Does anybody have objections against copying the first record that goes into the ReduceState?
2016-11-22 12:49 GMT+01:00 Aljoscha Krettek <aljos...@apache.org>: > That's right, yes. > > On Mon, 21 Nov 2016 at 19:14 Fabian Hueske <fhue...@gmail.com> wrote: > > > Right, but that would be a much bigger change than "just" copying the > > *first* record that goes into the ReduceState, or am I missing something? > > > > > > 2016-11-21 18:41 GMT+01:00 Aljoscha Krettek <aljos...@apache.org>: > > > > > To bring over my comment from the Github PR that started this > discussion: > > > > > > @wuchong <https://github.com/wuchong>, yes this is a problem with the > > > HeapStateBackend. The RocksDB backend does not suffer from this > problem. > > I > > > think in the long run we should migrate the HeapStateBackend to always > > keep > > > data in serialised form, then we also won't have this problem anymore. > > > > > > So I'm very much in favour of keeping data serialised. Copying data > would > > > only ever be a stopgap solution. > > > > > > On Mon, 21 Nov 2016 at 15:56 Fabian Hueske <fhue...@gmail.com> wrote: > > > > > > > Another approach that would solve the problem for our use case > (object > > > > re-usage for incremental window ReduceFunctions) would be to copy the > > > first > > > > object that is put into the state. > > > > This would be a change on the ReduceState, not on the overall state > > > > backend, which should be feasible, no? > > > > > > > > > > > > > > > > 2016-11-21 15:43 GMT+01:00 Stephan Ewen <se...@apache.org>: > > > > > > > > > -1 for copying objects. > > > > > > > > > > Storing a serialized data where possible is good, but copying all > > > objects > > > > > by default is not a good idea, in my opinion. > > > > > A lot of scenarios use data types that are hellishly expensive to > > copy. > > > > > Even the current copy on chain handover is a problem. > > > > > > > > > > Let's not introduce even more copies. > > > > > > > > > > On Mon, Nov 21, 2016 at 3:16 PM, Maciek Próchniak <m...@touk.pl> > > wrote: > > > > > > > > > > > Hi, > > > > > > > > > > > > it will come with performance overhead when updating the state, > > but I > > > > > > think it'll be possible to perform asynchronous snapshots using > > > > > > HeapStateBackend (probably some changes to underlying data > > structures > > > > > would > > > > > > be needed) - which would bring more predictable performance. > > > > > > > > > > > > thanks, > > > > > > maciek > > > > > > > > > > > > > > > > > > On 21/11/2016 13:48, Aljoscha Krettek wrote: > > > > > > > > > > > >> Hi, > > > > > >> I would be in favour of this since it brings things in line with > > the > > > > > >> RocksDB backend. This will, however, come with quite the > > performance > > > > > >> overhead, depending on how fast the TypeSerializer can copy. > > > > > >> > > > > > >> Cheers, > > > > > >> Aljoscha > > > > > >> > > > > > >> On Mon, 21 Nov 2016 at 11:30 Fabian Hueske <fhue...@gmail.com> > > > wrote: > > > > > >> > > > > > >> Hi everybody, > > > > > >>> > > > > > >>> when implementing a ReduceFunction for incremental aggregation > of > > > > SQL / > > > > > >>> Table API window aggregates we noticed that the > HeapStateBackend > > > does > > > > > not > > > > > >>> store copies but holds references to the original objects. In > > case > > > > of a > > > > > >>> SlidingWindow, the same object is referenced from different > > window > > > > > panes. > > > > > >>> Therefore, it is not possible to modify these objects (in order > > to > > > > > avoid > > > > > >>> object instantiations, see discussion [1]). > > > > > >>> > > > > > >>> Other state backends serialize their data such that the > behavior > > is > > > > not > > > > > >>> consistent across backends. > > > > > >>> If we want to have light-weight tests, we have to create new > > > objects > > > > in > > > > > >>> the > > > > > >>> ReduceFunction causing unnecessary overhead. > > > > > >>> > > > > > >>> I would propose to copy objects when storing them in a > > > > > HeapStateBackend. > > > > > >>> This would ensure that objects returned from state to the user > > > behave > > > > > >>> identical for different state backends. > > > > > >>> > > > > > >>> We created a related JIRA [2] that asks to copy records that go > > > into > > > > an > > > > > >>> incremental ReduceFunction. The scope is more narrow and would > > > solve > > > > > our > > > > > >>> problem, but would leave the inconsistent behavior of state > > > backends > > > > in > > > > > >>> place. > > > > > >>> > > > > > >>> What do others think? > > > > > >>> > > > > > >>> Cheers, Fabian > > > > > >>> > > > > > >>> [1] > > https://github.com/apache/flink/pull/2792#discussion_r88653721 > > > > > >>> [2] https://issues.apache.org/jira/browse/FLINK-5105 > > > > > >>> > > > > > >>> > > > > > > > > > > > > > > > > > > > > >
