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https://issues.apache.org/jira/browse/BEAM-644?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kenneth Knowles updated BEAM-644:
---------------------------------
Description:
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.
was:
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 always falls
within D of the original timestamp.
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.
> 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
> Assignee: Kenneth Knowles
>
> 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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