I prefer the ProcessFunction and side outputs solution over split() and
select() which I've never liked primarily due to the lack of type safety
and it also doesn't really seem to fit with the rest of Flink's API.
On the late data question I strongly prefer the late data concept being
explicit in the API. Could we not also do something like:
WindowedStream<> windowedStream = input
.keyBy(...)
.window(...);
DataStream<> mainOutput = windowedStream
.apply(...);
DataStream<> lateOutput = windowStream
.lateStream();
On Thu, Feb 23, 2017 at 7:08 AM, Gyula Fóra <gyf...@apache.org> wrote:
> Hi,
>
> Thanks for the nice proposal, I like the idea of side outputs, and it would
> make a lot of topologies much simpler.
>
> Regarding the API I think we should come up with a way of making side
> otuputs accessible from all sort of operators in a similar way. For
> instance through the RichFunction interface with a special collector that
> we invalidate when the user should not be collecting to it. (just a quick
> idea)
>
> I personally wouldn't deprecate the "universal" Split/Select API that can
> be used on any DataStream in favor of functionality that is only
> accessible trhough the process function/ or a few select operators. I think
> the Split/Select pattern is also very nice and I use it in many different
> contexts to get efficient multiway filtering (after map/co operators for
> examples).
>
> Regards,
> Gyula
>
> Aljoscha Krettek <aljos...@apache.org> ezt írta (időpont: 2017. febr. 23.,
> Cs, 15:42):
>
> > Hi Folks,
> > Chen and I have been working for a while now on making FLIP-13 (side
> > outputs) [1] a reality. We think we have a pretty good internal
> > implementation and also a proposal for an API but now we need to discuss
> > how we want to go forward with this, especially how we should deal with
> > split/select which does some of the same things side outputs can do. I'll
> > first quickly describe what the split/select API looks like, so that
> we're
> > all on the same page. Then I'll present the new proposed side output API
> > and then I'll present new API for getting dropped late data from a
> windowed
> > operation, which was the original motivation for adding side outputs.
> >
> > Split/select consists of two API calls: DataStream.split(OutputSelector)
> > and SplitStream.select(). You can use it like this:
> >
> > DataStreamSource<Integer> input = env.fromElements(1, 2, 3);
> >
> > final String EVEN_SELECTOR = "even";
> > final String ODD_SELECTOR = "odd";
> >
> > SplitStream<Integer> split = input.split(
> > new OutputSelector<Integer>() {
> > @Override
> > public Iterable<String> select(Integer value) {
> > if (value % 2 == 0) {
> > return Collections.singleton(EVEN_SELECTOR);
> > } else {
> > return Collections.singleton(ODD_SELECTOR);
> > }
> > }
> > });
> >
> > DataStream<Integer> evenStream = split.select(EVEN_SELECTOR);
> > DataStream<Integer> oddStream = split.select(ODD_SELECTOR);
> >
> > The stream is split according to an OutputSelector that returns an
> Iterable
> > of Strings. Then you can use select() to get a new stream that only
> > contains elements with the given selector. Notice how the element type
> for
> > all the split streams is the same.
> >
> > The new side output API proposal adds a new type OutputTag<T> and relies
> on
> > extending ProcessFunction to allow emitting data to outputs besides the
> > main output. I think it's best explained with an example as well:
> >
> > DataStreamSource<Integer> input = env.fromElements(1, 2, 3);
> >
> > final OutputTag<String> sideOutput1 = new OutputTag<>("side-output-1"){}
> ;
> > final OutputTag<Integer> sideOutput2 = new OutputTag<>("side-output-2"){}
> ;
> >
> > SingleOutputStreamOperator<String> mainOutputStream = input
> > .process(new ProcessFunction<Integer, String>() {
> >
> > @Override
> > public void processElement(
> > Integer value,
> > Context ctx,
> > Collector<String> out) throws Exception {
> >
> > ctx.output(sideOutput1, "WE GOT: " + value);
> > ctx.output(sideOutput2, value);
> > out.collect("MAIN OUTPUT: " + value);
> > }
> >
> > });
> >
> > DataStream<String> sideOutputStream1 =
> > mainOutputStream.getSideOutput(sideOutput1);
> > DataStream<Integer> sideOutputStream2 =
> > mainOutputStream.getSideOutput(sideOutput2);
> >
> > Notice how the OutputTags are anonymous inner classes, similar to
> TypeHint.
> > We need this to be able to analyse the type of the side-output streams.
> > Also notice, how the types of the side-output streams can be independent
> of
> > the main-output stream, also notice how everything is correctly type
> > checked by the Java Compiler.
> >
> > This change requires making ProcessFunction an abstract base class so
> that
> > not every user has to implement the onTimer() method. We would also need
> to
> > allow ProcessFunction on a non-keyed stream.
> >
> > Chen also implemented an API based on FlatMapFunction that looks like the
> > one proposed in the FLIP. This relies on CollectorWrapper, which can be
> > used to "pimp" a Collector to also allow emitting to side outputs.
> >
> > For WindowedStream we have two proposals: make OutputTag visible on the
> > WindowedStream API or make the result type of WindowedStream operations
> > more specific to allow a getDroppedDataSideOutput() method. For the first
> > proposal it would look like this:
> >
> > final OutputTag<String> lateDataTag = new OutputTag<>("side-output-1"){}
> ;
> >
> > DataStream<T> windowedResult = input
> > .keyBy(...)
> > .window(...)
> > .sideOutputLateData(lateDataTag)
> > .apply(...)
> >
> > DataStream<IN> lateData = windowedResult.getSideOutput(lateDataTag);
> >
> > For the second proposal it would look like this:
> >
> > WindowedOperator<T> windowedResult = input
> > .keyBy(...)
> > .window(...)
> > .apply(...)
> >
> > DataStream<IN> lateData = windowedResult.getSideOutput();
> >
> > Right now, the result of window operations is a
> > SingleOutputStreamOperator<T>, same as it is for all DataStream
> operations.
> > Making the result type more specific, i.e. a WindowedOperator, would
> allow
> > us to add extra methods there. This would require wrapping a
> > SingleOutputStreamOperator and forwarding all the method calls to the
> > wrapped operator which can be a bit of a hassle for future changes. The
> > first proposal requires additional boilerplate code.
> >
> > Sorry for the long mail but I think it's necessary to get everyone on the
> > same page. The question is now: how should we proceed with the proposed
> API
> > and the old split/select API? I propose to deprecate split/select and
> only
> > have side outputs, going forward. Of course, I'm a bit biased on this.
> ;-)
> > If we decide to do this, we also need to decide on what the side output
> API
> > should look like.
> >
> > Happy discussing! Feedback very welcome. :-)
> >
> > Best,
> > Aljoscha
> >
> > [1]
> >
> > https://cwiki.apache.org/confluence/display/FLINK/FLIP-
> 13+Side+Outputs+in+Flink
> >
>
--
Jamie Grier
data Artisans, Director of Applications Engineering
@jamiegrier <https://twitter.com/jamiegrier>
ja...@data-artisans.com
