On 3/28/22 20:17, Reuven Lax wrote:
On Mon, Mar 28, 2022 at 11:08 AM Robert Bradshaw <rober...@google.com>
wrote:
On Mon, Mar 28, 2022 at 11:04 AM Reuven Lax <re...@google.com> wrote:
>
> On Mon, Mar 28, 2022 at 10:59 AM Evan Galpin
<evan.gal...@gmail.com> wrote:
>>
>> I don't believe that the issue is Flink specific but rather
that Flink is one example of many potential examples. Enforcing
that watermark updates can only happen at bundle boundaries would
ensure that any data buffered while processing a single bundle in
a DoFn could be output ON_TIME, especially without any need for a
TimerSpec to explicitly hold the watermark for that purpose. This
is in reference to data buffered within a single bundle, and not
cross-bundle buffering such as in the case of GroupIntoBatches.
>
>
> Any in-flight data (i.e. data being processed that is not yet
committed back to the runner) must hold up the output watermark.
Since in the Beam model all records in a bundle are somewhat
atomic (e.g. if the bundle succeeds, none of of them should be
replayed in a proper exactly-once runner), I think this implicitly
means that any elements in an in-flight bundle must hold up the
watermark. This doesn't mean that the watermark can't advance
while the bundle is in flight -just that it can't advance past any
of the timestamps outstanding in the bundle.
Yes. The difficulty is that we don't have much visibility into
"timestamps outstanding in the bundle" so we have to take
min(timestamps of input elements in the bundle) which is not that
different from only having watermark updates at bundle boundaries.
Exactly.
Agree, this works exactly the same. The requirement is not to not update
the watermark, but not to update it past any on-time element in the
bundle. Not updating the watermark at all is one solution, computing
min(timestamps in bundle) works the same. Unfortunately, Flink does not
construct bundles in advance, it is more an ad-hoc concept. Therefore
the only way to hold the watermark is not to update it, because the
timestamps of elements that will be part of the bundle are not known.
Two more questions:
a) it seems that we are missing some @ValidatesRunner tests for this,
right?
b) should we relax the restriction of not allowing
outputWithTimestamp() output element before the current element? I think
it should be "before lowest element in the current bundle" or "before
output watermark, if not already late, or not droppable if late (uh,
this gets a little complicated :))". Not allowing outputting element
with timestamp lower than the current element seems to be just a
"safety-first" solution to the problem discussed here and is too
restrictive. It could be worked-around using getAllowedTimestampSkew(),
but that can cause errors.
>> Take for example a PCollection with 1 second Fixed windowing.
The PCollection holds payload bodies for an external API to which
requests will be made. A hypothetical runner creates a bundle
with Element A and Element B where Element A belongs to the window
[0:00:01, 0:00:02) and Element B belongs to the window [0:00:02,
0:00:03). Assume that the DoFn is going to buffer all elements in
a bundle so as to generate fewer round-trip requests to the
external API, and then output the corresponding responses. The
following is a high-level order of events that could result in
data being labelled as LATE:
>>
>> 1. Watermark is 0:00:00
>> 2. DoFn receives the bundle containing both Element A and Element B
>> 3. Element A is processed by the DoFn, buffering in-memory and
returning/completing
>> 4. Watermark is (maybe) updated after having processed the
element; let's assume in this example it is in fact updated to 0:00:02
>> 5. Element B (from the same bundle) is processed by the DoFn
>> 6. It's the end of the bundle, so now the in-memory buffered
entities are used to make a request to external API
>> 7. The API responses are gathered and intended to be output to
the same window from which the corresponding element with request
data originated (Element A and Element B carried this data)
>> 8. The response data associated with the request payload found
in Element A is output with the timestamp of Element A i.e.
something in the range of Element A's window [0:00:01, 0:00:02)
>> 9. The data in the prior step is considered LATE, strictly as a
result of updating the watermark to 0:00:02 in Step 4 above
>>
>> If Step 4 was moved to be the last step in the process (i.e. at
the bundle boundary) this issue would be avoided. I would also
argue that updating the watermark only after receiving a response
for an input Element is a more accurate depiction of having
completed processing for the element. All that said, I could buy
the argument that the above description might represent an
anti-pattern of sorts where response data should actually be
output with a timestamp corresponding to its receipt rather than
the timestamp of its corresponding input element carrying the
request body.
>>
>> - Evan
>>
>> On Mon, Mar 28, 2022 at 11:07 AM Reuven Lax <re...@google.com>
wrote:
>>>
>>> I agree with you that changing on-time elements to late
elements is incorrect, however I don't quite understand why doing
things on bundle boundaries helps. Is this specific to Flink?
>>>
>>>
>>>
>>> On Mon, Mar 28, 2022 at 1:07 AM Jan Lukavský <je...@seznam.cz>
wrote:
>>>>
>>>> Hi Robert,
>>>>
>>>> I had the same impression that holding the watermark between
bundles is
>>>> actually not part of the computational model. Now the
question is -
>>>> should it be?
>>>>
>>>> As you said, buffering and emitting in-memory buffered data means
>>>> possibly outputting data that arrived as ON_TIME, but is
outputted as
>>>> LATE (or droppable, which is even worse). My understanding is
that this
>>>> is why there is the (deprecated) getAllowedTimestampSkew()
method of
>>>> DoFn, but that only bypasses the check, does not solve the
issue (which
>>>> is why it is deprecated, I suppose). I strongly believe that
outputting
>>>> elements that switch from ON_TIME to LATE is a correctness
bug, because
>>>> it has the potential to violate causality (which is strongly
>>>> counter-intuitive in our universe :)). For some pipelines it can
>>>> definitely cause incorrect outputs.
>>>>
>>>> If we could ensure the output watermark gets updated only between
>>>> @FinishBundle and @StartBundle call then this problem would
go away. I
>>>> looked into the code of FlinkRunner and it seems to me that
we could
>>>> quite easily ensure this by not outputting watermark when a
bundle is
>>>> open and output it once it finishes. I didn't dig into that
too deep, so
>>>> I don't know if there would be any caveats, the question is
apparently,
>>>> if we could make these guarantees for other runners as well
and if we
>>>> could sensibly create a @ValidatesRunner test.
>>>>
>>>> WDYT?
>>>>
>>>> Jan
>>>>
>>>> On 3/25/22 23:06, Robert Bradshaw wrote:
>>>> > I do not think there is a hard and fast rule about updating
watermarks
>>>> > only at bundle boundaries. This seems perfectly legal for a
pure 1:1
>>>> > mapping DoFn. The issue is that DoFns are allowed to buffer
data and
>>>> > emit them in a later process (or finishBundle). If the
watermark has
>>>> > moved on, that may result in late data. We don't really
have a way for
>>>> > a DoFn to declare *it's* output watermark (i.e. "I promise
not to emit
>>>> > any data before this timestamp.")
>>>> >
>>>> > On Thu, Mar 24, 2022 at 8:10 AM Evan Galpin
<egal...@apache.org> wrote:
>>>> >> Thanks for starting this thread Jan, I'm keen to hear
thoughts and outcomes! I thought I would mention that answers to
the questions posed here will help to unblock a 2.38.0 release
blocker[1].
>>>> >>
>>>> >> [1] https://issues.apache.org/jira/browse/BEAM-14064
>>>> >>
>>>> >> On Thu, Mar 24, 2022 at 5:28 AM Jan Lukavský
<je...@seznam.cz> wrote:
>>>> >>> Hi,
>>>> >>>
>>>> >>> this is follow-up thread started from [1]. In the thread
there is mentioned multiple times that (in stateless ParDo), the
output watermark is allowed to advance only on bundle boundaries
[2]. Essentially that would mean that anything in between calls to
@StartBundle and @FinishBundle would be processed in single
instant in (output) event-time. This makes perfect sense.
>>>> >>>
>>>> >>> The issue is that it seems that not all runners actually
implement this behavior. FlinkRunner for instance does not have a
"natural" concept of bundles and those are created in a more
ad-hoc way to adhere with the DoFn life-cycle (see [3]). Watermark
updates and elements are completely interleaved without any
synchronization with bundle "open" or "close". If watermark
updates are allowed to happen only on boundaries of bundles, then
this seems to break this contract.
>>>> >>>
>>>> >>> The question therefore is - should we consider
FlinkRunner as non-compliant with this aspect of the Apache Beam
model or is this an "optional" part that runners are free to
implement at will? In the case of the former, do we miss some
@ValidatesRunner tests for this?
>>>> >>>
>>>> >>> Jan
>>>> >>>
>>>> >>> [1]
https://lists.apache.org/thread/mtwtno2o88lx3zl12jlz7o5w1lcgm2db
>>>> >>>
>>>> >>> [2]
https://lists.apache.org/thread/7foy455spg43xo77zhrs62gc1m383t50
>>>> >>>
>>>> >>> [3]
https://github.com/apache/beam/blob/14862ccbdf2879574b6ce49149bdd7c9bf197322/runners/flink/src/main/java/org/apache/beam/runners/flink/translation/wrappers/streaming/DoFnOperator.java#L786
>>>> >>>
>>>> >>>
