A vote is the last resort. Consensus through discussion is much nicer. And I think we are making progress.
We went for the lightweight version in the batch API, because - there are few cases that are affected (only functions with side effect state) - you can always switch lightweight -> failsafe in the future (only hardens guarantees), but not the other way around (dropping guarantees) without breaking existing code. If you are strong on that point, I do not want to be a blocker for this. I only ask to make a well informed decision, as this behavior is as much part of the API as the classname of the DataStream. On Wed, May 20, 2015 at 3:41 PM, Gyula Fóra <gyula.f...@gmail.com> wrote: > I would go for the Failsafe option as a default behaviour with a clearly > documented lightweight (no-copy) setting, but I think having a Vote on this > would be the proper way of settling this question. > > On Wed, May 20, 2015 at 3:37 PM, Aljoscha Krettek <aljos...@apache.org> > wrote: > > > I think that in the long run (maybe not too long) we will have to > > change our stateful operators (windows, basically) to use managed > > memory and spill to disk. (Think jobs that have sliding windows over > > days or weeks) Then then the internal operators will take care of > > copying anyways. The problem Gyula mentioned we cannot tackle other > > than by defining how user code must behave. > > > > On Wed, May 20, 2015 at 3:30 PM, Stephan Ewen <se...@apache.org> wrote: > > > It does not mean we have to behave the same way, it is just an > indication > > > that well-defined behavior can allow you to mess things up. > > > > > > The question is now what is the default mode: > > > - Failsafe/Heavy (always copy) > > > - Performance/Lightweight (do not copy) > > > > > > > > > On Wed, May 20, 2015 at 3:29 PM, Stephan Ewen <se...@apache.org> > wrote: > > > > > >> This is something that we can clearly define as "should not be done". > > >> Systems do that. > > >> I think if you repeatedly emit (or mutate) the same object for example > > in > > >> Spark, you get an RDD with completely messed up contents. > > >> > > >> On Wed, May 20, 2015 at 3:27 PM, Gyula Fóra <gyf...@apache.org> > wrote: > > >> > > >>> If the preceding operator is emitting a mutated object, or does > > something > > >>> with the output object afterwards then its a problem. > > >>> > > >>> Emitting the same object is a special case of this. > > >>> > > >>> On Wed, May 20, 2015 at 3:09 PM, Stephan Ewen <se...@apache.org> > > wrote: > > >>> > > >>> > The case you are making is if a preceding operator in a chain is > > >>> repeatedly > > >>> > emitting the same object, and the succeeding operator is gathering > > the > > >>> > objects, then it is a problem > > >>> > > > >>> > Or are there cases where the system itself repeatedly emits the > same > > >>> > objects? > > >>> > > > >>> > On Wed, May 20, 2015 at 3:07 PM, Gyula Fóra <gyf...@apache.org> > > wrote: > > >>> > > > >>> > > We are designing a system for stateful stream computations, > > assuming > > >>> long > > >>> > > standing operators that gather and store data as the stream > evolves > > >>> > (unlike > > >>> > > in the dataset api). Many programs, like windowing, sampling etc > > hold > > >>> the > > >>> > > state in the form of past data. And without careful understanding > > of > > >>> the > > >>> > > runtime these programs will break or have unnecessary copies. > > >>> > > > > >>> > > This is why I think immutability should be the default so we can > > have > > >>> a > > >>> > > clear dataflow model with immutable streams. > > >>> > > > > >>> > > I see absolutely no reason why we cant have the non-copy version > > as an > > >>> > > optional setting for the users. > > >>> > > > > >>> > > > > >>> > > On Wed, May 20, 2015 at 2:21 PM, Paris Carbone <par...@kth.se> > > wrote: > > >>> > > > > >>> > > > @stephan I see your point. If we assume that operators do not > > hold > > >>> > > > references in their state to any transmitted records it works > > fine. > > >>> We > > >>> > > > therefore need to make this clear to the users. I need to check > > if > > >>> that > > >>> > > > would break semantics in SAMOA or other integrations as well > that > > >>> > assume > > >>> > > > immutability. For example in SAMOA there are often local metric > > >>> objects > > >>> > > > that are being constantly mutated and simply forwarded > > periodically > > >>> to > > >>> > > > other (possibly chained) operators that need to evaluate them. > > >>> > > > > > >>> > > > ________________________________________ > > >>> > > > From: Gyula Fóra <gyf...@apache.org> > > >>> > > > Sent: Wednesday, May 20, 2015 2:06 PM > > >>> > > > To: dev@flink.apache.org > > >>> > > > Subject: Re: [DISCUSS] Re-add record copy to chained operator > > calls > > >>> > > > > > >>> > > > "Copy before putting it into a window buffer and any other > group > > >>> > buffer." > > >>> > > > > > >>> > > > Exactly my point. Any stateful operator should be able to > > implement > > >>> > > > something like this without having to worry about copying the > > object > > >>> > (and > > >>> > > > at this point the user would need to know whether it comes from > > the > > >>> > > network > > >>> > > > to avoid unnecessary copies), so I don't agree with leaving the > > copy > > >>> > off. > > >>> > > > > > >>> > > > The user can of course specify that the operator is mutable if > he > > >>> wants > > >>> > > > (and he is worried about the performance), But I still think > the > > >>> > default > > >>> > > > behaviour should be immutable. > > >>> > > > We cannot force users to not hold object references and also it > > is a > > >>> > > quite > > >>> > > > unnatural way of programming in a language like java. > > >>> > > > > > >>> > > > > > >>> > > > On Wed, May 20, 2015 at 1:39 PM, Stephan Ewen < > se...@apache.org> > > >>> > wrote: > > >>> > > > > > >>> > > > > I am curious why the copying is actually needed. > > >>> > > > > > > >>> > > > > In the batch API, we chain and do not copy and it is rather > > >>> > > predictable. > > >>> > > > > > > >>> > > > > The cornerpoints of that design is to follow these rules: > > >>> > > > > > > >>> > > > > 1) Objects read from the network or any buffer are always > new > > >>> > objects. > > >>> > > > > That comes naturally when they are deserialized as part of > that > > >>> (all > > >>> > > > > buffers store serialized) > > >>> > > > > > > >>> > > > > 2) After a function returned a record (or gives one to the > > >>> > collector), > > >>> > > > it > > >>> > > > > if given to the chain of chained operators, but after it is > > >>> through > > >>> > the > > >>> > > > > chain, no one else holds a reference to that object. > > >>> > > > > For that, it is crucial that objects are not stored by > > >>> > reference, > > >>> > > > but > > >>> > > > > either stored serialized, or a copy is stored. > > >>> > > > > > > >>> > > > > This is quite solid in the batch API. How about we follow the > > same > > >>> > > > paradigm > > >>> > > > > in the streaming API. We would need to adjust the following: > > >>> > > > > > > >>> > > > > 1) Do not copy between operators (I think this is the case > > right > > >>> now) > > >>> > > > > > > >>> > > > > 2) Copy before putting it into a window buffer and any other > > group > > >>> > > > buffer. > > >>> > > > > > > >>> > > > > > > >>> > > > > > > >>> > > > > > > >>> > > > > > > >>> > > > > > > >>> > > > > > > >>> > > > > > > >>> > > > > On Wed, May 20, 2015 at 1:22 PM, Aljoscha Krettek < > > >>> > aljos...@apache.org > > >>> > > > > > >>> > > > > wrote: > > >>> > > > > > > >>> > > > > > Yes, in fact I anticipated this. There is one central place > > >>> where > > >>> > we > > >>> > > > > > can insert a copy step, in OperatorCollector in > > OutputHandler. > > >>> > > > > > > > >>> > > > > > On Wed, May 20, 2015 at 11:17 AM, Paris Carbone < > > par...@kth.se> > > >>> > > wrote: > > >>> > > > > > > I guess it was not intended ^^. > > >>> > > > > > > > > >>> > > > > > > Chaining should be transparent and not break the > > >>> correct/expected > > >>> > > > > > behaviour. > > >>> > > > > > > > > >>> > > > > > > > > >>> > > > > > > Paris? > > >>> > > > > > > > > >>> > > > > > > On 20 May 2015, at 11:02, Márton Balassi < > > mbala...@apache.org > > >>> > > > >>> > > > wrote: > > >>> > > > > > > > > >>> > > > > > > +1 for copying. > > >>> > > > > > > On May 20, 2015 10:50 AM, "Gyula Fóra" < > gyf...@apache.org> > > >>> > wrote: > > >>> > > > > > > > > >>> > > > > > > Hey, > > >>> > > > > > > > > >>> > > > > > > The latest streaming operator rework removed the copying > of > > >>> the > > >>> > > > outputs > > >>> > > > > > > before passing them to chained operators. This is a major > > >>> break > > >>> > for > > >>> > > > the > > >>> > > > > > > previous operator semantics which guaranteed > immutability. > > >>> > > > > > > > > >>> > > > > > > I think this change leads to very indeterministic program > > >>> > behaviour > > >>> > > > > from > > >>> > > > > > > the user's perspective as only non-chained outputs/inputs > > >>> will be > > >>> > > > > > mutable. > > >>> > > > > > > If we allow this to happen, users will start disabling > > >>> chaining > > >>> > to > > >>> > > > get > > >>> > > > > > > immutability which defeats the purpose. (chaining should > > not > > >>> > affect > > >>> > > > > > program > > >>> > > > > > > behaviour just increase performance) > > >>> > > > > > > > > >>> > > > > > > In my opinion the default setting for each operator > should > > be > > >>> > > > > > immutability > > >>> > > > > > > and the user could override this manually if he/she > wants. > > >>> > > > > > > > > >>> > > > > > > What do you think? > > >>> > > > > > > > > >>> > > > > > > Regards, > > >>> > > > > > > Gyula > > >>> > > > > > > > > >>> > > > > > > > > >>> > > > > > > > >>> > > > > > > >>> > > > > > >>> > > > > >>> > > > >>> > > >> > > >> > > >
