Regarding the WatermarkGenerator (WG) interface itself. The proposal is
basically to turn emitting into a "flatMap", we give the
WatermarkGenerator a "collector" (the WatermarkOutput) and the WG can
decide whether to output a watermark or not and can also mark the output
as idle. Changing the interface to return a Watermark (as the previous
watermark assigner interface did) would not allow that flexibility.
Regarding checkpointing the watermark and keeping track of the minimum
watermark, this would be the responsibility of the framework (or the
KafkaConsumer in the current implementation). The user-supplied WG does
not need to make sure the watermark doesn't regress.
Regarding making the WG a "rich function", I can see the potential
benefit but I also see a lot of pitfalls. For example, how should the
watermark state be handled in the case of scale-in? It could be made to
work in the Kafka case by attaching the state to the partition state
that we keep, but then we have potential backwards compatibility
problems also for the WM state. Does the WG usually need to keep the
state or might it be enough if the state is transient, i.e. if you have
a restart the WG would loose its histogram but it would rebuild it
quickly and you would get back to the same steady state as before.
Best,
Aljoscha
On 27.04.20 12:12, David Anderson wrote:
Overall I like this proposal; thanks for bringing it forward, Aljoscha.
I also like the idea of making the Watermark generator a rich function --
this should make it more straightforward to implement smarter watermark
generators. Eg, one that uses state to keep statistics about the actual
out-of-orderness, and uses those statistics to implement a variable delay.
David
On Mon, Apr 27, 2020 at 11:44 AM Kostas Kloudas <kklou...@gmail.com> wrote:
Hi Aljoscha,
Thanks for opening the discussion!
I have two comments on the FLIP:
1) we could add lifecycle methods to the Generator, i.e. open()/
close(), probably with a Context as argument: I have not fully thought
this through but I think that this is more aligned with the rest of
our rich functions. In addition, it will allow, for example, to
initialize the Watermark value, if we decide to checkpoint the
watermark (see [1]) (I also do not know if Table/SQL needs to do
anything in the open()).
2) aligned with the above, and with the case where we want to
checkpoint the watermark in mind, I am wondering about how we could
implement this in the future. In the FLIP, it is proposed to expose
the WatermarkOutput in the methods of the WatermarkGenerator. Given
that there is the implicit contract that watermarks are
non-decreasing, the WatermarkOutput#emitWatermark() will have (I
assume) a check that will compare the last emitted WM against the
provided one, and emit it only if it is >=. If not, then we risk
having the user shooting himself on the foot if he/she accidentally
forgets the check. Given that the WatermarkGenerator and its caller do
not know if the watermark was finally emitted or not (the
WatermarkOutput#emitWatermark returns void), who will be responsible
for checkpointing the WM?
Given this, why not having the methods as:
public interface WatermarkGenerator<T> {
Watermark onEvent(T event, long eventTimestamp, WatermarkOutput
output);
Watermark onPeriodicEmit(WatermarkOutput output);
}
and the caller will be the one enforcing any invariants, such as
non-decreasing watermarks. In this way, the caller can checkpoint
anything that is needed as it will have complete knowledge as to if
the WM was emitted or not.
What do you think?
Cheers,
Kostas
[1] https://issues.apache.org/jira/browse/FLINK-5601
On Tue, Apr 21, 2020 at 2:25 PM Timo Walther <twal...@apache.org> wrote:
Thanks for the proposal Aljoscha. This is a very useful unification. We
have considered this FLIP already in the interfaces for FLIP-95 [1] and
look forward to update to the new unified watermark generators once
FLIP-126 has been accepted.
Regards,
Timo
[1] https://github.com/apache/flink/pull/11692
On 20.04.20 18:10, Aljoscha Krettek wrote:
Hi Everyone!
We would like to start a discussion on "FLIP-126: Unify (and separate)
Watermark Assigners" [1]. This work was started by Stephan in an
experimental branch. I expanded on that work to provide a PoC for the
changes proposed in this FLIP: [2].
Currently, we have two different flavours of Watermark
Assigners: AssignerWithPunctuatedWatermarks
and AssignerWithPeriodicWatermarks. Both of them extend
from TimestampAssigner. This means that sources that want to support
watermark assignment/extraction in the source need to support two
separate interfaces, we have two operator implementations for the
different flavours. Also, this makes features such as generic support
for idleness detection more complicated to implemented because we again
have to support two types of watermark assigners.
In this FLIP we propose two things:
Unify the Watermark Assigners into one Interface WatermarkGenerator
Separate this new interface from the TimestampAssigner
The motivation for the first is to simplify future implementations and
code duplication. The motivation for the second point is again code
deduplication, most assigners currently have to extend from some base
timestamp extractor or duplicate the extraction logic, or users have to
override an abstract method of the watermark assigner to provide the
timestamp extraction logic.
Additionally, we propose to add a generic wrapping WatermarkGenerator
that provides idleness detection, i.e. it can mark a stream/partition
as
idle if no data arrives after a configured timeout.
The "unify and separate" part refers to the fact that we want to unify
punctuated and periodic assigners but at the same time split the
timestamp assigner from the watermark generator.
Please find more details in the FLIP [1]. Looking forward to
your feedback.
Best,
Aljoscha
[1]
https://cwiki.apache.org/confluence/display/FLINK/FLIP-126%3A+Unify+%28and+separate%29+Watermark+Assigners
[2] https://github.com/aljoscha/flink/tree/stephan-event-time
