[WIP] Sequential Union Implementation#54578

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ericm-db wants to merge 15 commits intoapache:masterfrom

ericm-db:sequential-union-implementation

Commits on Mar 2, 2026

Add SequentialUnionManager for managing sequential source processing
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Step 1: Add SequentialUnionExecution with basic child tracking

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Step 2: Add followedBy API and SparkStrategies support

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Add followedBy stub for Spark Connect

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Step 3: Implement offset control for sequential execution

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Wire StreamingQueryManager to instantiate SequentialUnionExecution

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Fix child-to-source mapping initialization in SequentialUnionExecution

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Return startOffset as endOffset for inactive sources (not None)

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Add debug logging and disable premature auto-transition

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Add AvailableNow preparation and auto-transition logic (WIP)

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Complete core sequential union implementation with AvailableNow prep

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Delete redundant SequentialUnionManager and update handoff doc

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3/5 tests passing
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Add Kafka sequential union test and clean up debug logs
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Fix NPE in Kafka metrics and add union child test
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