to think about synchronization because no one will ever use this 
> object concurrently."
> So, while I sympathize with the desire to let people say "I know that 
> this entire stream pipeline has been carefully controlled such as to 
> not have statefulness distort its results", I think in reality, this 
> will quickly turn into "statefulness is OK" in most people's minds.  
> With the attended inevitable foot-shooting.

recognizing what is stateful and what is stateless is enough
(says the guy the "concurrency in Java" teacher in me :)

+1 for 'each', foreach is already used for the enhanced for loop "for(:)",
in that case, forEachOrdered() should be eachOrdered().


> On 3/21/2013 9:57 PM, Joe Bowbeer wrote:
>> I'm traveling now and won't be able to respond promptly but this topic
>> has been raised a couple of times already. Feel free to copy and paste
>> my response from previous discussions:)
>> Rephrasing, I'm OK with non-interference but I object to banning
>> stateful in sequential ops.
>> I think there should be a one-one correspondence between any for loop
>> and a sequential forEach.
>> Can you compare your restrictions with those in Scala and Groovy? Scala
>> in particular, because it is more strictly defined, and I'm pretty sure
>> I've combined stateful expressions with functional forms in Scala, to
>> good effect. (One of the benefits of being multi-paradigmatic?)
>> In addition, I'm wary of the new form of forEach. If anything, I'd like
>> its name to be simpler, e.g., each, not longer.
>> Joe
>> On Mar 21, 2013 3:48 PM, "Brian Goetz" <brian.goetz at
>> <mailto:brian.goetz at>> wrote:
>>     Doug and I have been revisiting sequential() and parallel().  I
>>     think there's a nice simplification here.
>>     The original motivation for sequential() was because we originally
>>     had .into(collection), and many collections were not thread-safe so
>>     we needed a means to bring the computation back to the current
>>     thread.  The .sequential() method did that, but it also brought back
>>     a constraint of encounter ordering with it, because if people did:
>>         stuff.parallel().map(...).__sequential().into(new 
>> ArrayList<>());
>>     not respecting encounter order would violate the principle of least
>>     astonishment.
>>     So the original motivation for sequential() was "bring the
>>     computation back to the current thread, in order."  This was doable,
>>     but has a high price -- a full barrier where we buffer the contents
>>     of the entire stream before doing any forEach'ing.
>>     Most of the time, sequential() only appears right before the
>>     terminal operation.  But, once implemented, there was no reason to
>>     constrain this to appear right before the forEach/into, so we 
>> didn't.
>>     Once we discovered a need for .parallel(), it made sense it be the
>>     dual of .sequential() -- fully unconstrained.  And again, the
>>     implementation wasn't *that* bad -- better than .sequential().  But
>>     again, the most desirable position for .parallel() is right after
>>     the source.
>>     Then we killed into() and replaced it with reduction, which is a
>>     much smarter way of managing ordering.  Eliminating half the
>>     justification for .sequential().
>>     As far as I can tell, the remaining use cases for .sequential() are
>>     just modifiers to forEach to constrain it, in order, to the current
>>     thread.
>>     As in:
>>        ints().parallel().filter(i -> isPrime(i))
>>              .sequential().forEach(System.__out::println)
>>     Which could be replaced by
>>     .__forEachSequentialAndOrderedInC__urrentThread(), with a suitably
>>     better name.  Which could further be simplified to ditch the "in
>>     current thread" part by doing some locking in the implementation,
>>     which brings us to .forEachOrdered(action).  Which nicely
>>     complements .collectUnordered, and the two actually stand better
>>     with their duals present (reduce is by default ordered; forEach is
>>     by default unordered.)
>>     The "put it anywhere" behavior of .parallel was completely
>>     bootstrapped on the "put it anywhere" nature of .sequential; we
>>     never really set out to support transitions in the API.
>>     So, pulling the rug out from under the house of cards, I think we
>>     can fall back to:
>>     1.  Modify semantics of .sequential and .parallel to apply globally
>>     to the entire pipeline.  This works because pipelines are fully lazy
>>     anyway, so we don't commit to seq-ness/par-ness until we hit the
>>     terminal op.  So they are functional versions of "set the seq/par
>>     bit in the source".  And that simplifies the specification of
>>     seq/par down to a single property of the entire pipeline -- much
>>     easier to spec.
>>     2.  Add .forEachOrdered.  For sequential streams, this is just
>>     .forEach.  For par streams, we use a lock to avoid concurrent
>>     invocation of the lambda, and loosen up the current behavior from
>>     "full barrier" to "partial barrier", so that when the next chunk is
>>     available, we can start working right away.  This is easy to
>>     accomplish using the existing AbstractTask machinery.
>>     Before we go there, does anyone have use cases for .sequential() /
>>     .parallel() that *don't* put the parallel right after the source, or
>>     the sequential right before a forEach?

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