Status

Current state: Under Discussion

Discussion thread: here

JIRA: here

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

The Stream Table join is inflexible how it handles late data in its current state. There is only one option from the stream side and only recently is there a second option on the table side. This semantic gap leads to incorrect output in some cases as the stream can only process data as it comes in.

If the table side uses a materialized version store the value is the latest by stream time rather than by offset within its defined grace period. This proposal would bring the stream side into alignment with that flexibility.

Public Interfaces

public class Joined {
   /**
	* Set the grace period on the stream side of the join. Records will enter a buffer before being processed. Late records in the grace period will be processed in timestamp order, records out of the grace period will be dropped.
	*
	* @param gracePeriod the duration of the grace period. If zero, no buffer will be used. 
	* @return new {@code Joined} instance configured with the gracePeriod
	*/
	public Joined withGracePeriod(Duration gracePeriod);
}

Proposed Changes

To allow users to configure the buffertime we will add one new api. The grace period will be set on the Joined object using a duration. This Joined object is currently only used for the stream table join and will be optional to set. The grace period will only affect the stream buffer and the table grace period, if set, will remain unchanged.

If a grace period is not set the join will execute as before, using the same logic in the stream table join node. If a grace period of zero is set the join will drop all late records from the stream side operations. If the grace period is non zero, the record will enter a stream buffer and will dequeue when the record timestamp is greater than stream time plus the grace period.

The buffer will be checked whenever stream time is advanced. From then the record will be evicted and sent down to the next processor, the join node. The buffer will use an implementation of TimeOrderedKeyValueBuffer .

Compatibility, Deprecation, and Migration Plan

Old joins will not be affected, in order to use the new feature users will need to set a grace period. Nothing needs to be deprecated.

Test Plan

The testing should be covered with unit test and integration test. The only other tests that would be necessary, would be benchmarking to assess the  performance impact on the stream buffer on the join.

Rejected Alternatives

• Only allowing grace period for versioned tables. This would be unnecessarily restrictive especially if the table is not going to change and there is no need for the versioning. Or if the table is already materialized for another use that does not want versioning