Allow null consume in BatchPushSource

[srkukarni]

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Motivation

BatchSource records allow sources to return a null record to indicate that the batch is done.
For BatchPushSource, since we are using LinkedBlockingQueue, user's cannot simply pass a null value. Thus we need a special mechanism to indicate the end of a batch.

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[jerrypeng]

A small optimization: Just declare a final variable and re-use

[srkukarni]

done

[david-streamlio]

Wouldn't it be easier to allow users to place a null value inside their existing Record class and then test for that condition rather than creating a NullRecord class? e.g.

private static final boolean isNull(Record rec) { return (rec == null) || (rec.getValue() == null); }

Then you could just call this method instead of using the instanceof NullRecord check

[jerrypeng]

The thing is, is a record with a null value the same as returning null? Can a record have a null value but have other fields e.g. key with valid values

[david-streamlio]

The Null object is currently only inserted if the record passed in is null, so there aren't any fields.

[david-streamlio]

Letting users to continue to use their own Record types and using a null value inside the record seems like a more intuitive approach to me

[srkukarni]

That is the approach thats taken here. Users will be using their own record types and when they are done with the task, will do consume(null) to signify the end of the task. The NullRecord is strictly private class as an internal impl detail ofBatchPushSource