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