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In this FLIP, we propose to optimize performance for the above use-cases by allowing an operator to effectively switch its execution mode from batch to stream based on "backlog" status of the records emitted by the source operators.

NOTE: this FLIP focuses only on the capability to switch from batch to stream mode. If there is any extra API needed to support switching from stream to batch mode, we will discuss them in a follow-up FLIP.

Behavior changes when switching from batch mode to stream mode

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For simplicity, we assume the Flink job uses default configuration values for all configurations (e.g. scheduler-mode) except execution.runtime-mode. And the operator reads records from a source which switches from isBacklog=true to isBacklog=false before it reaches EOF.TODO: we will update the FLIP to clarify what happens when isBacklog switches from false to true.

1) Scheduling strategy

Batch mode:

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