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https://issues.apache.org/jira/browse/BEAM-644?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16823895#comment-16823895
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Deniz Saner commented on BEAM-644:
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Thanks [~kenn]! As we couldn't afford to loose data, we simply built a small
service, which reads batched messages from pubsub and republishes them as
singular items with the timestamp attribute set correctly. Doing so, we could
eliminate any use of withAllowedTimestampSkew from our code base. I'm just
writing this here so that other people may take this as a starting point.
> Primitive to shift the watermark while assigning timestamps
> -----------------------------------------------------------
>
> Key: BEAM-644
> URL: https://issues.apache.org/jira/browse/BEAM-644
> Project: Beam
> Issue Type: New Feature
> Components: beam-model
> Reporter: Kenneth Knowles
> Priority: Major
>
> There is a general need, especially important in the presence of
> SplittableDoFn, to be able to assign new timestamps to elements without
> making them late or droppable.
> - DoFn.withAllowedTimestampSkew is inadequate, because it simply allows one
> to produce late data, but does not allow one to shift the watermark so the
> new data is on-time.
> - For a SplittableDoFn, one may receive an element such as the name of a log
> file that contains elements for the day preceding the log file. The timestamp
> on the filename must currently be the beginning of the log. If such elements
> are constantly flowing, it may be OK, but since we don't know that element is
> coming, in that absence of data, the watermark may advance. We need a way to
> keep it far enough back even in the absence of data holding it back.
> One idea is a new primitive ShiftWatermark / AdjustTimestamps with the
> following pieces:
> - A constant duration (positive or negative) D by which to shift the
> watermark.
> - A function from TimestampedElement<T> to new timestamp that is >= t + D
> So, for example, AdjustTimestamps(<-60 minutes>, f) would allow f to make
> timestamps up to 60 minutes earlier.
> With this primitive added, outputWithTimestamp and withAllowedTimestampSkew
> could be removed, simplifying DoFn.
> Alternatively, all of this functionality could be bolted on to DoFn.
> This ticket is not a proposal, but a record of the issue and ideas that were
> mentioned.
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