Kenneth Knowles created BEAM-2535:
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Summary: Allow explicit output time independent of firing
specification for all timers
Key: BEAM-2535
URL: https://issues.apache.org/jira/browse/BEAM-2535
Project: Beam
Issue Type: New Feature
Components: beam-model, sdk-java-core
Reporter: Kenneth Knowles
Assignee: Kenneth Knowles
Today, we have insufficient control over the event time timestamp of elements
output from a timer callback.
1. For an event time timer, it is the timestamp of the timer itself.
2. For a processing time timer, it is the current input watermark at the time
of processing.
But for both of these, we may want to reserve the right to output a particular
time, aka set a "watermark hold".
A naive implementation of a {{TimerWithWatermarkHold}} would work for making
sure output is not droppable, but does not fully explain window expiration and
late data/timer dropping.
In the natural interpretation of a timer as a feedback loop on a transform,
timers should be viewed as another channel of input, with a watermark, and
items on that channel _all need event time timestamps even if they are
delivered according to a different time domain_.
I propose that the specification for when a timer should fire should be
separated (with nice defaults) from the specification of the event time of
resulting outputs. These timestamps will determine a side channel with a new
"timer watermark" that constrains the output watermark.
- We still need to fire event time timers according to the input watermark, so
that event time timers fire.
- Late data dropping and window expiration will be in terms of the minimum of
the input watermark and the timer watermark. In this way, whenever a timer is
set, the window is not going to be garbage collected.
- We will need to make sure we have a way to "wake up" a window once it is
expired; this may be as simple as exhausting the timer channel as soon as the
input watermark indicates expiration of a window
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