Add watermark generator #959
Merged
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Signed-off-by: Chen Dai [email protected]
Description
Watermark is a monotonically increasing timestamp of the oldest work not yet completed. The work can be any grouping operation, such as aggregate or join, that accumulate stream events to a table and maintain the state. In other words, watermark is the way of how we reason about the completeness of accumulated window state.
There are several aspects of watermark implementation, including watermark generation, watermark emit frequency and watermark propagation. This PR is focused on the watermark generation which has no dependency on how we integrate with query plan later.
What's covered in this PR is the common generate strategy: Bounded Out-Of-Order Watermark Generator which allows a fixed delay for disordered data.
Issues Resolved
#953
Check List
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