Hi,
I'll try to summarize the mailing list threads to clarify why I think
this addition is needed (and actually necessary):
a) there are situations where the order of input events matter
(obviously any finite state machine)
b) in streaming case, this can be handled by the current machinery
(e.g. holding elements in state, sorting all elements with timestamp
less than input watermark, dropping latecomers)
c) in batch case, this can be handled the same way, but
i) due to the nature of batch processing, that has extreme
requirements on the size of state needed to hold the elements (actually,
in extreme, that might be the whole input, which might not be feasible)
ii) although it is true, that watermark might (and will) fall behind
in streaming processing as well so that similar issues might arise there
too, it is hardly imaginable that it will fall behind as much as several
years (but it is absolutely natural in batch case) - I'm talking about
regular streaming processing, not some kappa like architectures, where
this happens as well, but is causes troubles ([1])
iii) given the fact, that some runners already use sort-merge
groupings, it is actually virtually for free to also sort elements
inside groups by timestamps, the runner just has to know, that it should
do so
I don't want to go too far into details to keep this focused, but the
fact that runner would know that it should sort by timestamp before
stateful pardo brings additional features that are currently unavailable
- e.g. actually shift event time smoothly, as elements flow through, not
from -inf to +inf in one shot. That might have positive effect on timers
being fired smoothly and thus for instance being able to free some state
that would have to be held until the end of computation otherwise.
Therefore, I think it is essential for users to be able to tell runner
that a particular stateful pardo depends on order of input events, so
that the runner can use optimizations available in batch case. The
streaming case is mostly unaffected by that, because all the sorting can
be handled the usual way.
Hope this helps to clarify why it would be good to introduce (some way)
to mark stateful pardos as "time sorted".
Cheers,
Jan
[1]
https://www.ververica.com/resources/flink-forward-san-francisco-2019/moving-from-lambda-and-kappa-architectures-to-kappa-at-uber
Hope these thoughts help
On 11/8/19 11:35 AM, Jan Lukavský wrote:
Hi Max,
thanks for comment. I probably should have put links to discussion
threads here in the vote thread. Relevant would be
- (a pretty lengthy) discussion about whether sorting by timestamp
should be part of the model - [1]
- part of the discussion related to the annotation - [2]
Regarding the open question in the design document - these are not
meant to be open questions in regard to the design of the annotation
and I'll remove that for now, as it is not (directly) related.
Now - main reason for this vote is that there is actually not a clear
consensus in the ML thread. There are plenty of words like "should",
"could", "would" and "maybe", so I wanted to be sure there is
consensus to include this. I already run this in production for
several months, so it is definitely useful for me. :-) But that might
not be sufficient.
I'd be very happy to answer any more questions.
Thanks,
Jan
[1]
https://lists.apache.org/thread.html/4609a1bb1662690d67950e76d2f1108b51327b8feaf9580de659552e@%3Cdev.beam.apache.org%3E
[2]
https://lists.apache.org/thread.html/dd9bec903102d9fcb4f390dc01513c0921eac1fedd8bcfdac630aaee@%3Cdev.beam.apache.org%3E
On 11/8/19 11:08 AM, Maximilian Michels wrote:
Hi Jan,
Disclaimer: I haven't followed the discussion closely, so I do not
want to comment on the technical details of the feature here.
From the outside, it looks like there may be open questions. Also, we
may need more motivation for what we can build with this feature or
how it will become useful to users.
There are many threads in Beam and I believe we need to carefully
prioritize the Beam feature set in order to focus on the things that
provide the most value to our users.
Cheers,
Max
On 07.11.19 15:55, Jan Lukavský wrote:
Hi,
is there anything I can do to make this more attractive? :-) Any
feedback would be much appreciated.
Many thanks,
Jan
Dne 5. 11. 2019 14:10 napsal uživatel Jan Lukavský <je...@seznam.cz>:
Hi,
I'd like to open a vote on accepting design document [1] as a
base for
implementation of @RequiresTimeSortedInput annotation for stateful
DoFns. Associated JIRA [2] and PR [3] contains only subset of
the whole
functionality (allowed lateness ignored and no possibility to
specify
UDF for time - or sequential number - to be extracted from data).
The PR
will be subject to independent review process (please feel free to
self-request review if you are interested in this) after the
vote would
eventually succeed. Missing features from the design document
will be
added later in subsequent JIRA issues, so that it doesn't block
availability of this feature.
Please vote on adding support for @RequiresTimeSortedInput.
The vote is open for the next 72 hours and passes if at least
three +1
and no -1 PMC (binding) votes are cast.
[ ] +1 Add support for @RequiresTimeSortedInput
[ ] 0 I don't have a strong opinion about this, but I assume
it's ok
[ ] -1 Do not support @RequiresTimeSortedInput - please provide
explanation.
Thanks,
Jan
[1]
https://docs.google.com/document/d/1ObLVUFsf1NcG8ZuIZE4aVy2RYKx2FfyMhkZYWPnI9-c/edit?usp=sharing
[2] https://issues.apache.org/jira/browse/BEAM-8550
[3] https://github.com/apache/beam/pull/8774
