Description

As another main component for stream data windowing, this PR adds Window Trigger along with Stream Context.

Stream Context

A stream context holds useful context information for all stream processing components. It’s also easy to store and restore between executions by put all state in one place.

Window Trigger

A window trigger determines whether to trigger window state output (a window pane). Common trigger implementation includes:

• After Watermark: trigger if the window is below watermark.
• Count: trigger based on a counter. A special case is per-event trigger for materialized view semantics.
• Time: trigger when its time comes.

Note that window trigger is easy to be confused with watermark. Watermark is the way to reason about the completeness of window state. Window trigger upon user configuration can decide to trigger output no matter the window state is complete or not.

Issues Resolved

#951

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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