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From Stephan Ewen <se...@apache.org>
Subject Re: [DISCUSS] Re-add record copy to chained operator calls
Date Wed, 20 May 2015 13:29:33 GMT
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 <gyfora@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 <sewen@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 <gyfora@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 <parisc@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 <gyfora@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 <sewen@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 <
> > aljoscha@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 <parisc@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 <mbalassi@apache.org>
> > > > wrote:
> > > > > > >
> > > > > > > +1 for copying.
> > > > > > > On May 20, 2015 10:50 AM, "Gyula Fóra" <gyfora@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
> > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >
>

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