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From "Ben Chambers (JIRA)" <>
Subject [jira] [Commented] (BEAM-644) Primitive to shift the watermark while assigning timestamps
Date Thu, 22 Sep 2016 17:30:20 GMT


Ben Chambers commented on BEAM-644:

I'm not sure if this will clarify, but there are two parts:
1. A function which for a given input timestamp (or input element) computes an output timestamp.
2. A duration which declares the maximum distance the output timestamp could be behind the
input timestamp.

Consider a case where elements are read from a stream with timestamps based on when they were
But each element also has a timestamp that indicates when it actually happened.
And we know that there is at most a 60 minute delay between when it happened and when it was

We want to adjust the timestamps of elements to be when they actually happened. But this may
shift timestamps up to 60 minutes into the past. Naively, this will make data later, since
the watermark logic doesn't know about the shift. But, with this proposal we'd write something
that couples the shift up to 60 minutes into the past with a declaration that we'll shift
at most up to 60 minutes into the past. The first applies to each element while the second
relates the output watermark to the input watermark.

Similar reasoning could apply the other direction as well (shifting the output watermark forward)
if we knew that we'd always adjust timestamps forward by at least a certain amount.

> Primitive to shift the watermark while assigning timestamps
> -----------------------------------------------------------
>                 Key: BEAM-644
>                 URL:
>             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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