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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 execute as a normal join as each record comes it will try to join to the point of time in the versioned table. If the grace period is non zero, the record will enter a stream buffer and will dequeue when the record timestamp is lees than or equal to than stream time plus the grace period.  Late records, out of the grace period, will be executed right as they come in.

The buffer will be checked whenever stream time is advanced. Stream time will be advanced whenever a new record enters the buffernode from the stream. From then the record records will be evicted and sent down to the next processor, the join node. The buffer will use an on disk implementation of TimeOrderedKeyValueBuffer.

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When a failure occurs the buffer will try to recover from an OffsetCheckpoint if possible. If not it will reload the buffer from the a compacted change-log topic.

Adding a buffer to the stream will increase the latency of the join by at least the grace period. It will also take the time to serialize and store the record as well as retreive retrieve and deserialize.

Compatibility, Deprecation, and Migration Plan

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