[WIP] Use trigger state to know if a window is new

[arunpandianp]

No description provided.

[gemini-code-assist]

Summary of Changes

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This pull request introduces optimizations for state management within Apache Beam's windowing system, specifically targeting new windows. By leveraging trigger state to determine if a window is being processed for the first time, the system can avoid redundant state reads and writes, leading to improved performance, especially in distributed execution environments like Windmill. The changes include adding methods to identify new windows and to mark state as locally empty, along with corresponding logic in the ReduceFnRunner to utilize these optimizations.

Highlights

• New Window Identification: Introduced a mechanism to identify 'new' windows using trigger state, allowing for optimized state management by preventing unnecessary state reads and writes for windows processed for the first time.
• Optimized Watermark Hold Clearing: Added a setKnownEmpty method to the WatermarkHoldState interface and its implementations, enabling local marking of watermark state as empty without requiring immediate backend clearing, which can reduce I/O operations.
• Conditional State Clearing in ReduceFnRunner: Modified ReduceFnRunner to leverage the new isNew window check, conditionally clearing state for new windows to improve performance, especially in distributed runners like Windmill.
• Sentinel Bit for Trigger State: Enhanced TriggerStateMachineRunner to use a sentinel bit when persisting trigger state, allowing the system to accurately distinguish between a truly new window (no state ever persisted) and a window with previously cleared or empty trigger state.
• New Test Coverage: Added new test cases to ReduceFnRunnerTest and WindmillStateInternalsTest to validate the new window optimization and the setKnownEmpty functionality.

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Changelog

• runners/core-java/src/main/java/org/apache/beam/runners/core/InMemoryStateInternals.java
  • Added setKnownEmpty() method to InstantState implementation.
• runners/core-java/src/main/java/org/apache/beam/runners/core/ReduceFnRunner.java
  • Added getTriggerRunner() and getContextFactory() methods for testing purposes.
  • Implemented conditional state clearing for new windows within the processElement method using triggerRunner.isNew().
• runners/core-java/src/main/java/org/apache/beam/runners/core/WatermarkHold.java
  • Added setKnownEmpty() method to allow local marking of watermark holds as empty without clearing them in the backend.
• runners/core-java/src/main/java/org/apache/beam/runners/core/triggers/FinishedTriggersBitSet.java
  • Imported @Nullable for nullability annotations.
  • Added equals() and hashCode() methods for proper object comparison.
• runners/core-java/src/main/java/org/apache/beam/runners/core/triggers/TriggerStateMachineRunner.java
  • Added isNew() method to determine if a window is new based on its trigger state.
  • Added getFinishedBits() method for testing.
  • Modified persistFinishedSet() to write a sentinel bit for empty trigger states to distinguish new windows from seen-but-empty ones.
  • Added isEquivalent() helper method for comparing BitSets, accounting for the sentinel bit.
• runners/core-java/src/test/java/org/apache/beam/runners/core/ReduceFnRunnerTest.java
  • Imported BitSet and StateStyle classes.
  • Added testNewWindowOptimization() to verify the new window state clearing logic and sentinel bit handling.
• runners/google-cloud-dataflow-java/worker/src/main/java/org/apache/beam/runners/dataflow/worker/windmill/state/WindmillWatermarkHold.java
  • Implemented the setKnownEmpty() method for Windmill-specific watermark holds, optimizing state management.
• runners/google-cloud-dataflow-java/worker/src/test/java/org/apache/beam/runners/dataflow/worker/windmill/state/WindmillStateInternalsTest.java
  • Added testWatermarkSetKnownEmptyBeforeRead() to verify setKnownEmpty behavior before reading.
  • Added testWatermarkSetKnownEmptyPersist() to confirm setKnownEmpty results in no-op persistence for cleared holds.
• sdks/java/core/src/main/java/org/apache/beam/sdk/state/WatermarkHoldState.java
  • Added a default setKnownEmpty() method to the WatermarkHoldState interface, marked as internal.

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