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We will provide unit tests to validate the proposed changes.

Rejected Alternatives

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Merge RecordEvaluator and stoppingOffsetsInitializer (currently provided via KafkaSourceBuilder's setBounded() or setUnbounded()) into one class.

For example, we can add a KafkaStopCursor class (similar to PulsarSource's StopCursor) which contains all possible stopping criteria (e.g. based on the offset, the de-serialized message, and the ConsumerRecord).

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This alternative has the following disadvantages compared to the proposed approach:

a1) It introduces backward incompatible changes to KafkaSource. This is because we will need to replace KafkaSourceBuilder::setBounded(...) with the new API.

b2) KafkaStopCursor can not be shared with other source types because different sources have different raw message formats. For example, KafkaSource uses offset and ConsumerRecord, whereas PulsarSource uses MessageId and Message. In comparison, the RecordEvaluator proposed in this FLIP (as well as the proposed implementation changes in the SourceReaderBase) could be used by other sources (e.g. PulsarSource) to detect EOF based on de-serialized records.

c3) The implementation of this alternative approach will likely be harder to maintain. Note that users might want to stop the job based on the offset, de-serialized message, or both. The offset-based criteria should ideally be evaluated before records are de-serialized for performance-reason; and the criteria based on the de-serialized record should be evaluated after the record is de-serialized. Thus these criteria ideally should be evaluated at difference position in the code path. It could be inconvenient to achieve this goal while having these logics in the same class.

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Let user specify eofRecordEvaluator via StreamExecutionEnvironment::fromSource(...).withEofRecordEvaluator(...)

The advantage of this approach that the feature can be used by all connectors without we having to change the implementation (e.g. KafkaSourceBuilder, PulsarSourceBuilder) of individual connectors. Thus it improves the connector developers' experience.

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