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