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 > >>> > > > > > > > >>> > > > > > > > >>> > > > > > > >>> > > > > > >>> > > > > >>> > > > >>> > > >>> > >> > >> >
